# Bottom Balancing



## Roy Von Rogers (Mar 21, 2009)

There should be no debate on this any longer, for its the only way to do a lithium pack correctly.

I may want to add that after discharging the cells to 2.7v per cell, to wait at least 24 hours, I like to wait 48, because some cells will come back up a bit, and to bring those down to 2.7v till all of them are equal, and wait another 24 to make sure.

It takes time to do this right, spend the time, so later on you wont have to buy more cells, cause you were in too much of a hurry to get your ev up and running.

Roy


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## Elithion (Oct 6, 2009)

Your argument follows very nicely from this one premise:

"No single cell should touch the top end of the charge curve, ever. No single cell should touch the bottom of the charge curve, but if it does, all other cells should do so at the same time."

As you presented that premise without substantiation, would you care to tell us very precisely how you derived it?

Thanks


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## EVfun (Mar 14, 2010)

Since you are making this bold statement of fact would you care to share your working experience with LiFePO4 battery packs?



Roy Von Rogers said:


> There should be no debate on this any longer, for its the only way to do a lithium pack correctly.
> 
> I may want to add that after discharging the cells to 2.7v per cell, to wait at least 24 hours, I like to wait 48, because some cells will come back up a bit, and to bring those down to 2.7v till all of them are equal, and wait another 24 to make sure.
> 
> ...


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## EVfun (Mar 14, 2010)

skooler said:


> (The following will be controversial!) Cell drift doesn't exist, except for unequal losses in capacity over time.


Please tell me what I have experienced then...

I bought 42 ThunderSky cells in February 2010. Initially, 40 where run in one car for about 6 months. Over the winter I removed them and installed 32 of them in my EV Buggy. I drove it like that for 1 year. Over the next winter I added 6 of the 8 cells back to the Buggy pack to increase the voltage. The pack runs without a BMS or any connections to the pack except at the ends. It is a top balanced pack so I can expect all the cells to reach 3.5 volts at the same time. The 6 cells I added back had quite a habit of creeping up in voltage. 

I would start with all the cells charging to 3.50 volts average (range 3.47-4.55, always the same cells low and high.) The 6 cells I added back where in the bottom part of that range (3.47-3.50) initially. After a couple months those 6 cells where all up to 3.7 to 3.9 volts, the rest of the cells slightly lower for the same end of charge voltage. Using a resistor I removed 0.2 amp hours from the 6 cells as a block. In following charges the cells where once again near the bottom of the pack voltage range at the end of a charge. Over a couple months they cells again showed the same increase in end of charge voltage, and where once again knocked down about 0.2 amp hour as a block to lower them back into range. I watched this happen 3 times, over about 3 months each time.


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## TTmartin (Feb 7, 2012)

EVfun said:


> Please tell me what I have experienced then...
> 
> I bought 42 ThunderSky cells in February 2010. Initially, 40 where run in one car for about 6 months. Over the winter I removed them and installed 32 of them in my EV Buggy. I drove it like that for 1 year. Over the next winter I added 6 of the 8 cells back to the Buggy pack to increase the voltage. The pack runs without a BMS or any connections to the pack except at the ends. It is a top balanced pack so I can expect all the cells to reach 3.5 volts at the same time. The 6 cells I added back had quite a habit of creeping up in voltage.
> 
> I would start with all the cells charging to 3.50 volts average (range 3.47-4.55, always the same cells low and high.) The 6 cells I added back where in the bottom part of that range (3.47-3.50) initially. After a couple months those 6 cells where all up to 3.7 to 3.9 volts, the rest of the cells slightly lower for the same end of charge voltage. Using a resistor I removed 0.2 amp hours from the 6 cells as a block. In following charges the cells where once again near the bottom of the pack voltage range at the end of a charge. Over a couple months they cells again showed the same increase in end of charge voltage, and where once again knocked down about 0.2 amp hour as a block to lower them back into range. I watched this happen 3 times, over about 3 months each time.


In my humble opinion, 
I think that would be classed as uneqlossy repayment over time due to adding the 6 cells. The cells in the pack have not been treated the same over time. Also the pack is not bottom balanced and also not (re- bottom balanced) when the 6 cells were added.

If I'm talking Rubbish Please Explain

I have no experience what so ever in looking after lithium cells but I've ordered 72, 100ah sinopoly cells for my TT and am trying to learn as much as I can before they arrive, hopefully early June.
At this point in time everything I have read on these forums and else where suggest for me, bottom balance as per mikes post is the way to go, but I'm open to any advice as I don't want to end up with a very expensive mistake


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## Ziggythewiz (May 16, 2010)

TTmartin said:


> I think that would be classed as uneqlossy repayment
> 
> If I'm talking Rubbish Please Explain


uneqlossy is not even a word.



TTmartin said:


> At this point in time everything I have read on these forums and else where suggest for me, bottom balance as per mikes post is the way to go, but I'm open to any advice as I don't want to end up with a very expensive mistake


 



Ziggythewiz said:


> 70 or 80% DOD is recomended to prolong cell life, while charging is nearly always to 90-100% SOC. It's easier to stay away from the bottom than it is to stay away from the top, so better protection on the top end is more useful.
> Potential overdischarge would happen when you are in the car, able to notice and respond to any warnings, limp triggers, or disable features you've installed. Potential overcharge would happen when the car is unattended, usually when you are having dinner, watching the game, or sleeping. That makes it more important to protect from overcharging.
> Top balancing is easier
> Cells (at least CALBs) usually arrive at ~60% SOC. It's faster to charge them to nearly 100% than to drain them to nearly 0% just to charge them again.
> There are more purpose built battery chargers available than dischargers.


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## GerhardRP (Nov 17, 2009)

skooler said:


> <snip>
> Note how the voltage spikes at the start and begins to fall out rapidly at the end.<snip>
> Mike


This is a discharge curve. 
Does the voltage similarly "spike" on charge. Or alternatively, how sharp is the knee on charging. Or another alternative form of the question is: can you detect one single cell being fully charged by monitoring the full pack voltage during charge [assuming no failed cell]?


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## Elithion (Oct 6, 2009)

GerhardRP said:


> Does the voltage similarly "spike" on charge.


The Open Circuit Voltage? Yes, identically: on a first order, the OCV vs SOC curve is fixed, and independent of current.

The terminal voltage? Mostly. But it depends on the IR drop due to the charging and discharging current.




GerhardRP said:


> can you detect one single cell being fully charged by monitoring the full pack voltage during charge?


In general no, because the pack voltage does not divide equally among cells, due to unbalance, variations is cell capacity, variations in cell DC resistance. The exceptions are:


Top balanced pack, at 0 current, 100 % SOC (professional EVs)
Mid balanced pack, at 0 current, 50 % SOC (professional HEVs)
Bottom balanced pack, at 0 current, 0 % SOC (No-BMS school of thought)
In those 3 cases, and those 3 cases only, all the cell voltages are identical, so you can deduce the cell voltage from measuring just the pack voltage and dividing by the number of cells.

But you asked specifically about during charging. Then, your answer is: only if the pack is top balanced.


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## EVfun (Mar 14, 2010)

TTmartin said:


> In my humble opinion,
> I think that would be classed as uneqlossy repayment over time due to adding the 6 cells. The cells in the pack have not been treated the same over time. Also the pack is not bottom balanced and also not (re- bottom balanced) when the 6 cells were added.


LOL, I like that 

The thing is, my pack is top balanced, the 6 added cells where top balanced a little farther up the curve, then added to the pack and then their top balance point was matched to the pack top balance point (using the same resistor to pull the 6 down together.) So at the launch they all agreed to finish up at 3.50 volts, with a range of 3.47-3.55 volts, at the end of charge.

So I am seeing some type of change in the way cells charge because of previous different treatment. It is taking them out of top balance. What can that be if not some type of difference in charge efficiency or self discharge? Either of those would be a deal breaker for the concept of no drift in LiFePO4 cells.


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## onegreenev (May 18, 2012)

> uneqlossy is not even a word.


It is now.


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## dragonsgate (May 19, 2012)

As far as I could find Une Glossy is some kind of French makeup. On Topic. This sounds like equalization of the cells so my question is how often do you have to balance the batteries? Is it a one shot deal?


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## onegreenev (May 18, 2012)

So you have a cell that can hold 40ah and another that can hold 45ah. Now you bottom balance both to 2.6 volts each. Then charge them up and stop when you have 38ah put into them both since you are charging them in a string. What you have is two cells that both have exactly 38ah within both cells. No more no less. What you will find if you monitor the pack is that the one cell that can only hold 40ah will have a little higher end voltage than the other but both still have exactly 38ah. That cell will show different behaviors while driving also but it still started with exactly 38ah and when your done driving you will have taken out exactly 38ah. 

Each cell in a pack is slightly different and will show a wandering behavior under loads but by no means does it change the end result. It does not cause drift, self discharge nor imbalances unless of course you have a load on a few cells which many actually have.

Bottom balance your cells to 2.7 volts. This has been confirmed and validated and repeated. There is no doubt. 

Charge your pack to 3.5 or if you want 3.45 which is well in the safe zone. Without a BMS you can actually safely charge your pack and not worry about sending cells off the charts. If by chance you do have a cell that is OFF the charts you need to replace it. Simple. 

I fully concur with Mike and know what happens when you do drive an improperly bottom balanced pack to the end of charge where one or more cells reaches empty and you drive those into reversal. You WILL kill the cell and bloat it beyond belief fast and there is no recovery. 

Charging a few cells into the 3.7 volt range with little amperage is not going to damage your cells.


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## onegreenev (May 18, 2012)

dragonsgate said:


> As far as I could find Une Glossy is some kind of French makeup. On Topic. This sounds like equalization of the cells so my question is how often do you have to balance the batteries? Is it a one shot deal?


Bottom balancing should only need to be done once unless you need to marry in a cell or if you put in a parasitic load then you'd need to do it a lot. Don't put in any parasitic loads. Loads that come from the entire pack are fine as long as you don't leave it sit with that load continuing. But being bottom balanced you may be ok if you let the pack drain. Best to just disconnect all loading when sitting for a week or more. If you forget you could have trouble.


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## jddcircuit (Mar 18, 2010)

EVfun said:


> Please tell me what I have experienced then...
> 
> I bought 42 ThunderSky cells in February 2010. Initially, 40 where run in one car for about 6 months. Over the winter I removed them and installed 32 of them in my EV Buggy. I drove it like that for 1 year. Over the next winter I added 6 of the 8 cells back to the Buggy pack to increase the voltage. The pack runs without a BMS or any connections to the pack except at the ends. It is a top balanced pack so I can expect all the cells to reach 3.5 volts at the same time. The 6 cells I added back had quite a habit of creeping up in voltage.
> 
> I would start with all the cells charging to 3.50 volts average (range 3.47-4.55, always the same cells low and high.) The 6 cells I added back where in the bottom part of that range (3.47-3.50) initially. After a couple months those 6 cells where all up to 3.7 to 3.9 volts, the rest of the cells slightly lower for the same end of charge voltage. Using a resistor I removed 0.2 amp hours from the 6 cells as a block. In following charges the cells where once again near the bottom of the pack voltage range at the end of a charge. Over a couple months they cells again showed the same increase in end of charge voltage, and where once again knocked down about 0.2 amp hour as a block to lower them back into range. I watched this happen 3 times, over about 3 months each time.


EVfun,
I am very intrigued by your experience. Thank you for sharing. If the older cells are shrinking in capacity at a faster rate than the newer ones or if there is some other environmental reason for the apparent drift is an interesting one.

Using either top or bottom balancing doesn't seem to be much of an issue to me. The main issue for me is the lack of resolution in pack voltage to quantify possible differences in cells.

I guess the question for me is how much measurement resolution is needed to detect or react to what level of mismatch cases.

Regards
Jeff


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## TTmartin (Feb 7, 2012)

TTmartin said:


> In my humble opinion,
> I think that would be classed as uneqlossy repayment over time due to adding the 6 cells. The cells in the pack have not been treated the same over time. Also the pack is not bottom balanced and also not (re- bottom balanced) when the 6 cells were


Oop's how embarrassing, what's worse is I thought I'd read it through before posting
Not much hope for my cells when they arrive
Enough said I think


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## kennybobby (Aug 10, 2012)

*Excellent Thread!*



skooler said:


> ...what I recommend for charging of LiFePO4 cells. Rather than repeating myself every time I thought I'd post what I think is right... Cheers,
> 
> Mike


This is a great go-to post with all the basis procedure for how-to-do-it. 

We did it this way for Paul's Celica (Weighs ~ 3000 lbs) and he is up and running down the road. It appears that his 44 cell 100Ahr pack comes off the 160 Volt (3.65) cut-off charger in the mornings at 150 Volts (3.41) but quickly drops to 144 (3.27) in the first mile or so. Then follows a very linear sag burning about Weight/10 or 300 Wh/mile down to pack of 136 V (3.1). By then he had better be getting close to home because at 132 (3.0) he is 'out of gas' and will need a tow...


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## skooler (Mar 26, 2011)

Elithion said:


> Your argument follows very nicely from this one premise:
> 
> "No single cell should touch the top end of the charge curve, ever. No single cell should touch the bottom of the charge curve, but if it does, all other cells should do so at the same time."
> 
> ...


Hi Elithion,

I deliberately didn't get *too* techy so not to scare of any newbies!...

As a result of actively testing lifepo4 cells and discussing with others who have done the same (not with huge, expensive prismatics but smaller cylindrical cells). I can make 3 destructive conclusions bout the chemistry:



Overcharging any single cell will significantly reduce capacity or destroy it.
Overdischarging any single cell on its own will reduce its capacity, by how much depends on a few environmental factors, how deeply discharged and point 3
Overdischarging any single cell and then passing current through it (how much depends on the capacity remaining in the cell) will cause it to go into a state of reversal. This is identifiable when a negative voltage can be read across the cell terminals where a positive one would normally be read. the cell will have a significantly reduced capacity and likely be destroyed.
So by having a pack that is not bottom balanced will make point number 3 much more likely to happen. 



By bottom balancing, all of the cells are fully discharged at the same time so there is no potential for (much) current to flow and cause reversal. 



If a pack is not bottom balanced, there is very likely to be potential in some of the cells that can be drawn through a depleted cell and cause reversal.



For anyone who wants to try this. Buy four small, cheap LiFePO4 cells on ebay, only need to be a couple of AH. Fully charge (3.5v+) three of them and discharge (2.5v) another. Put them into a four series string and load the pack with something 12v (bulb or something). Monitor the voltage of the depleted cell and watch it go negative as the pack depletes.


You can then measure the capacity of each of the cells. The easiest way to do it is to fully charge all for cells individually. Put them back in series and discharge again. 


If you can get the reversed cell to hold a charge, you have done well. If it does, measure the voltage of it compared to the other three while it is being discharged lower by any chance?


Obviously if you have something like a powerlab you can test the capacity using that.



Hope this helps with the understanding.


Cheers,


Mike


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## Elithion (Oct 6, 2009)

Hello Mike,

To summarize, your premises are:

1) "No single cell should touch the top end of the charge curve, ever."

2) "No single cell should touch the bottom of the charge curve, but if it does, all other cells should do so at the same time."

> Overcharging any single cell will significantly reduce capacity or destroy it.

Not quite: it mostly increases its resistance. But your premise doesn't follow from this point.

> Overdischarging any single cell on its own will reduce its capacity

That's just restating your first point. And your premise still doesn't follow it.

> Overdischarging any single cell and then passing current through it ... will cause it to go into a state of reversal.

Very true. But, again, your premise doesn't follow from this point either.

> If a pack is not bottom balanced, there is very likely to be potential in some of the cells that can be drawn through a depleted cell and cause reversal.

True. (Though that can also happen if a pack _is_ bottom balanced.)
Again, your premise doesn't follow from this point.

---

You presented your premises as if they were facts.

In absence of corroborating evidence, please allow me to think of them as your opinions instead of facts. And I do respect your opinions. 

Carry on!


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## onegreenev (May 18, 2012)

What other premise do you need. There is plenty of FACT to substantiate what is said. Facts don't just come from a LAB. All are reproducible. Try it sometime.


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## skooler (Mar 26, 2011)

dragonsgate said:


> how often do you have to balance the batteries? Is it a one shot deal?


Hi Dragonsgate,

Bottom balance once on install and follow the steps in the first post.

Check for that lowest capacity cell (first to reach 3.5v) every few hundred cycles. Ff it has changed then change you charge cutoff voltage.

Only when you suspect something has gone wrong or you wish to change the pack in any way should you need to bottom balance again.


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## onegreenev (May 18, 2012)

Davide,

In response to your graphs.


> In absence of corroborating evidence, please allow me to think of them as your opinions instead of facts.


. May we toss it back at ya?


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## Roy Von Rogers (Mar 21, 2009)

Confucius says....

Man with hammer, always looking for nail.

Roy


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## onegreenev (May 18, 2012)

Hammer away.


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## skooler (Mar 26, 2011)

Elithion said:


> Hello Mike,
> 
> To summarize, your premises are:
> 
> ...


Maybe I haven't written clearly or yo have misunderstood my points.

I don't see why you don't think my points follow?

Either way, let me just state the following.

I personally prefer bottom balancing, It protects the pack from overdischarge (which I think is most likely) but not so much from overcharge although this is easily managed by undercharging the cells slightly

Nothing wrong with top balancing, it protects the pack from overcharge but not overdischarge, perhaps getting slightly more capacity from the cells but at a slightly higher risk of something going wrong.

BMS have there uses. I think in 95% of EV conversions they are an added expense that is not necessary (sorry!). The only use I can really see is when one needs to squeeze every possible watt of energy from the cells without burning them up, probably where space and weight are issues (motorcycles?). Although there are some that would say that a BMS increases the risk of fire. I don't think this is the case when implemented correctly with degree of common sense.

Better of spending the funds on additional cells to make up for the slight undercharge on a bottom balance in my opinion.

I think BMS do have a place in this market, but only for a select few that have a requirement for them. They can be replaced by good education, a little bit of manual monitoring, a top or bottom balance and even a few extra cells.


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## GerhardRP (Nov 17, 2009)

skooler said:


> Hi All,
> <snip>
> The below graph is of a single Sinopoly 60AH (B) cell being discharged at a little over 30A (0.5c) measured capacity was exactly 60AH. This cell had been over-discharged and abused in the past so probably not the best example! They normally come in closer to 70A
> 
> ...


My earlier question about recognizing overcharge, answered by Davide was meant to be rhetorical. I think it can be done. During the main phase of charging, there is a linear rise of voltage vs. Coulombs delivered. It should be trivial for a modern microprocessor based system to recognize that the total measured voltage slope is larger than expected and therefore the charge should be terminated.

Gerhard


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## Elithion (Oct 6, 2009)

GerhardRP said:


> It should be trivial for a modern microprocessor based system to recognize that the total measured voltage slope is larger than expected and therefore the charge should be terminated.


Bingo! .


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## fb_bf (Jul 6, 2011)

I have to speak up here because I did buy a BMS, an Elithion Pro. I did it because while I was very much into building an electric car, I knew eventually I would rather just drive it and wouldn't want to have to check my batteries every so often. I read all of the posts about people who had over charged their batteries because of the bms, and decided that most of those occurrences probably came from operator error. When I installed my Elithion, I was careful to follow all of the instructions. I have the proper cutoff relays, the extra 12 volt power source that runs during charging, and just in case, an Elcon charger set to turn itself off if the pack voltage exceeds 3.7 volts * 38. I tested all of the turn-off methods before charging, and then on the first pack charge, (after top balancing) I watch the whole thing through my computer. I have had this system now for 1 and 1/2 years, and haven't had any issues. I simply come home, plug in my car and the next morning the BMS has finished the charge and top balanced the pack. I ran the pack to the end, set to 2.7 volts. The bms set it's fault line, and I had my controller set to go to a limp home mode, (max allowable 80 amps). This all worked as well. This winter, the BMS warned me a few times of low voltage because of the cold. That is also programmable. I also got a SOC meter out of it. I built a circuit to run my gas gauge off of its SOC output. It correlates very well (as in I think it is dead on) to my miles driven after a full charge. If I forget to re-set my trip odometer I have my gas gauge to go by. I now should go out and check my cell voltages, because at the end of the charge it seems it takes longer for the balance to finish. All I need to do is hook up the computer and read the cell voltages reported by the BMS. No individual voltmeters needed. I understand the bottom balance arguments, and I think that it is a perfectly good way to go if you're willing to put more time into your car. This thread hasn't gone so far as to bash bms systems like some others, and I glad to see that in forum posts. I just wanted to post a very positive experience with a bms system.


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## Russco (Dec 23, 2008)

Purchased a group of CALB 100 AH cells from Portland. Spec. sheets show a tested 118 AH and factory instructions state cells are 50% charged.

Everyone has their opinion of charging lithium cells: Mine is to bottom charge to equal and use a cell log 8 for monitoring LV during driving and HV during charging. 

Your opinion may vary. That's fine.

Reading over the DIY for the past several years, I have an idea of recommended charging of the new cells. Please correct me if my procedure is wrong.

First Step: Take a single cell and connect to a resistive load that draws around 10 amps to discharge the cell. Connect just the first channel of the Cell Log 8 to shut off the load automatically when the cell reaches 2.7 volts.

Second Step: Repeat the first step two more times after allowing the cell to rest.

Third Step: Repeat for the remainder of cells.

Fourth Step: Connect all cells in series in the vehicle and set the charger for (number of cells) (3.500v) Set the Cell Log 8 to shut off when any cell reaches 3.5 volts on charging and 3.0 volts when driving. 

Fifth step: Place a 10 amp load across the series string and monitor cell voltage with a 4-1/2 digit DVM. All cells should hit 2.7 volts at the same time. If there are some early sprinters out front, bleed a little juice off that cell with a small 2 amp load.

How's that sound? Thank you for your help.


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## Zak650 (Sep 20, 2008)

If you have a JLD404 or JLD 5740, a contactor that you probably have for the car anyway, two 10' pieces of 3/8 rebar you can setup a 160 amp bottom balancer, here's a video from my thread on this system. Set the cells up in parallel and the rebar in series, at 3.2V the rebar gets to 160 deg F.




Zak's 160 amp bottom balancer in batteries and charging


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## Ziggythewiz (May 16, 2010)

Russco said:


> Fifth step: Place a 10 amp load across the series string and monitor cell voltage with a 4-1/2 digit DVM. All cells should hit 2.7 volts at the same time. If there are some early sprinters out front, bleed a little juice off that cell with a small 2 amp load.
> 
> How's that sound? Thank you for your help.


Sounds a bit off. First, you can't bottom charge. You bottom discharge. And that last bit you're discharging when you should be charging.


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## EVfun (Mar 14, 2010)

skooler said:


> Hi Dragonsgate,
> 
> Bottom balance once on install and follow the steps in the first post.
> 
> ...


Could you do an actual bottom balance check at some point? What I mean is, not verifying that the same cell is the first to the top, but a series discharge after 500 or so cycle to make sure they all still agree on the bottom, based on voltage under a light load at near dead. 

I posted earlier about how I have seen some top end drift with my top balanced pack. Clearly I can't look at bottom and learn anything, as they have never been there (even my capacity checks end at 3.00 volts at 0.2C discharge rate.) 

If you, or even better several, people would recheck their bottom balance with a discharge test after putting hundreds of cycles on their pack I would be most interested in the results. Some of the best cases for me to compare to would involve packs that have had cells added, but solid matched packs are needed for comparison purposes.


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## octagondd (Jan 27, 2010)

Cells slowly lose capacity with usage, but what I want to know is, does a cell lose more capacity based on its SOC? Does it hurt the cell more to draw 200-300 amps at 80 or 90%DOD as opposed to 50%DOD?

I think I found some general lipo studies a while ago that basically state this in their cycle testing saying that recharging around 50-70%DOD gave optimal cycle life. Too shallow of cycles hurt life and too deep hurt life. I am not sure if that holds true with LiFePo or not.


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## arber333 (Dec 13, 2010)

Hi Can you help me with a problem?

I am using DIY BMS modules to assist with bottom balancing. I discharged the pack to 2,8V and charged untill one cell hit 3,6V couple of times and they went together untill full discharge quite nicely. I drive 65km to work and 50 of those are highway. Speeds are 110km/h and discharge is cca 130A.

However day before yesterday i was returning home and i noticed one cell no. 22 voltage was falling below 2,6V at 130A load. I managed to get home and checked this cell. It bounced back to 3V. Nothing to cry about.

I charged them trough the day. First thing i noticed was the soc reading was 92% and cell no. 22 has shut the charger off. I went to work and by the time i arrived there i only had cca 50% SOC left!!! After work i drove back. 15km before home I noticed cell 22 and 23 voltages were falling. I went off the highway and drove at cca 50A - 90A. Nothing changed and just before home my BMS lost comm with those batteries because the modules need at least 2,2V of power to work. I checked cells in my garage and i found them quite warm. They cooled much slower then the rest. But when they cooled down they had 3,1V again. WTF!!! 
My front box lid is lined with neopren rubber to press the cells in their place. Could this cause the cells to heat up? But why only two of them?

Do you have a solution? 

Arber


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## Zak650 (Sep 20, 2008)

arber333 said:


> Hi Can you help me with a problem?
> 
> I am using DIY BMS modules to assist with bottom balancing. I discharged the pack to 2,8V and charged untill one cell hit 3,6V couple of times and they went together untill full discharge quite nicely. I drive 65km to work and 50 of those are highway. Speeds are 110km/h and discharge is cca 130A.
> 
> ...


It's beyond me why people think that any electronic circuit or components are more reliable than no circuit at all. If you bottom balance your cells and do not attach any bms device to them then charge them together in series. There is only one thing attached to them to make them exceed the set voltage and that's the charger. Compare that system to each bms component attached to each cell plus the control module and all it's connections in addition to the charger. The sheer numeric possibilities of a failure overwhelms the simplicity of a single component, the charger.
After bottom balancing the cells and connecting them in series, keep an eye on the voltages of all the cells as you charge them. The one with the least capacity will reach the highest voltage first. Your charger should shut off when the that cell reaches 3.45 to 3.5 volts. There are probably several cells that are of fairly close voltage at this point make note of them and watch them periodically.

You have to think of these cells as something that tend to measure at 3.2 volts in a rested state. They are like a basket ball with only a slight amount of air pressure. It's basically a sphere that doesn't change it's size or shape a great deal with varying pressures. Increase the pressure enough and the weakest spot will bulge or decrease the pressure and a dimple will appear. 

Keeping away from the extremes is cell nirvana and happiness will reign. Live at either edge and chaos will have it's day.


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## arber333 (Dec 13, 2010)

Zak650 said:


> It's beyond me why people think that any electronic circuit or components are more reliable than no circuit at all. If you bottom balance your cells and do not attach any bms device to them then charge them together in series. There is only one thing attached to them to make them exceed the set voltage and that's the charger. Compare that system to each bms component attached to each cell plus the control module and all it's connections in addition to the charger. The sheer numeric possibilities of a failure overwhelms the simplicity of a single component, the charger.
> After bottom balancing the cells, charge them and keep an eye on the voltages of all the cells. The one with the least capacity will reach the highest voltage first. Your charger should shut off when the cell with the least capacity reaches 3.45 to 3.5 volts. There are probably several cells that are of fairly close voltage at this point make note of them and watch them periodically.


Well, i am doing exactly that . And my BMS is actually only voltage measuring assist. Though it can send the pulse to disconnect charger when one battery reaches 3,6V. Additionaly i use the modules to discharge cells together by 1A shunting untill 2,8V which is my zero point. 
The problem is not BMS, as it actually warned me about two cell voltages falling below 2,5V. I had to ignore it to reach home but the strange thing is, i measured both cells just after and they were at 2,3V and climbing a little. However after 1hour they reached 3,1V!!! How, if they were empty? The rest of the pack was steady at 3,2V. As i said i discharge them at cca 130A (up to 300A for a brief time). Oh yes, the cells are Sinopoly 200A black.

A

A


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## Zak650 (Sep 20, 2008)

Any connection, device, electronic component, or circuit that is connected between the terminals of a battery cell has the ability to cause an electron flow between the terminals of that cell. Multiply that possibility times the number of cells with similar circuits and this is what makes a different number of electrons pass through each of the other cells in the pack and thus unbalances the pack.

Z


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## IamIan (Mar 29, 2009)

skooler said:


> (The following will be controversial!) Cell drift doesn't exist, except for unequal losses in capacity over time.


Goes too far for my tastes.

I'd be happier with something like "Is very small" ... or ... "smaller than ___" ... etc.

Proving a absolute negative like the way you wrote it ...is virtually impossible ... and yet you state it casually.

Just my 2 bits ... and my own personal preferences.

- - - - - - - - - 

As for the Bottom vs Top Balancing :

To me it's either a personal preference ... and a person always does one or the other even when it is not the best option for a given scenario.

Or , the person knows the pros and cons of both methods and will use either method interchangeably as they best fit the context of a given scenario without personal bias one way or the other.

- - - - - - - - - - 

As for BMS vs no-BMS :

What I find is that most people who advocate 'no-BMS' in general when asked about the details ... are not actually advocating no-BMS ... instead they are advocating Human manual BMS done at some interval ... and an active human BMS at some level ... because they trust Humans to error less often at these tasks than the computer systems of some BMS.

Which computers and such do fail ... but I don't always share others faith in a human having fewer errors ... especially without personal knowledge of the particular human involved.


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## EVfun (Mar 14, 2010)

Zak650 said:


> Any connection, device, electronic component, or circuit that is connected between the terminals of a battery cell has the ability to cause an electron flow between the terminals of that cell. Multiply that possibility times the number of cells with similar circuits and this is what makes a different number of electrons pass through each of the other cells in the pack and thus unbalances the pack.


Do you have any evidence that the BMS failed the pack, as opposed to the BMS warning about a failure within the pack?

The bottom-balance-no-BMS group likes to tell me about how over-discharge is virtually inevitable, but so far not one has been able to share end of discharge pack voltages on a deep cycle after a significant period of cyclic use. Personally, my latest work was top balance no BMS and my next setup will most likely be top balance shunt reg. 

If I set my controller minimum voltage to cell count times 2.5 I can draw 5C without issue for 75% of the charge, I will feel power loss between about 10% and 25% SOC, and I can't use the bottom 10% in any practical way for driving. I can't get a end of discharge voltage lower than 3 volts with a test load of 0.05C after the minimum voltage limit has rendered the pack useless. On the plus side, I can accurately determine pack balance on any charge cycle. How far do they vary from 3.50 volts when the charger is holding the pack at 3.5v * cell count?

As I recall, the last poll on the BMS issue shows that half of Li battery users here are using a BMS. Could it be they post less because they have fewer issues? I'm starting to wonder.


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## Zak650 (Sep 20, 2008)

EVfun said:


> Do you have any evidence that the BMS failed the pack, as opposed to the BMS warning about a failure within the pack?
> 
> The bottom-balance-no-BMS group likes to tell me about how over-discharge is virtually inevitable, but so far not one has been able to share end of discharge pack voltages on a deep cycle after a significant period of cyclic use. Personally, my latest work was top balance no BMS and my next setup will most likely be top balance shunt reg.
> 
> ...


I'm strictly talking about the possibility of any device to fail. Simplicity and fewer items to fail is the main advantage of top or bottom balancing with no BMS. The advantage of bottom balancing is if some parasitic load is left on the cells in the pack stand a better chance of survival. Top balancing protects the cells better at the top end of charge.


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## onegreenev (May 18, 2012)

> Could it be they post less because they have fewer issues? I'm starting to wonder.


As stated before I and We state the bottom balance issue because we DON'T have troubles. Why would we say do this if we had troubles. We bottom balanced and our troubles just went away. 

Pete


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## nucleus (May 18, 2012)

arber333 said:


> Well, i am doing exactly that . And my BMS is actually only voltage measuring assist. Though it can send the pulse to disconnect charger when one battery reaches 3,6V. Additionaly i use the modules to discharge cells together by 1A shunting untill 2,8V which is my zero point.
> The problem is not BMS, as it actually warned me about two cell voltages falling below 2,5V. I had to ignore it to reach home but the strange thing is, i measured both cells just after and they were at 2,3V and climbing a little. However after 1hour they reached 3,1V!!! How, if they were empty? The rest of the pack was steady at 3,2V. As i said i discharge them at cca 130A (up to 300A for a brief time). Oh yes, the cells are Sinopoly 200A black.
> 
> A
> ...


I think you go off the rails a bit with the voltage measurement while you drive the car... That voltage bounces around depending on the discharge rate. The only reliable voltage measurement on the LiFePO4 cells is rested, after they have been sitting for 24 hours.

You say "How, if they were empty?" - but they weren't empty were they!? 

*The voltage under load varies greatly, and has little relation to state of charge.*

If I were you I would cut the charge off at 3.35 volts.


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## EVfun (Mar 14, 2010)

onegreenev said:


> As stated before I and We state the bottom balance issue because we DON'T have troubles. Why would we say do this if we had troubles. We bottom balanced and our troubles just went away.
> 
> Pete


As I recall, the latest poll on Battery Management System shows that half of Li battery users here are using a BMS. Though this case is an exception, I'm not seeing enough posts about BMS problems that suggest they are causing significant issues. I used one at one point and I never had an issue. I had good access to the cell tops so I didn't have the need (human BMS.) I have seen cell voltage creep though. Never seen an explanation.


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## onegreenev (May 18, 2012)

I won't disagree with that. I too don't see but then there are countless others doing conversions that just flat out don't bother with the forums except to glean information totally under the radar. I'd say there may actually be more users of BMS systems than not but I have also not seen any real solid reasons for needing them at least for the LiFePO4 cells. I fully understand the issues with BMS systems and what they are to do or supposed to do. 

I just choose NO BMS and speak it. I don't agree with the reasons people tout for top balancing. Not that others have not been successful with top balancing but I just don't buy the argument. 

Anyway it all boils down to personal preference. Giving as much information both for and against and top or bottom is very useful. 

One nice thing is that there ARE fewer fires being reported now that the bottom balance issue and no BMS has come to light. Been watching very carefully.


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## IamIan (Mar 29, 2009)

Zak650 said:


> I'm strictly talking about the possibility of any device to fail. Simplicity and fewer items to fail is the main advantage of top or bottom balancing with no BMS. The advantage of bottom balancing is if some parasitic load is left on the cells in the pack stand a better chance of survival. Top balancing protects the cells better at the top end of charge.


I'd agree.

But ... I'd add in the odds of the Human user / builder in question to fail / error ... when you don't have a computer doing a safety check for them.

Both the computer and the Human have possibility to fail or error.

I trust some humans more than some electronics for odds of error / failure ... and I trust some electronics more than some humans for odds of error / failure.


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## octagondd (Jan 27, 2010)

octagondd said:


> Cells slowly lose capacity with usage, but what I want to know is, does a cell lose more capacity based on its SOC? Does it hurt the cell more to draw 200-300 amps at 80 or 90%DOD as opposed to 50%DOD?
> 
> I think I found some general lipo studies a while ago that basically state this in their cycle testing saying that recharging around 50-70%DOD gave optimal cycle life. Too shallow of cycles hurt life and too deep hurt life. I am not sure if that holds true with LiFePo or not.


Does anyone have any thoughts or data on my question here? Is capacity loss greater at a larger DOD?

Here is my theory. Top balancing with shunt balancing BMS is absolutely doing its job of protecting the life of the cells. In fact, it may be doing too good of a job, and it may be a detriment to the pack in the future. If, (and this is a big, huge giant IF,) the capacity loss is larger at deeper discharge levels, then a cell that is 10% larger will be experiencing a slightly smaller capacity loss on a daily basis than a small cell. 

ie. Let's say you have a 100AH cell and a 110AH cell and one cycle removes .01% of the capacity. 100.00 AH cell after 100 cycles may now have 99.00 AH. One might expect the 110AH cell to have 108.9AH after the same 100 cycles, but what if it loses .008% per cycle instead of .01% because it is slightly larger and therefore not as deep in the DOD as the small cell? It would then be at 109.12AH instead of 108.9AH. This sounds nice on the surface because you have had a lower percentage capacity loss, but then you charge, and now your shunt balancer has to stay on a bit longer because your smallest cell has decreased its capacity at a greater percentage than your largest and reaches its "full" voltage quicker than the larger cell. Every cycle increases the gap and although you are still using your smallest cell at the normal rate of capacity loss, your shunt BMS has to work harder and harder each time to keep from overcharging the small cell. Has anyone with a shunt system noticed anything like this?

Does this make any sense or am I just talking out of my ass? Its ok if you think I am. I am just throwing stuff at the wall and seeing if it sticks.


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## EVfun (Mar 14, 2010)

octagondd said:


> Does anyone have any thoughts or data on my question here? Is capacity loss greater at a larger DOD?
> 
> Here is my theory. Top balancing with shunt balancing BMS is absolutely doing its job of protecting the life of the cells. In fact, it may be doing too good of a job, and it may be a detriment to the pack in the future. If, (and this is a big, huge giant IF,) the capacity loss is larger at deeper discharge levels, then a cell that is 10% larger will be experiencing a slightly smaller capacity loss on a daily basis than a small cell.


I think that fully charging will take more life out of a cell than a difference in discharge depth of 10% when the smallest cell is rarely going below 20% SOC. When you have a bottom balanced pack it is my understanding that you fully charge the smallest cells every cycle because it is that voltage jump that shuts the charger off. So I suspect that what you describe may be more of an issue for a bottom balanced pack -- in 5-10 years.


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## jddcircuit (Mar 18, 2010)

EVfun said:


> I think that fully charging will take more life out of a cell than a difference in discharge depth of 10% when the smallest cell is rarely going below 20% SOC. When you have a bottom balanced pack it is my understanding that you fully charge the smallest cells every cycle because it is that voltage jump that shuts the charger off. So I suspect that what you describe may be more of an issue for a bottom balanced pack -- in 5-10 years.


I agree with your logic. It does sound like the weakest cell could potentially be getting more stress during each charge cycle with a bottom balanced pack that is using pack voltage to terminate charge. This is perhaps one of the biggest cons against bottom balance methodologies.

When they say they are under charging their pack they mean under charging most of it I guess.

Does anyone know of a BMS that removes a fixed amount of charge (1AH lets say) from the cell that was first to full during charging? This method would distribute the frequency of times an individual cell entered into the high state of charge region across all cells. A different cell would be first to full each charge cycle in theory and perhaps increase pack cycle life if Depth of Charge is a significant factor in capacity degradation.

Regards
Jeff


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## onegreenev (May 18, 2012)

If you charge your pack to 3.5 and you have a cell that reaches 3.6 by the end of charge what is the problem? All cells are below the top. If however you have a cell that is in the 4 volt range then you need to replace the cell. Shunting with a BMS will not change the fact that you are still limited to the lowest capacity cell. Just replace it and be happy. Be sure your pack is well balanced. When you are talking 100ths or 1000ths of a volt difference your'e nit picking. 

Go drive your car and enjoy it.


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## octagondd (Jan 27, 2010)

EVfun said:


> I think that fully charging will take more life out of a cell than a difference in discharge depth of 10% when the smallest cell is rarely going below 20% SOC. When you have a bottom balanced pack it is my understanding that you fully charge the smallest cells every cycle because it is that voltage jump that shuts the charger off. So I suspect that what you describe may be more of an issue for a bottom balanced pack -- in 5-10 years.


I imagine almost everyone here is undercharging, regardless of balancing solution or BMS, so even shunting BMS top balancers are staying away from the end of the charge curve if they are using 3.65 or 3.7. I also have my doubts about 10-30 amp charge creating as much capacity loss as 200-300 amp discharge at a large DOD. As the graphs attest, using the cells at higher C-rates than 1C will give you fewer usable AHs. Over the long haul I imagine this also reduces capacity faster.

I believe most people who bottom balance do it as explained earlier in the thread. After the bottom balance, on the first charge you notate the pack voltage when the first cell hits 3.5 or 3.6 (depending on cell type). I have yet to get to this portion of my project, so it is all theory to me at the moment.

Not including your 6 troublesome cells, how many cycles would you say you have on your main pack and what DOD do you typically go to before re-charge? Have you noticed any difference in any cells. Did you do a capacity test on your cells before use by any chance or did you get a capacity list?


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## EVfun (Mar 14, 2010)

onegreenev said:


> If you charge your pack to 3.5 and you have a cell that reaches 3.6 by the end of charge what is the problem? All cells are below the top. If however you have a cell that is in the 4 volt range then you need to replace the cell. Shunting with a BMS will not change the fact that you are still limited to the lowest capacity cell. Just replace it and be happy. Be sure your pack is well balanced. When you are talking 100ths or 1000ths of a volt difference your'e nit picking.


There is some distance between 3.6 volts and 4 volts. At what point do you pull the smallest cell? (3.7 volts, 3.8 volts...) Most people don't have much of a stash of spare cells.


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## octagondd (Jan 27, 2010)

jddcircuit said:


> I agree with your logic. It does sound like the weakest cell could potentially be getting more stress during each charge cycle with a bottom balanced pack that is using pack voltage to terminate charge. This is perhaps one of the biggest cons against bottom balance methodologies.
> 
> When they say they are under charging their pack they mean under charging most of it I guess.
> 
> ...


As stated in this thread, when the smallest cell gets to 3.5 on the initial charge after bottom balance, the charge voltage is notated for the pack and that is the cutoff, so the smallest cell will intially only get to 3.5. As the pack loses capacity over 100s of cycles then the smallest cell might inch up a bit, but checking once every 6 months or year just is not that much of a problem.


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## EVfun (Mar 14, 2010)

octagondd said:


> Not including your 6 troublesome cells, how many cycles would you say you have on your main pack and what DOD do you typically go to before re-charge? Have you noticed any difference in any cells. Did you do a capacity test on your cells before use by any chance or did you get a capacity list?


That is tricky to answer because there is even disagreement on what constitutes a cycle. If a cycle represents a full charge then it's clearly over 1000 and most cycles are pretty harsh (5C to 7C peak currents with an itchy right foot.) The average DOD is likely only 50% though. I have not individually tested cells, only top balanced and then tested as a pack. I finish the discharge at 0.2C until the first cell hits 3.00 volts. The capacity loss at this point seems to be about 3% over 2.5 years. Any increase in internal resistance is slight, perhaps increasing from 1.5 to 1.6 milliohms. It may be less, temperature is a much bigger factor than any cell aging. In the cold internal resistance can rise to about 2 milliohms in my 60 amp hour Thunder Sky cells. 

What I know is that the cells are remaining in alignment. When the first cell hits 3.000 the next cell is 3.04. They are always the same 2 cells (number 2 is lower than number 1 until just below 3.1 volts, slightly higher internal resistance with slightly higher capacity.) The next few smallest cells continue being the same cells and with the same spread. The strongest cell is 3.082-3.084 volts. On the charge side, aside from the added creepers, the small scatter remains consistent and what cells are higher and lower doesn't change. I've been very happy with the little pack.


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## IamIan (Mar 29, 2009)

octagondd said:


> Does anyone have any thoughts or data on my question here? Is capacity loss greater at a larger DOD?


Thoughts , yes ... see bellow.

Data to verify / quantify ... not yet.

- - - - - 

I would expect the closer one is the edges 100% DoD or 0% DoD ... or 100% SoC or 0% SoC ... 0% SoE , or 100% SoE ... how ever you want to call it... the less room chemically there is for larger currents in that direction ... at near full less room for charging ... at near empty less room for discharge... that is not much of a stretch at that point and fairly common concept.

The 3 parts of the battery ... Anode , Cathode , Electrolyte ... do not each do their part of the electro-chemical reaction instantly ... the chemical reactions do not happen faster than light speed for example.

Also from 0% to 1% is not all of the molecules taking a 0% to 1% transition ... no ... some of the molecules completely change over from one state to the other ... among the truly massive number of molecules in the cell... even at 1% SoC or 99% DoD or 1% SoE , there are individual molecules that are already in the chemical form they will be in when the whole of the battery is at 100% SoC or 0% DoD or 100% SoE ... etc... what we refer to by those things ( SoE, SoC, DoD ) is referring to the average for all of the battery... but that is made of up the steps each molecule in the battery takes... they don't take 10% steps ... they either chemical convert to form B , or the stay as form A.

Because of those two reasons I would expect their to be a electrochemical gradient among the billions and billions of molecules at each of the 3 main parts ( anode , cathode, electrolyte )... that gradient might be visualized as peaks and valise on a 2D plane , like a topographical map ... but those peaks and valise are actually in the 3D space of the cell medium ... because each of those cell parts is also not 1 molecule thick... but many many molecules thick.

The details of the specific battery and conditions will alter how fast this gradient equalizes in the cell medium , and how potentially harmful some amount of gradient might be.

- - - - - - - - 

Another experimental way to help visualize the gradient diffusion ... is with effects like 'surface charge' ... where the total SoC , DoD , SoE ... of a given cell is not significantly changing ... but the real time terminal voltage is as the gradient diffuses through the cell medium.


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## octagondd (Jan 27, 2010)

IamIan said:


> Thoughts , yes ... see bellow.
> 
> Data to verify / quantify ... not yet.
> 
> ...


I think most people would say large currents at low SOC would be more harmful to capacity, which is why I began down this road in my theory. With top balancing, the further you get in DOD, the further apart two cells will be. At 70% DOD of a 100 AH cell, a 110 AH cell will be at 63.7%. So it is protecting the capacity loss in the large cell even more than the normal loss in the small cell. With a bottom balance, it is the opposite. The large cell is at a greater DOD during discharge, so it will lose its capacity% quicker, but I don't care because it will bring it closer in line with my smaller cell. Thats the theory anyways. I know it is much more complicated then that with internal resistance and all the chemical stuff you mention, but this is my laymen's theory of capacity loss.


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## onegreenev (May 18, 2012)

> John Hardy
> 21 minutes ago
> An addition to Anne’s thoughtful exhortations on having good electrical connections in your battery pack: I just had an intermittent problem with my battery test rig with a resistance in the charger circuit fooling the charger into terminating early.
> BTW I now have over 400 cycles on a set of CALB SE 40s. I am undercharging and under discharging slightly on all cycles apart from every 50th where I am doing a maximum effort charge and discharge to measure capacity. At cycle 400 there was about a 4.5% capacity loss compared with cycle 50. Just like the Headways there is no discernible voltage drift: minimum variance between all eight cells at some point in the charge/discharge cycle has remained consistently around 11 – 14 millivolts with no balancing since a bottom balance on assembly.


More or less they will loose capacity during use. Even by staying off the top and bottom but I will still stay off the top and bottom. My Leaf has shown no discernible loss of capacity even after 2 years and 23,500 miles. The Leaf stays off the top and bottom too.


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## octagondd (Jan 27, 2010)

onegreenev said:


> More or less they will loose capacity during use. Even by staying off the top and bottom but I will still stay off the top and bottom. My Leaf has shown no discernible loss of capacity even after 2 years and 23,500 miles. The Leaf stays off the top and bottom too.


That appears to be looking like standard capacity loss. Did he say what his full test specs were? Is it in a thread here? I am curious if he is doing a test based on factory charge and discharge specs, or a more real world DIY electric car type of test. He probably has to do constant discharge, but if you set it at 1.5C or 2C you could probably get a reasonable idea.


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## onegreenev (May 18, 2012)

No, he is not posting here at all. You do know that there really are not as many on this site as you may think. There may have been lots of registrations but really not many doing much here. I'll get back with you.


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## onegreenev (May 18, 2012)

His site must be getting hit pretty hard from folks checking out his testing. Check it out when you can get in. 

http://www.tovey-books.co.uk


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## octagondd (Jan 27, 2010)

onegreenev said:


> No, he is not posting here at all. You do know that there really are not as many on this site as you may think. There may have been lots of registrations but really not many doing much here. I'll get back with you.


Yah, I actually haven't been here in a while myself, but something brought me over and I saw a few threads that looked interesting. Once people get their conversion done, then there isn't much to check on, although any advances in the various components is nice to keep with.


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## onegreenev (May 18, 2012)

Well for me its more than just coming in and building a conversion. Its about education and teaching and sharing REAL information. Glad your back to learn. Things are going slow. I think the economy is more to blame.


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## octagondd (Jan 27, 2010)

onegreenev said:


> Well for me its more than just coming in and building a conversion. Its about education and teaching and sharing REAL information. Glad your back to learn. Things are going slow. I think the economy is more to blame.


I agree, the economy is certainly not helping, especially since the batteries have a large up front cost. Does John Hardy have a website where he is keeping his data for his test. I was thinking of doing something similar, but with the small RC batteries and that Powerlabs cycle tester. If I used a 5C 2AH cell I could get more cycles in much quicker. I know it is not the same as the format of cells we have, but the chemistry is the same.


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## onegreenev (May 18, 2012)

Don't know but you could contact him and ask. He is a great person and doing some interesting testing. A few of his videos have been on EVTV. Don't know where to find them except there. Nothing on VIMEO either. 

Pete 

Make up a plan of attack for your testing and be sure you have a control as well. I am sure you can put together a test and follow through with the test. Been thinking of that myself but decided I don't want to leave a running test at my place just in case it burns down. 

I don't have a good place to safely do a long term test.


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## IamIan (Mar 29, 2009)

octagondd said:


> I think most people would say large currents at low SOC would be more harmful to capacity, which is why I began down this road in my theory. With top balancing, the further you get in DOD, the further apart two cells will be. At 70% DOD of a 100 AH cell, a 110 AH cell will be at 63.7%. So it is protecting the capacity loss in the large cell even more than the normal loss in the small cell. With a bottom balance, it is the opposite. The large cell is at a greater DOD during discharge, so it will lose its capacity% quicker, but I don't care because it will bring it closer in line with my smaller cell. Thats the theory anyways. I know it is much more complicated then that with internal resistance and all the chemical stuff you mention, but this is my laymen's theory of capacity loss.


It does seem to be a common belief that the bottom is more susceptible than the top... and more important to avoid.

The only part for me at least is ... I don't yet have data to confirm and quantify how the two compare ... in Magnitude of risk , at different levels of rate , for different battery levels ( SoC , DoD, SoE ) ... thus I am personally hesitant to call one worse than the other without that data.

Additionally ... Without data about the % of battery time spent at different levels it also seems to me to be difficult to quantify the comparative exposure to each risk ... even if the top were to be quantified to say 1/2 as damaging per unit time of exposure as the bottom ... if the battery is at the top 3x more of the time ... the net effect is still statistically more harmful in the long run.

In the mean time ... until I have more quantified data ... the idea / theory of the gradient and diffusion rate seems sound enough to me to take some basic precautions about battery level and rates at the top and bottom.


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## Zak650 (Sep 20, 2008)

One way to look at the potential dangers at the bottom of the charge is does any cell in the pack have any power left in it to drive current through the other cells in the pack with less of a charge? Let's say you had a pack of 26 cells A-Z, they were all perfectly bottom balanced and driven down to 2.5 volts per cell. You removed cell H and replaced it with a fully charged cell and applied a heavy load. The fully charged cell H would probably kill all the other cells because it has the power(charge, amp/hours) to do so. A fully charged CA180 cell will put out 2000 amps, the other cells without much left in charge are defenseless. The load will travel through all the cells and kill all but cell H.

Instead:
If the original cell H is not replaced and a load is applied none of the cell has any more power left than any other to drive current through the pack so current simply stops flowing.


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## onegreenev (May 18, 2012)

The only time your charge is going to be a problem is when the charger fails to terminate. Then it matters not if your pack is top or bottom balanced. It will continue to charge until they just flat out bloat and blow and burn. Under normal charging and bottom balanced you will have a few cells maybe go higher than your nominal cut off voltage but if you have any that actually reach into the 4 volt range during the charge compared to the rest in your pack you need to change that cell as it is not properly matched. 

Many wish to not replace that cell and just purchase a shunt BMS and hope for the best. It is not that the cell is actually bad but mismatched to the pack. If its top balanced and you drive a touch too far you can kill that cell in seconds driving hundreds of amps of current throught the cell. 

In a charge situation your only looking at 10s of amps. Its easy to see where the most danger lies. So with that, bottom balancing is best and safest.


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## EVfun (Mar 14, 2010)

onegreenev said:


> The only time your charge is going to be a problem is when the charger fails to terminate. Then it matters not if your pack is top or bottom balanced. It will continue to charge until they just flat out bloat and blow and burn.


I had that charger problem once. Sitting for a few extra hours just above 3.5 volts didn't seem to do any harm. No cell made it to 3.8 volts (I don't think any exceeded 3.7 volts.) Some people have left cells charging near 3.6 volts for hours, perhaps even days, when balancing a pack. "Days" scares me, but doesn't seem to have caused problems. This is part of why I top balance. 

My pack is decently matched so setting the controller minimum pack voltage to 95 volts (2.5 * 38 cells) makes if painfully obvious before I could drive the pack into the ground. The pack voltage goes below that point at 150 amps (2.5C) discharge rates well before any cell is dead. I suspect that if your bottom balanced pack is matched enough that no cell is over 3.8 volts when the largest cell hits 3.5 volts then it is matched well enough that if top balanced the pack voltage would sag excessively before any cell goes flat.

I can't really "buy into" bottom balancing mostly because the only way to know they are still balanced is a deep discharge, which is rare. No one yet has posted information about a discharge test of a bottom balanced pack after 1+ years and 500+ cycles. With a top balance I can can check balance any cycle where I want to run around with a DMM in the last 15 minutes of a charge. Remember, I've seen some sort of "drift" (like thing) with 6 cells that where added back to my pack (and one charger timer failure.)


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## Siwastaja (Aug 1, 2012)

EVfun said:


> Remember, I've seen some sort of "drift" (like thing) with 6 cells that where added back to my pack


New cells may actually gain extra capacity on the first 10-100 cycles so maybe this explains the drift you have seen. Cell aging curve is not linear, so cells with different usage history age differently.


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## onegreenev (May 18, 2012)

EVfun said:


> I had that charger problem once. Sitting for a few extra hours just above 3.5 volts didn't seem to do any harm. No cell made it to 3.8 volts (I don't think any exceeded 3.7 volts.) Some people have left cells charging near 3.6 volts for hours, perhaps even days, when balancing a pack. "Days" scares me, but doesn't seem to have caused problems. This is part of why I top balance.
> 
> My pack is decently matched so setting the controller minimum pack voltage to 95 volts (2.5 * 38 cells) makes if painfully obvious before I could drive the pack into the ground. The pack voltage goes below that point at 150 amps (2.5C) discharge rates well before any cell is dead. I suspect that if your bottom balanced pack is matched enough that no cell is over 3.8 volts when the largest cell hits 3.5 volts then it is matched well enough that if top balanced the pack voltage would sag excessively before any cell goes flat.
> 
> I can't really "buy into" bottom balancing mostly because the only way to know they are still balanced is a deep discharge, which is rare. No one yet has posted information about a discharge test of a bottom balanced pack after 1+ years and 500+ cycles. With a top balance I can can check balance any cycle where I want to run around with a DMM in the last 15 minutes of a charge. Remember, I've seen some sort of "drift" (like thing) with 6 cells that where added back to my pack (and one charger timer failure.)


So when you hit that 2.5 volts under acceleration your controller reduces the amperage to keep them at 2.5 no matter what. Ouch. That must hurt performance when you jump on it. 

Well heck thats easy. If all your cells are at 2.5 static when you stop you are well into the bottom curve and they are balanced. I think there is way too much nit picking when it comes to this balance crap. I have taken my pack to a crawl as have others and one that most here really don't like. My cells were all at the same rate and the one that most here hate has recently done it again with his and all his cells were the same. No drift or imbalance. 

I know how fast you can loose a cell on the bottom while driving if one of the cells hits empty before the others. Actually I lost two. Since then no losses and bottom balanced. I do drive my car to the ends but I do have a pretty low limit to allow me to pump the juice during the accelerations. I do know what its like to limit the voltage to 2.5 volts and the controller does a bang up job keeping that there and it does a bang up job slugging along with little current but it keeps me from ever going under 2.5 volts. Well it did. I now have that sag limit at 2 volts. So if I hit the acceleration hard it will throttle back the current. But I will never go below 2 volts. So my pack stays balanced. 

Pete


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## EVfun (Mar 14, 2010)

onegreenev said:


> So when you hit that 2.5 volts under acceleration your controller reduces the amperage to keep them at 2.5 no matter what. Ouch. That must hurt performance when you jump on it.


No, until the pack gets low the cells only sag to 2.8 volts at 5C. Normally I set the low voltage warning light at 103 volts (2.71 vpc) and the controller minimum voltage to 95 volts (2.50 vpc.) I rarely see the 103 volt light (cool or getting low.) Somewhere near dead the 95 volt limit cuts power back hard.

I have a short range pack and I did a test when I first put it in my buggy. This is back when it was a 32 cell pack, not the 38 cell pack the above number are based on. I could feel the power cut a little at 28 miles, not annoying but noticeable to the driver. At 30 miles it was getting annoying, but city driving was acceptable. At 32 miles I could only manage residential neighborhoods. Voltage sag not only cuts power but moves the power and down to lower rpms. It was very unhappy and I was convinced to pull over and tow it about a half mile home. The resting cell voltages ranged from 3.064 to 3.162 (that note is still tacked to the garage wall.)

The next version of the pack will be a compact 16 gallon pack (8 * 14 * 33 = exactly 16 gallons) made up of 39 cells so the target numbers will change again. Cable drop will almost cease to exist with about 6 feet of 1/0 cable and 38 braided connectors, each 2.4 inches long with about 1/0 worth of braid. I will likely use 98 volt pack minimum and 106 volts for the warning light.


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## dougingraham (Jul 26, 2011)

octagondd said:


> I was thinking of doing something similar, but with the small RC batteries and that Powerlabs cycle tester. If I used a 5C 2AH cell I could get more cycles in much quicker. I know it is not the same as the format of cells we have, but the chemistry is the same.


The hobby Lipo cell chemistry is not the same. The closest that the RC guys use is the 26650 size A123 cells (3.2V 2.3AH). Testing with those would be similar. With the Lipo pouch cells not so much.


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## octagondd (Jan 27, 2010)

dougingraham said:


> The hobby Lipo cell chemistry is not the same. The closest that the RC guys use is the 26650 size A123 cells (3.2V 2.3AH). Testing with those would be similar. With the Lipo pouch cells not so much.


hmm I thought I saw some 5C rated LifePo4 cells on Hobby King or some site like that.


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## dougingraham (Jul 26, 2011)

octagondd said:


> hmm I thought I saw some 5C rated LifePo4 cells on Hobby King or some site like that.


You are right. They list quite a selection of packs as LiFePO4 based. Although it would bankrupt you if you tried to build a pack out of them for your car. About 1.3 wh/$ compared to 2.3 wh/$ for the GBS cells I purchased. But your goal is to do some testing and this would let you do that. The Zippy's were a better deal than the Turnigy brand.


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## drgrieve (Apr 14, 2011)

EVfun said:


> With a top balance I can can check balance any cycle where I want to run around with a DMM in the last 15 minutes of a charge.


And this is the flaw of top balancing during charging. Just like bottom balancing you can only measure SOC at static voltage - which takes at least 24 hours of rest. 48 hours is better.

If you want to truly top balance your pack you need to measure the voltage at rest over 24 hours after a charge.


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## EVfun (Mar 14, 2010)

drgrieve said:


> And this is the flaw of top balancing during charging. Just like bottom balancing you can only measure SOC at static voltage - which takes at least 24 hours of rest. 48 hours is better.
> 
> If you want to truly top balance your pack you need to measure the voltage at rest over 24 hours after a charge.


That's rather funny as the manufacturer defines full charge as charging to 3.65 volts and holding that voltage until the current drops to 3 amps (0.05C and I have 60 amp hour cells.)


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## Roy Von Rogers (Mar 21, 2009)

EVfun said:


> That's rather funny as the manufacturer defines full charge as charging to 3.65 volts and holding that voltage until the current drops to 3 amps (0.05C and I have 60 amp hour cells.)


 
Just because it claims to be a 60ah cells, that's doesn't mean it is so. Could be a 65ah,60ah or 55ah.

You cant know till you remove the capacity and record it, only then will you know what exactly your ah are.

If you empty all the cells to about 2.7v per cell, and wait 24 to 48hrs to make sure they all at the same low voltage, and then charge and put an ah counter on the pack, will you know what the max ah is on the pack.

You will know when the first cell reaches 3.5/3.65v. 

Or you can do one cell at the time.

Roy


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## Ziggythewiz (May 16, 2010)

drgrieve said:


> And this is the flaw of top balancing during charging. Just like bottom balancing you can only measure SOC at static voltage - which takes at least 24 hours of rest. 48 hours is better.
> 
> If you want to truly top balance your pack you need to measure the voltage at rest over 24 hours after a charge.


Except that you don't care about the resting voltage. You want the charge voltage to be balanced. It's really not tough to get them all within 1/10% and if you stay out of the last 5% who cares?


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## GizmoEV (Nov 28, 2009)

Ziggythewiz said:


> Except that you don't care about the resting voltage. You want the charge voltage to be balanced. It's really not tough to get them all within 1/10% and if you stay out of the last 5% who cares?


The problem is that when current if flowing or has recently stopped the diffusion rate of each cell is different giving you false voltage VS SOC readings. If you follow the charging procedure of charging to 3.6V and stop when the current drops to 0.05C you are merely getting the cells to a close approximation of 100% SOC. If you wait for the voltage to settle down then the cell is, hopefully, below 3.4V and not over charged but then the precision of the voltage measurement, time since charge, and temperature all affect the measurement. Bottom balancing eliminates all of that and gives you a pack in a known state of balance rather than a *presumed known* state of balance. If I could get my charger to terminate based on voltage AND current I would definitely go to a bottom balanced pack.


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## IamIan (Mar 29, 2009)

GizmoEV said:


> The problem is that when current if flowing or has recently stopped the diffusion rate of each cell is different giving you false voltage VS SOC readings. If you follow the charging procedure of charging to 3.6V and stop when the current drops to 0.05C you are merely getting the cells to a close approximation of 100% SOC. If you wait for the voltage to settle down then the cell is, hopefully, below 3.4V and not over charged but then the precision of the voltage measurement, time since charge, and temperature all affect the measurement. Bottom balancing eliminates all of that and gives you a pack in a known state of balance rather than a *presumed known* state of balance. If I could get my charger to terminate based on voltage AND current I would definitely go to a bottom balanced pack.


Why would bottom balancing eliminate the effected described of the diffusion rate and real time ( presumption ) based on the voltage while current is flowing?

It seems to me the same mechanism you describe giving potentially false approximations of SoC while current is flowing to charge , will do the same thing on discharge while current is flowing ... either way , it takes time at zero current flow for the rested terminal voltage to stabilize.

Either way , the presumption might be a fairly close approximation to the real thing ... but I don't see why bottom balance would eliminate this particular effect ??


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## skooler (Mar 26, 2011)

A very interesting video from Damien Maguire. It supports a lot of the statements in this thread.

http://www.youtube.com/watch?v=N2WARL8Y0Pk


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## GizmoEV (Nov 28, 2009)

IamIan said:


> Why would bottom balancing eliminate the effected described of the diffusion rate and real time ( presumption ) based on the voltage while current is flowing?
> 
> It seems to me the same mechanism you describe giving potentially false approximations of SoC while current is flowing to charge , will do the same thing on discharge while current is flowing ... either way , it takes time at zero current flow for the rested terminal voltage to stabilize.
> 
> Either way , the presumption might be a fairly close approximation to the real thing ... but I don't see why bottom balance would eliminate this particular effect ??


Because when you bottom balance each cell is taken to a given voltage such as 2.7V and left to sit for several hours. If the voltage is different after the sitting time it is then charged or discharged as needed until all the cells in the pack stay at the target voltage. The time sitting with nothing connected to the cells gives ample time for the diffusion to occur.

Now, the reason that the differences in diffusion don't matter on charge or discharge is that on charge, charging is stopped a little before the first battery reaches 100% SOC so there is no over charge. On discharge, the whole pack is reaching empty at the same time. Even if discharge depresses the voltage of some cells more than others, by the time it could be pushed to 0V the whole pack runs out of enough energy to keep the pack voltage above the controller's cutoff voltage so nothing bad happens to the pack.

Hope that makes sense.


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## skooler (Mar 26, 2011)

GizmoEV said:


> Because when you bottom balance each cell is taken to a given voltage such as 2.7V and left to sit for several hours. If the voltage is different after the sitting time it is then charged or discharged as needed until all the cells in the pack stay at the target voltage. The time sitting with nothing connected to the cells gives ample time for the diffusion to occur.
> 
> Now, the reason that the differences in diffusion don't matter on charge or discharge is that on charge, charging is stopped a little before the first battery reaches 100% SOC so there is no over charge. On discharge, the whole pack is reaching empty at the same time. Even if discharge depresses the voltage of some cells more than others, by the time it could be pushed to 0V the whole pack runs out of enough energy to keep the pack voltage above the controller's cutoff voltage so nothing bad happens to the pack.
> 
> Hope that makes sense.


Spot on - well explained


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## IamIan (Mar 29, 2009)

GizmoEV said:


> Because when you bottom balance each cell is taken to a given voltage such as 2.7V and left to sit for several hours. If the voltage is different after the sitting time it is then charged or discharged as needed until all the cells in the pack stay at the target voltage. The time sitting with nothing connected to the cells gives ample time for the diffusion to occur.
> 
> Now, the reason that the differences in diffusion don't matter on charge or discharge is that on charge, charging is stopped a little before the first battery reaches 100% SOC so there is no over charge. On discharge, the whole pack is reaching empty at the same time. Even if discharge depresses the voltage of some cells more than others, by the time it could be pushed to 0V the whole pack runs out of enough energy to keep the pack voltage above the controller's cutoff voltage so nothing bad happens to the pack.
> 
> Hope that makes sense.


Yes , that makes sense.

I took your first post / statement a bit too literal.

- - - - - - 


Here it seems you assume that the person doing the bottom balance is waiting with no load on each cell the time needed for the voltage to finish stabilizing... before they add a load to charge or discharge a bit again ... wait for __ hours or so again ... ect... repeat ... likely to take a long time to get all the cells , if one is resting the ___ Hours or so as indicated , between bits of fine tuning.

If the person doing the Top balance did the same kind of waiting resting period you are having the bottom balancing person do ... then they would have just as valid of rested voltage readings as the bottom balance person had.

ie ... bottom balancing does not skip this issue ... if someone trying to bottom balance a pack skipped the rest period it would skew their balancing accuracy similarly to how it could skew someone top balancing.

- - - - - - - 

Any one cell hit top or bottom first should also be equal to the person who is in the process of top or bottom balancing ... if both do the same rest period and the same per cell end fine tuning... for the purposes of creating the pack balance ( top or bottom ) in the first place.

And if the bottom balancing person does not do the rest period they will also suffer from the inaccuracies described from the voltage not yet stabilizing.

- - - - - - - 

The diffusion ( time for voltage to stabilize after a charge or discharge load has been removed ) ... is about the internal to one cell distribution ... I haven't seen data that shows a 'no harm' state from just avoiding 0V ... if anything the data I've seen seem to instead indicate otherwise.

ie:
the person who does 100% DoD cycles will see degradation in fewer cycles than the person who does 80% DoD cycles ... etc... that degradation comes from the sum of the smaller scale at the individual molecule level inside each cell... where harm is done 1 molecule at a time.

So it might not be as much harm ... but the data I've seen seems to strongly indicate that it is not a 'no harm' thing.


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## 1-ev.com (Nov 4, 2010)

HMMM. 
Do you guys know what happened to CALB USA http://calibpower.com/ ?

It looks like no page and no link from Chinese website http://en.calb.cn/ where it said "CALB USA" , but registration shows it valid until 2014...http://who.godaddy.com/whois.aspx?domain=calibpower.com&prog_id=GoDaddy


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## EVfun (Mar 14, 2010)

IamIan said:


> If the person doing the Top balance did the same kind of waiting resting period you are having the bottom balancing person do ... then they would have just as valid of rested voltage readings as the bottom balance person had.
> 
> ie ... bottom balancing does not skip this issue ... if someone trying to bottom balance a pack skipped the rest period it would skew their balancing accuracy similarly to how it could skew someone top balancing.


You can't provide a similar rest period for the purpose of top balancing for a couple or reasons.

1. You are trying to get all the cells to rise together during charge. The whole point is that the charger "sees" a sharp rise in voltage that initiates the charge termination process (timer, current level, abrupt off, whatever is chosen.) The cells are all charged to about the same top level, varying slightly by internal resistance, but it isn't significant. If you add 4 milliohms to one cell it only effects the charging voltage at 10 amps by 0.04 volts. At 300 amp driving loads that same extra resistance causes 1.2 volts MORE sag than the other cells -- it EV useless before internal resistance is a charging issue.

2. The cells have a nearly flat resting voltage unless very low on charge. Full is somewhere between 3.38 and 3.40 volts. You can get them to rest a little higher than that, but then they are overcharged and that is bad for them (it is something top balancing is supposed to make unlikely for any cell.) You would end up trying to balance based on a charged voltage to within a few thousandths of a volt, which means you are going to need extended rest time and tight temperature control and still get a less accurate balance. This is not practical.


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## IamIan (Mar 29, 2009)

EVfun said:


> You can't provide a similar rest period for the purpose of top balancing for a couple or reasons.


I disagree ... see bellow:

First and foremost ... it seems silly to me ... to disallow a rest period for top balancing ... and then fault top balancing for not using the rested terminal voltage... maybe that's just me.

The initial claim ... that started this particular aspect of the discussion .... made in post #78 pointed out the potential to make a mistake by presuming a cell's SoC using the terminal voltage while current is flowing and not the terminal voltage after the cell had rested.

Then went on to claim bottom balancing eliminated all of that and was not a presumed SoC ... but a known SoC.

It was not until after clarification was asked for that a correction was made ... and the rest period under no current flow was added to the bottom balancing method used ... and in so doing it nullifies the earlier claim that it eliminated that potential for a error in presumed SoC from voltage while current is flowing.

Weather current is flowing on a bottom balanced pack or a top balanced pack there is a difference between the loaded ( charge or discharge ) voltage and the rested voltage ... this effect is not avoided by people trying to top balance their packs ... and it is also not voided by people trying to bottom balance their packs... no mater which one someone is trying to do there is still a difference between a rested and a loaded terminal voltage... the issue is not avoided.



EVfun said:


> 1. You are trying to get all the cells to rise together during charge. The whole point is that the charger "sees" a sharp rise in voltage that initiates the charge termination process (timer, current level, abrupt off, whatever is chosen.) The cells are all charged to about the same top level, varying slightly by internal resistance, but it isn't significant. If you add 4 milliohms to one cell it only effects the charging voltage at 10 amps by 0.04 volts. At 300 amp driving loads that same extra resistance causes 1.2 volts MORE sag than the other cells -- it EV useless before internal resistance is a charging issue.


Voltage sag from V=IR is an issue ... but that is not the change in voltage being previously discussed about the diffusion rates in the cells... that is a different aspect.

Also ... the V=IR effect also happens on both charging and discharging and is an effect that would influence ones ability to accurately bottom or top balance a given pack ... neither method is immune to this effect.

Once amps of current flow go to zero ... the V=IR effect is gone ... for both ... those trying to top balance and those trying to bottom balance.



EVfun said:


> 2. The cells have a nearly flat resting voltage unless very low on charge. Full is somewhere between 3.38 and 3.40 volts. You can get them to rest a little higher than that, but then they are overcharged and that is bad for them (it is something top balancing is supposed to make unlikely for any cell.) You would end up trying to balance based on a charged voltage to within a few thousandths of a volt, which means you are going to need extended rest time and tight temperature control and still get a less accurate balance. This is not practical.


The extended rest time , temperature control , etc ... are needed to be employed weather someone is top balancing or bottom balancing... failure to do so on either type of balancing effort only reduces the accuracy of the per cell balance ... either top or bottom.

A top balanced pack is just getting each cell toward the top together ... just like a bottom balanced pack ... in both cases you can start with a whole pack to get close ... and in both cases the last fine tuning is done each cell ... weather you are doing top or bottom balance the concept is the same ... getting a pack into an initial state of being bottom balanced does not get to skip the rest periods ... or the per cell fine adjustments any more than a getting a pack top balanced would.

To skip that rest period on either one ... would get back to the same issues brought up in post #78 about the potential for error between a presumed SoC and an actual SoC... and the difference in voltage from loaded to rested.

- - - - - - - - 

The minor cell voltage fluctuations of a rested cell ... for people trying to top balance a pack ... are also issues for people trying to bottom balance a pack ... every cell is not identical ... small variations mean even a bottom balanced pack will not have all the cells exactly the same terminal rested voltage within say 1mV or 1 nV ... etc ... that variation exists on both methods ... the magnitude of the variation is largely effected by the quality of the cells and then getting pack to thermal regulation , rest periods, etc....

- - - - - - - - 

That the rested terminal voltages are not exactly identical doesn't prevent someone from using that rested terminal voltage method to try and balance a pack ... either way ... getting it top balanced or bottom balanced.

The person who is trying to top balance tries to get them close to each other in rested terminal voltage ... so does the person trying to do a bottom balance.


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## EVfun (Mar 14, 2010)

What voltage do you propose to use for a rested top balance? How many percent difference in SOC is represented by 0.01 volt change at that voltage? At the bottom 0.01 volt is just a fraction of 1% in SOC. 

I'm pretty sure that diffusion and internal resistance are closely related (not the same, but not independent of each other.) If diffusion is keeping the Lithium ions from moving then it will make their entry into the electrolyte (and in turn the opposite plate) slow and that shows up as voltage sag -- typically measured as internal resistance. 

I'm confident you will get closer to the same SOC based on charging voltage at a fairly low current. I'd suggest 3.65 volts at 0.05C, since at least one manufacturer uses that to define a full charge.


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## kennybobby (Aug 10, 2012)

*Boost and Sag*

i agree with the premise that the Open Circuit Voltage (OCV) after a period of rest will give a better indication of the state of the cell, than either the boost voltage during charging or the sag voltage while under load. 

It seems to me that one of the side benefits of bottom balancing is the opportunity to measure the actual cell capacity before putting the cell into service--this lets you verify that you have a good cell of the proper capacity as that which was ordered and paid.

And one of the benefits of top balancing is the ability to fill all the cells to an evenly balanced state of charge which could result in a higher overall voltge for the assembled pack than might otherwise be possible.


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## IamIan (Mar 29, 2009)

EVfun said:


> What voltage do you propose to use for a rested top balance?


That is a whole other issue of it's own... and there are options , each with pros and cons... and many shades of gray.

Not what I was giving my 2 bits about earlier... But ... I don't mind giving a few bits on it... I'll try to summarize , as a more in depth discussion about this would make this a very long post... I'm sorry if the summary is overly vague.

#1> My personal preference for a top balance would not to use voltage by itself ... due to the limits of precision touched on previously.

#2> However the question at hand here is , what voltage I would use ... for that I would say it depends on the priorities of the specific application ... higher rested voltages are more certain of the cells topness but stress the cell more ... lower rested voltages reduce the degree of certainty of the topness but also reduce the stress on the cell ... which is more important is either a far more technical discussion on a specific application , or it is personal preference ... to a degree one can use higher precision volt meters to go down to smaller rested terminal variations from cell to cell ... this helps some but does not remove the initial issue of priorities.

#3> But ... If we are not talking specifically about me ... in a bit more 'most people' kind of case ... I would say ... the main purpose of the top balance is to have all the cells of an entire pack reach the top as closely together as one group as is reasonably possible ... for that , one can use just about any rested terminal voltage they want to match all the cells , to create the pack balance ... the vast majority of people I've read seem to advocate for cell voltages of around ~3.45V ... but there are some who prefer others , which just gets back to variations in priorities touched on in #2 above.



EVfun said:


> I'm confident you will get closer to the same SOC based on charging voltage at a fairly low current. I'd suggest 3.65 volts at 0.05C, since at least one manufacturer uses that to define a full charge.


That would be a fairly common loaded voltage I've seen suggested ... and by using a fairly low current one can reduce both the V=IR effects and the Diffusion effects ... but as was touched on back in Post #78 ... it wouldn't eliminate those effects ... and it does reduce the degree of certainty one presumes the SoC to be ... Perhaps not enough to matter to some people ... but that's either situation specific details , or personal preference.

One could also use a fairly low discharge current for doing the bottom balance... and there too they would reduce the V=IR and diffusion effects ... but would be reduced accuracy for the presumed SoC from not using a rested terminal voltage ... but again it might not matter to some people , or in some specific applications.



EVfun said:


> How many percent difference in SOC is represented by 0.01 volt change at that voltage? At the bottom 0.01 volt is just a fraction of 1% in SOC.


You answered your own question ... the amount indicated per 0.01V varies... and as you pointed out it varies more so at the ends ... but it is both top and bottom , not just the bottom.

That rate of change in amount per 0.01V , is itself often a fairly good indicator of the SoC... better than V alone.



EVfun said:


> I'm pretty sure that diffusion and internal resistance are closely related (not the same, but not independent of each other.) If diffusion is keeping the Lithium ions from moving then it will make their entry into the electrolyte (and in turn the opposite plate) slow and that shows up as voltage sag -- typically measured as internal resistance.


Related in the sense that they are all aspects of the electrochemical reactions in the cell ... yes.

Related as in , the same mechanism ... no.

Diffusion is not just about the normal battery chemical reaction chain between the anode cathode and electrolyte ... Diffusion is also about each one of those in and of themselves... and each molecule in each of those.

The chemical reaction is not instant ... each one step from 1 molecule to the next is not instant ... not light speed ... not even 1/10 light speed ... etc.

So it is not that diffusion is 'keeping' the lithium ions from moving ... but it is pointing out that the reactions that cause them to move ... and allow them to move in the first place ... require time... and are not perfectly uniform across all the molecules.

V=IR effects are more related to the materials resistance to the imposed voltage differential and establishing a electromagnetic field ... this mechanism happens far faster than the chemical reactions.

Not only is Diffusion referring to a slower mechanism than V=IR ... it is also referring to a mechanism that continues after the current at the terminals as gone to zero , and the V=IR effect is gone... no more current is flowing into or out of either terminal of the cell ... but the diffusion mechanism is still going on and still happening.

Diffusion is a logarithmically decaying mechanism ... unlike the linear mechanism of V=IR ... which is one more reason why diffusion takes a much much longer time to bottom out.

I have not yet seen significant amounts of evidence on any correlation between measured V=IR and measured Diffusion rates... I suspect V=IR measurements would be a poor indicator of Diffusion rates... They aren't the same mechanism.


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## EVfun (Mar 14, 2010)

IamIan said:


> ... the vast majority of people I've read seem to advocate for cell voltages of around ~3.45V ...
> 
> So it is not that diffusion is 'keeping' the lithium ions from moving ... but it is pointing out that the reactions that cause them to move ... and allow them to move in the first place ... require time... and are not perfectly uniform across all the molecules.
> 
> V=IR effects are more related to the materials resistance to the imposed voltage differential and establishing a electromagnetic field ... this mechanism happens far faster than the chemical reactions.


It is true that actual internal resistance is in the metal foils and especially the poor conductivity of iron phosphate (there is generally some graphite in the positive plates just to improve the conductivity of the phosphate.) However, slow diffusion shows up as local depletion under load, which results in voltage sag. While not the same, the manifestation is very similar. The key difference is that slow diffusion will allow voltage recovery and decreased voltage sag under load if the cell is given time to "rest." 

I cannot recommend ever using 3.45 volts for rested voltage for a LiFePO4 cell. These cells should not have a rested voltage that high -- they have been overcharged. Overcharging can result in increased internal resistance, or perhaps it's a decrease in diffusion. (Is the result of overcharging a thickening in the SEI layer?)


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## Roy Von Rogers (Mar 21, 2009)

I don't understand why we keep going over and over on this topic, its so simple anyone should get it by now.

Lets say you have a 100 cell pack, you remove all the capacity from the cells, were there is little energy left.

You put an ah counter on the pack and you charge it, when the first cell hits 3.5/3.65v you stop, you note the amp hours on the meter, that's your pack capacity, and your pack is in capacity balance, you have a balanced pack.

If you charge to the top, you can still use the max capacity of the pack, but the pack is *NOT* in balance, for you now have cells with various capacities. And if you ever drive it accidently to the bottom, you will reverse a cell and kill it.

Look at it like balancing weights, you have a board balanced in the center, you put equal amounts of weights on the left and right, you will be in balance.

Top balancing would be tantamount to putting more weight on one side, but moving the center over to compensate for such imbalance. It may still balance the board, but that doesn't make the amount of weight equal.

Balance is when all cells get emptied equally to the bottom.

Roy


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## GizmoEV (Nov 28, 2009)

IamIan said:


> If the person doing the Top balance did the same kind of waiting resting period you are having the bottom balancing person do ... then they would have just as valid of rested voltage readings as the bottom balance person had.
> 
> ie ... bottom balancing does not skip this issue ... if someone trying to bottom balance a pack skipped the rest period it would skew their balancing accuracy similarly to how it could skew someone top balancing.


What you are forgetting is that to hold a LiFePO4 cell high enough up the voltage curve to where a small change in SOC is a significant change in voltage requires flowing current. When bottom balancing no current needs to flow and a cell can be held at any point desired along the steep part of the voltage VS SOC curve. So no, a top balanced pack cannot be balanced my holding the voltage at something like 3.65vpc or 4.00vpc without overcharging the cells and still dealing with a flowing current. Giving a rest period will bring the cells back down to a much flatter part of the curve (assuming the cells didn't get over charged) and reduce the accuracy of the balancing.

You can see what I'm talking about in the graphs shown here: http://tovey-books.co.uk/testing.php. Note that the voltages of some cells traded places during the charging and subsequent stopping of charge. If they had all been "capped" at a fixed voltage they would definitely have been at different states of charge.


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## GizmoEV (Nov 28, 2009)

Roy Von Rogers said:


> I don't understand why we keep going over and over on this topic, its so simple anyone should get it by now.


You get it and others of us get it but not everyone has spend the same amount of time getting it. Every one also has different experiences which factor into the "Getting It" process. Furthermore, what if those of us who "Get It" discover that we really didn't quite get it because someone came along and asked a valid question or showed some evidence which showed us that we weren't quite right?


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## EVfun (Mar 14, 2010)

I've got real world experience with LiFePO4 cells. I've yet to see any compelling reason to bottom balance. I don't like the idea that the smallest cell is also the cell that hits the peak charging voltage every cycle, since we know even fully charging a cell increases wear (shortens cycle life in testing.) I don't trust my charger as much as myself, and closely matched cells seem to flog the controller minimum voltage well before any one cell is in danger anyway.


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## IamIan (Mar 29, 2009)

EVfun said:


> While not the same, the manifestation is very similar. The key difference is that slow diffusion will allow voltage recovery and decreased voltage sag under load if the cell is given time to "rest."


Slower diffusion = longer time for voltage to reach the rested voltage state... and a larger difference from the loaded ( charge or discharge ) voltage and the rested voltage.

I still have not yet seen the sufficient data to show a strong correlation between measured V=IR ... Voltage sag or boost under load ... and diffusion rates ... they are not the same mechanism ... Without data to show the correlation ... I am hesitant to presume there is one ... in either direction ... the V=IR correlating to diffusion rates ... or ... diffusion rates correlating to V=IR.



GizmoEV said:


> What you are forgetting is that to hold a LiFePO4 cell high enough up the voltage curve to where a small change in SOC is a significant change in voltage requires flowing current.


I'm not forgetting that ... but that aspect is also true about the bottom end of a cell as well.



GizmoEV said:


> When bottom balancing no current needs to flow and a cell can be held at any point desired along the steep part of the voltage VS SOC curve.


Incorrect.

Discharge any cell you like ... and the moment you take the discharging current off the cell the terminal voltage will change ... violating your claim here ... as the voltage will not be held.

First the voltage changes as the V=IR effect goes to zero with zero current ... then over a longer period of of time the diffusion rate of the cell will slowly see the cell voltage increase ... all while no current is flowing ... used to be called "growing amps".

The terminal voltage is not held.

These effects happen at both Top and bottom ... the Bottom is not immune to them.



GizmoEV said:


> So no, a top balanced pack cannot be balanced my holding the voltage at something like 3.65vpc or 4.00vpc without overcharging the cells and still dealing with a flowing current. Giving a rest period will bring the cells back down to a much flatter part of the curve (assuming the cells didn't get over charged) and reduce the accuracy of the balancing.


yes.
But ... the same aspect you describe here is also true of the bottom as well... just switch the voltage from a high V to the bottom low point of cell V.

If you use the V under discharge your accuracy is reduced due to V=IR and diffusion effects ... If you use the voltage the instant current goes to zero without resting ... you still have reduced pack balance accuracy due to diffusion effects ... The terminal voltage will come up after you let it rest enough for the diffusion rate to balance out to the rested voltage.

And pulling the cell artificially too low can shorten ones cycle life as well ... over charging at the top end is not the only candidate ... either end of the cell has issues ... the bottom is not immune.

And weather one is top or bottom balancing ... as long as voltage is used by itself as the indicator of SoC ... both will suffer from the inaccuracy cause by cell to cell variations... and both suffer from inaccuracies of V=IR and diffusion effects.



GizmoEV said:


> You can see what I'm talking about in the graphs shown here: http://tovey-books.co.uk/testing.php. Note that the voltages of some cells traded places during the charging and subsequent stopping of charge. If they had all been "capped" at a fixed voltage they would definitely have been at different states of charge.


And the same link also shows they went to different discharge voltages as well at the bottom end ... ie ... the bottom is not immune.

Had the discharge been to a fixed capped voltage the bottom would have had different states , similarly to how you point out the top would have different states ... the bottom is not immune.

Although this particular aspect also gets back to the issue of individual cell variation ... cells are not 100% identical ... there will be variations... even in the 100%SoC or 0%SoC states.


----------



## GizmoEV (Nov 28, 2009)

EVfun said:


> I've got real world experience with LiFePO4 cells. I've yet to see any compelling reason to bottom balance.


The biggest reasons I see is in the case of someone pushing the limits of their pack and in the case of something slowly draining the pack like a heater getting stuck on or something. At least we know of a few who have not killed their pack when they pushed the limits and the couple cases of Jack Rickard having a pack drained by a stuck on heater and the pack drained in the car that sat for several weeks without the maintenance switch being turned off. Of course we don't know what would have happened if either or both packs would have been top balanced.



EVfun said:


> I don't like the idea that the smallest cell is also the cell that hits the peak charging voltage every cycle, since we know even fully charging a cell increases wear (shortens cycle life in testing.) I don't trust my charger as much as myself, and closely matched cells seem to flog the controller minimum voltage well before any one cell is in danger anyway.


So by using a top balanced pack all of the cells are hitting peak charging voltage thus wearing all of them, presumably evenly. I don't know if anyone else has documented it but I seem to recall Jack Rickard demonstrating that a set of TS cells being carefully top balanced and then driving the GEM they were in until the pack voltage said "empty" and finding 2-3 cells damaged due to reversal.

I wonder how closely matched the cells need to be to not matter whether they are top or bottom balanced? It probably depends somewhat on how long the string is.

I think that the most important thing is that a person truly understand the pros and cons of top vs bottom balanced, how to do each properly, and to know the pros and cons of a cell level BMS of various types vs no BMS except the human version. Combine that with what charging equipment is used and its limitations and then choose the best for that person.


----------



## GizmoEV (Nov 28, 2009)

IamIan said:


> Incorrect.
> 
> Discharge any cell you like ... and the moment you take the discharging current off the cell the terminal voltage will change ... violating your claim here ... as the voltage will not be held.


Why do you insist that I'm talking about the voltage immediately after the discharge load is removed? I am not and neither is any one else who does bottom balancing of a pack. To top balance you have to maintain current to the pack. To bottom balance you remove the discharge load and wait for the voltage to settle, something you cannot do with a top balanced pack if you are going to go up the steep part of the voltage curve.

Try this for a procedure. Pick a voltage such as 2.50V as your cutoff voltage for an automatic discharger. If you want program in a hysteresis so that if the voltage bounces above 2.6-2.7V the discharger turns back on again and cuts off at 2.50V. Run this on every cell in the pack. By the time the last cell has been discharged the first one will have settled quite nicely to some voltage below 2.7V. Choose an arbitrary voltage below this point or pick the voltage of the first cell discharged, doesn't matter. Manually balance all other cells to this voltage. Each time let the cell sit a while unloaded. It will settle in on a voltage.

Now, try to do that with 3.65-4.00V for a top balanced pack. It won't work. When you wait for the voltage to settle it will drop back down every time. You might be able to get the cell to hold a voltage over 3.38V but then you have over charged the cell and caused it damage. The bottom balance method we are talking will not damage the cell like trying to top balancing would.



IamIan said:


> And the same link also shows they went to different discharge voltages as well at the bottom end ... ie ... the bottom is not immune.


Of course, but he wasn't trying to bottom balance the cells at that point either, was he? Again, to bottom balance the balance voltage is after there has not been a load on the cells for some time, not immediately after removing the load. That is the key.


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## IamIan (Mar 29, 2009)

GizmoEV said:


> Why do you insist that I'm talking about the voltage immediately after the discharge load is removed?


That was only one item I mentioned ... it is not the key stone or anything like that... and as pointed out there are other issues even ignoring that point.

But ... to answer your question ... Because it is a clear and easy example of the concept ... and clearly shows the error of the statement / claim that was made ... the voltage is not held ... the bottom experiences the same kinds of issues the top does ... and it is a transition point where one of the effects ( V=IR ) goes to zero, but other effects remain , even without current flow at the terminals.

Thus it is relevant enough that I thought it warranted being talked about.



GizmoEV said:


> I am not and neither is any one else who does bottom balancing of a pack. To top balance you have to maintain current to the pack. To bottom balance you remove the discharge load and wait for the voltage to settle, something you cannot do with a top balanced pack if you are going to go up the steep part of the voltage curve.


Try a apples to apples version of that.

When talking about rest voltages and diffusion rates and how it effects the accuracy of a pack's balance ... either one ... top or bottom ... treating both top balance and bottom balance the same is vital to the comparison ... if you want a rest period for accuracy at the bottom ... then the top gets a rest period as well ... if you don't want the top to get a rest period than neither does the bottom.

When you are then comparing apples to apples you will see what I have been saying ... that both top and bottom have this same diffusion effect as the voltage changes while the cell rests ... and the accuracy of the balance either one ... top balance or bottom balance ... will be effected ... the bottom is not immune , it still experiences the effect.

Originally back at the start of this part of the discussion we are in now ... in Post #78 you were referring to the potential for reduced accuracy while measuring the voltage while current was flowing , as a means to determine pack balance for a top balanced pack ... and unlike the claim that the bottom "eliminates all that" ... it does no such thing ... do the same type of terminal voltage under load at the bottom will also have the reduced accuracy of being able to determine the packs balance.



GizmoEV said:


> Now, try to do that with 3.65-4.00V for a top balanced pack. It won't work.


Incorrect.
The mechanism will work the same way it does for the bottom balance you described... of course it's at the top and not the bottom ... but the mechanisms described are the same... see explanations of your points you made next bellow.



GizmoEV said:


> When you wait for the voltage to settle it will drop back down every time.


And similar thing on the bottom balance ... voltage will rise up every time you take the current off of it.... every time.



GizmoEV said:


> You might be able to get the cell to hold a voltage over 3.38V but then you have over charged the cell and caused it damage.


And similar thing on the bottom balance ... rested voltage will not hold bellow __V unless you've over discharged it and potentially caused damage ... shortened cycle life , etc... every time.



GizmoEV said:


> The bottom balance method we are talking will not damage the cell like trying to top balancing would.


See above ... apples to apples.

The bottom can do damage... doesn't have to.
The top doesn't have to do damage... can do damage.

Both have the potential to do damage ... neither one is immune to these effects ... both don't have to cause damage.



GizmoEV said:


> Of course, but he wasn't trying to bottom balance the cells at that point either, was he?


my point was that ... you were exclusively focusing on the top doing something ... and describing it as if it were a knock against the top ... while not mentioning at all that the bottom was doing the very same effect you were describing about the top.

ie ... top and bottom both experience the same effect you were describing in the example you provided ... neither one eliminated it.



GizmoEV said:


> Again, to bottom balance the balance voltage is after there has not been a load on the cells for some time, not immediately after removing the load. That is the key.


Apples to apples ... if you do that at the bottom to remove the V=IR and diffusion accuracy issues ... than do the same thing for the top... for it to also remove the V=IR and diffusion accuracy issues.

They ( top and bottom ) are both effected by both of these effects ... V=IR and diffusion... bottom itself does not eliminate either one.

The current goes to zero is what eliminates the V=IR effect on balance accuracy ... and it does so for both top and bottom.

The rest period is what eliminates the diffusion effect on balance accuracy ... and it does so for both the top and the bottom.


----------



## onegreenev (May 18, 2012)

Ian,

Where are you getting your information?


----------



## IamIan (Mar 29, 2009)

onegreenev said:


> Ian,
> 
> Where are you getting your information?


Books ... chemistry classes ... personal experience ... experiments I've done ... data posted by others on this site and other sites ... etc... etc.

I've been making the following points in a variety of ways ... by all means tell me which one of these you think is in error ... and why? ... or which one you would like data to support it?


#1> Cell Voltage changes while resting ( diffusion ) ... happens both on top and bottom ... neither one eliminates it.

#2> Voltage changes with V=IR ... happens both on top and bottom ... neither one eliminates it. 

#3> Cells are not 100% identical ... and variation happens ... neither top nor bottom eliminates it.

#4> Items #1 , #2, #3 above can effect the accuracy of the a pack balanced by measuring cell terminal voltage ... and do so for both top balanced and bottom balanced packs... neither one eliminates it.


----------



## onegreenev (May 18, 2012)

Im not understanding your application to top balancing and using static voltage to attain that. In my testing when you charge to 3.65 volts per cell and hold that until the current is like 4 amps your considered full. Upon resting for 24 hours the voltage then drops to 3.35 volts across the board. Problem is that all my cells balance out to that same static voltage but some cells only reached 3.4 or 3.5 volts while a couple reached 3.7 volts but all rest at 3.35 volts. They are not balanced but look balanced. Having a static voltage after 24 hours of higher than 3.4 volts is too high. That is not in the books or online. 

You should check out the last blog posting from Jack that addresses this once again. Also should check out his PDF on how to care for and charge your pack. He does provide a well written paper. 

http://blog.evtv.me/2013/06/2277/#wp-comments
http://media3.evtv.me/cellcare.pdf


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## IamIan (Mar 29, 2009)

onegreenev said:


> Im not understanding your application to top balancing and using static voltage to attain that.


Sorry for the misunderstanding.

That is not my application ... I do not personally prefer to use cell terminal voltage by itself as a means of determining cell balance for balancing a pack ... top or bottom... I dislike that method for a variety of reasons... but that's an entirely different can of worms.

The point I was making about the top balance and the rested voltage ... is that the voltage diffusion ( reason for a rested cell reading at all ) happens at both the top and the bottom ... neither one being at the top nor being at the bottom eliminate this effect.



onegreenev said:


> In my testing when you charge to 3.65 volts per cell and hold that until the current is like 4 amps your considered full.


We could have an entirely new discussion about the pros and cons of different methods of determining that one has reached 'full'... and what is considered 'full'... but what you describe is a common method.



onegreenev said:


> Upon resting for 24 hours the voltage then drops to 3.35 volts across the board. Problem is that all my cells balance out to that same static voltage but some cells only reached 3.4 or 3.5 volts while a couple reached 3.7 volts but all rest at 3.35 volts. They are not balanced but look balanced. Having a static voltage after 24 hours of higher than 3.4 volts is too high. That is not in the books or online.


3.65 per cell can not be the actual per cell if you are also getting cells at 3.7 ( above 3.65 ) , and 3.4 or 3.5 ( bellow 3.65 ) ... so the initial condition is not actually happening ... unless you meant 3.65 as an 'average' voltage ... not a per cell voltage.

As for the rested voltage ... ~100mV of precision is fairly limited... as is efforts to generalize for all cells.

I would bet that if you fully charged a cell ... let it do it's rest until the voltage leveled off ... discharged a small amount like 1% ... let it rest again ... that you will not actually have the same rested terminal voltage on that same cell to within 1mV of precision.

The note to make is this is for that specific cell ... trying to generalize this rested voltage to all other cells will be inaccurate ... because not all cells are 100% identical ... although the others will vary ... if the same experiment is repeated... but not necessarily to the 100% identical rested voltage.



onegreenev said:


> You should check out the last blog posting from Jack that addresses this once again. Also should check out his PDF on how to care for and charge your pack. He does provide a well written paper.
> 
> http://blog.evtv.me/2013/06/2277/#wp-comments
> http://media3.evtv.me/cellcare.pdf


Thanks for the tip ... will check it out.


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## onegreenev (May 18, 2012)

Correct, the 3.65 was the average for my pack. But because I bottom balanced the top varies a bit but not dangerously so. I normally check my voltage at three decimal points to the right. So 3.650 volts per cell average then hold to a specified amperage to allow a tiny bit of extra AH into the cell. By the way, its not much at all. The new norm is more like 3.550 or even 3.500 volts per cell average for a bottom balanced pack. 

As for the original argument, yes there is diffusion from top down and all point in between and at varying temps and currents too. That is just the natural course of charge and discharge under loads. There is no exact SOC but you can generalize pretty good if you allow a few seconds of rest. Knowing that its not a full 24 hours the difference is not that great. So working in a working parameter a good general SOC can be made if you check voltages at least three decimal points out. Better if you can do 4. 

I'd love to do a thread on states of full and how to get there. It will be short and sweet. Its pretty easy if you first determine what is considered full.


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## GizmoEV (Nov 28, 2009)

IamIan said:


> I've been making the following points in a variety of ways ... by all means tell me which one of these you think is in error ... and why? ... or which one you would like data to support it?
> 
> 
> #1> Cell Voltage changes while resting ( diffusion ) ... happens both on top and bottom ... neither one eliminates it.


#1 is in error because you insist that there is no rest period when doing a proper bottom balance. #1 simply doesn't apply to what I described. That is my whole point but you keep ignoring it. Of course there is a logarithmic increase in voltage when discharging a cell. That is the whole point of the rest time. The rest is sufficiently long enough to get the SOC of the cells sufficiently close in SOC. The cell is not at 0% SOC because the voltage is greater than 0V.

Top balancing and bottom balancing is not an apples to apples comparison and yet you keep trying to make it so and make what I said say something I didn't say. That is what the disconnect is. Try it, discharge a good LiFePO4 cell to 2.50V using a low current like 0.01C or less. Remove the load and let the cell sit for 24 hours. Measure the voltage and record it. Let it sit a week and measure the terminal voltage again. As long as the conditions remained the same as the week prior you will see little if any voltage change. If there is a voltage change it will only be indicative of a small fraction of a percent of the SOC. Why don't you add this into your list of experiments you said you have done?

Bottom balancing isn't something that is done very often. With a good pack of cells, once a year is overkill.

As for the 100%SOC resting voltage of a LiFePO4 cell at 20-25°C it is between 3.38V and 3.40V. When I come across the technical paper I found it in I'll post the link. As I recall it is closer to 3.38V. I have not seen it posted in any manufacturer spec sheet. The closest was when the SE cell spec sheet listed 3.40V as the "float" voltage.


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## EVfun (Mar 14, 2010)

> Posted by GizmoEV
> Now, try to do that with 3.65-4.00V for a top balanced pack. It won't work.





IamIan said:


> Incorrect.
> The mechanism will work the same way it does for the bottom balance you described... of course it's at the top and not the bottom ... but the mechanisms described are the same... see explanations of your points you made next bellow.


That seems to be the key of your misunderstanding. There is NO WAY for a LiFePO4 cell to display a *resting* voltage between 3.65 and 4.00 volts. The overcharge required to result in that rested voltage will have destroyed the cell.

You can have a cell display a resting voltage of 2.5 volts, or 2.8 volts, or even 2.0 volts (though I don't recommend going that far over the discharge curve.) Even if you leave the cell sit at that voltage for 3 days it would be hard to measure any cell damage on subsequent cycles. There are reports that voltages under 1.5 volts can damage the plates over time by allowing side corrosion reactions.

Short term damage seems to result from significant time over 3.8 volts, any time over 4.3 volts, any cell reversal, and allowing internal overheating due to excess discharge or charge current. Outside of those imminently bad things there are certainly actions that decrease life, but don't destroy the cell in short order.


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## Ziggythewiz (May 16, 2010)

onegreenev said:


> Problem is that all my cells balance out to that same static voltage but some cells only reached 3.4 or 3.5 volts


Which is still basically full, so who cares? Did you bother to measure the difference between the ones hitting 3.4 and the ones hitting 3.7?

It's not enough to get down the street, so why all the fuss?


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## IamIan (Mar 29, 2009)

onegreenev said:


> As for the original argument, yes there is diffusion from top down and all point in between and at varying temps and currents too. That is just the natural course of charge and discharge under loads.


Give that man a cookie. 

Of course given the resistance and fighting against this I've been hit with again and again ... it would seem like it is some kind of new idea ... and not the old hat that it is.



onegreenev said:


> I'd love to do a thread on states of full and how to get there. It will be short and sweet. Its pretty easy if you first determine what is considered full.


If you start one ... and I miss it ... give me a bump so I can swing by to take a look.

- - - - - - - - - - - 



GizmoEV said:


> IamIan said:
> 
> 
> > I've been making the following points in a variety of ways ... by all means tell me which one of these you think is in error ... and why? ... or which one you would like data to support it?
> ...


Ok first I'm assuming it's a typo and you meant to write something like ... that the logarithmic increase in voltage ( while resting ) just after discharging stopped... because the voltage does not increase when actively discharging.... the rest assumes my assumption is correct about the typo.

Green you agree with indicated point #1 ... and Red you disagree the with same point ... Green gets a cookie 

You've misunderstood what I was describing previously ... I'll try to explain again.

I am not insisting there is no rest period ... I am saying , if you do no rest period for A then you do no rest period for B ... it is the way to do a proper test ... apples to apples ... and when you do the rest period on both you get the logarithmic voltage change during the rest for both ( top and bottom )... ie , it is the rest period itself that is removing the diffusion effect , not the top or the bottom.

I understand you seem to be describing a apples to oranges comparison ... you seem to only want to give a rest to one and you seem to want to insist on no rest for the other ... I am not ignoring that ... I have been pointing to exactly that repeatedly ... I suggest an apples to apples comparison and one will see #1 referenced above , applies to both top and bottom.

It is not the bottom that removes the Diffusion ... its the rest period you are using ... and such a rest period will do the same kind of thing for the top for diffusion as it does the bottom.

It is not the bottom that removes the V=IR ... its the current going to zero... and having the current go to zero will have the same kind of effect for the top as it does the bottom.



GizmoEV said:


> Top balancing and bottom balancing is not an apples to apples comparison and yet you keep trying to make it so and make what I said say something I didn't say.


I'm sorry you got that impression... that was not my intent.

I'm not saying top and bottom are the same ... I'm saying the effects on reduced accuracy of making a balanced pack are effected by the Diffusion, V=IR, cell variation ... and those effects happen at both the top and the bottom ... neither top nor bottom eliminate these effects.

The things that do eliminate these effects will do so for the top or the bottom.



GizmoEV said:


> Why don't you add this into your list of experiments you said you have done?


Because .. It would require time travel in order for me to currently or in the future add an item to things I have already done in the past ... and my flux capacitor is on back order. 

Seriously though ... diffusion rates is something I have done some experiments on and I currently am doing other experiments on ... which is related to the experiment you suggest ... my experiments are not yet complete ... even when they are ... I won't know if the data is significant until I sift through it afterwards.



GizmoEV said:


> Bottom balancing isn't something that is done very often. With a good pack of cells, once a year is overkill.


I've been told ... and seen packs doing the same kind of balance retention for a good top balance with good cells as well.

- - - - - - - - - - - 



EVfun said:


> IamIan said:
> 
> 
> > GizmoEV said:
> ...


You misunderstood what I wrote.
That ( rested voltage ) is not what I was referring to.

See the Bold Underlined Pink part of the quote ... where it points to the explanation ... and if you go to those explanation points ... you see that 3.65 and 4.00 resting voltage ( itself ) , is not what I was claiming or referring to in the explanation.



EVfun said:


> Short term damage seems to result from significant time over 3.8 volts, any time over 4.3 volts, any cell reversal, and allowing internal overheating due to excess discharge or charge current. Outside of those imminently bad things there are certainly actions that decrease life, but don't destroy the cell in short order.


Now think about that on the actual functional scale of the chemical reactions in the cell ... ie each molecule ... keeping in mind the diffusion effects creating a differential gradient in the medium.

But that's another issue I have been toying with ... related to what you describe here ... but different from what was being discussed previously.


----------



## EVfun (Mar 14, 2010)

IamIan said:


> Now think about that on the actual functional scale of the chemical reactions in the cell ... ie each molecule ... keeping in mind the diffusion effects creating a differential gradient in the medium.
> 
> But that's another issue I have been toying with ... related to what you describe here ... but different from what was being discussed previously.


I don't just think about the chemical reactions, I charge and discharge LiFePO4 cells. Diffusion, for the most part, looks like internal resistance that changes if you wait. Particularly on discharge, diffusion limits are why a cell can be given a some rest and then display less voltage sag under load. Good LiFePO4 cells have a lot less of that than even AGM lead acid batteries. For an EV capable LiFePO4 cell it just isn't that significant at typical charging rates. If you plan to charge at 1C or better then much about your charging considerations needs to be changed.

Further, my experience tells me that charged and rested voltage doesn't work for top balancing LiFePO4. There is only a couple hundredths of a voltage between 95% SOC and 102% SOC (perhaps it's 101% or 105%, but clearly overcharged based on resting voltage.) Charging voltage works, and at low charge rates both diffusion and internal resistance are minor issues.


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## IamIan (Mar 29, 2009)

EVfun said:


> Charging voltage works, and at low charge rates both diffusion and internal resistance are minor issues.


A fair position.

We don't see eye to eye on all of it ... as we touched on in the previous posts ... but , almost no one ever does , 100% agree on everything about a topic ... if anything , that's a good thing.


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## Siwastaja (Aug 1, 2012)

IamIan said:


> A fair position.
> 
> We don't see eye to eye on all of it ... as we touched on in the previous posts ... but , almost no one ever does , 100% agree on everything about a topic ... if anything , that's a good thing.


The charging "problem" goes away by accepting that you quick charge only to 90% or so and then slow down charging. Then, internal resistance becomes a non-issue.

Of course, further research on how to charge as much as possible in the shortest possible time without sacrificing cell life, is valuable, but you need to recognize it as a special case.

Reducing charging current when the first cell hits the limit is the most typical solution and it works. That being said, most typical charge currents are so low to begin with (1/5C or less), and typical cells so close to each other in Ri, that the effect you describe cannot be seen at all.

Any of this of course does not have anything to do with "bottom balancing".


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## IamIan (Mar 29, 2009)

Siwastaja said:


> That being said, most typical charge currents are so low to begin with (1/5C or less), and typical cells so close to each other in Ri, that the effect you describe cannot be seen at all.
> 
> Any of this of course does not have anything to do with "bottom balancing".


I disagree.

It is one thing to say it is good enough ... for whatever some person's standard is for , 'good enough'.

It is entirely different to say that the effects I've described cannot be seen at all ... that is incorrect.

As I have stated previously ... the effects I described ... effect both top and bottom... and that is why it does have something to do with "bottom balancing".

That all having been said ... they are your batteries do with them as you wish.


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## GizmoEV (Nov 28, 2009)

IamIan said:


> GizmoEV said:
> 
> 
> > Why don't you add this into your list of experiments you said you have done?
> ...


Not it wouldn't. You have a list from the past, you do the experiments and add to the list.

To top balance to a voltage such as 3.6V requires a current until such time that the cell is overcharged. Assuming that the cell doesn't get overcharged then there will always be a current applied to the terminals thus there is no rest period because then the voltage will decay when the charge current is removed. The voltage will decay to a point not very far up the steep part of the curve.

To bottom balance the cell can be discharged to say 2.5V with say a 0.01C discharge current and then be left to sit and the voltage will settle to a voltage point on the steep part of the discharge curve.

It is this difference in how top balancing has to be done and how bottom balancing should be done that eliminates the problem of diffusion rates in the cell. This is what many of us have been saying all along but some how you don't want to take what we said and keep saying things we are not saying. It is the FACT that rest time can be used on a bottom balance and that it cannot be used on the top balance. Go do the experiments and you will see what we are talking about.


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## IamIan (Mar 29, 2009)

GizmoEV said:


> IamIan said:
> 
> 
> > GizmoEV said:
> ...


Next time take note of the  in comments ... it gives additional information , that can be useful in putting a specific comment in context.


Here you describe a action to do ... not and action that is done.
Past vs Future.

A list that I said ( Past tense ) describing things that I have ( past tense ) done ( past tense ) ... is completely in the past.

The list that I said is in the past ... it can not be changed without time travel to the past ... it was what it was ... at no point in the future will the list I said in the past change ... the past is done ... only a future reversion of the list or a current version of the list list can change , but those will then not be the list I said in the past.



GizmoEV said:


> It is this difference in how top balancing has to be done and how bottom balancing should be done that eliminates the problem of diffusion rates in the cell. This is what many of us have been saying all along but some how you don't want to take what we said and keep saying things we are not saying. It is the FACT that rest time can be used on a bottom balance and that it cannot be used on the top balance. Go do the experiments and you will see what we are talking about.


You misunderstand ... again I will try to explain.

This entire time I have over and over again ... in a variety of different ways been trying to explain this same point I have been making to you ... re-read previous posts if you like ... or read this one more carefully.

It is the rest period itself ... and only the rest period itself that is removing the diffusion effects.

It's an example of cause and effect.

The rest period itself is the cause of the loss of the diffusion effect.

The bottom is not the cause.

The top is not the cause.

------
I am not tell you , that you're balancing it wrong.
I am and have been pointing out ... what is the cause of the effect in question ( diffusion effect ).

The ONLY reason the diffusion effects are removed from a bottom balance done as you have suggested ... as I have repeated several times ... because of the rest period itself... and ONLY because of the rest period itself.

The ONLY reason the diffusion effect are not removed from the top balance as you have described ... as I have repeated several times ... because of the lack of a rest period itself ... and ONLY because of the lack of a rest period itself.

------

It is not Bottom balancing itself that removes the diffusion effect ... it is the rest period itself that removes the diffusion effect ... and that rest period will remove the diffusion effect from either top or bottom ... weather you use it top or bottom does not change that it is the rest period itself , that is the cause.

-----
as for "you don't want to take what we said"
100% incorrect.
read more carefully.

I have been taking exactly what you have said ... if you think otherwise ... please reference what gave you that impression ... and I can try to explain that to you again.

I have repeatedly over and over again tried to make the same point about what you describe ... to which you seem intent on skipping past the point I make about it ... that is not me 'not taking' what you said ... it is me making a point about what you said.


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## octagondd (Jan 27, 2010)

EVfun said:


> I've got real world experience with LiFePO4 cells. I've yet to see any compelling reason to bottom balance. I don't like the idea that the smallest cell is also the cell that hits the peak charging voltage every cycle, since we know even fully charging a cell increases wear (shortens cycle life in testing.) I don't trust my charger as much as myself, and closely matched cells seem to flog the controller minimum voltage well before any one cell is in danger anyway.


I am now arguing for the sake of arguing, so please don't take this as bottom balance evangelism. I was once in your camp (top balance, no BMS) and was afraid of my charger burning my car down, therefore it makes sense to balance the cell voltages at the top when I am not in the vehicle and control the voltages at the bottom when I am driving the vehicle. In fact, I may still operate this way for a while while I am testing my almost finished EV out. I individually top balanced each TS cell to 3.7V. --End Disclaimer--

You said you do not want the small cell hitting the 3.5V each charge correct? Maybe you are not understanding the process of bottom balance. You measure the total voltage of the system on the first charge when the first cell hits 3.5, and use that total as the cutoff or CV stage for the charger. This cell, regardless of capacity, should reach 3.5V each charge and the charger should shutoff or go CV relatively quickly around that total voltage. So the small cell hits 3.5V which is probably about 95% capacity and the larger cells are at 93 or 94% capacity. As the pack slowly decreases in capacity, the voltages remain. 95% will still be 3.5V if that is the correlation we are assuming here.

In your top balance situation, all of your cells are reaching the same percentage capacity, let's say 95%, so more of your cells are in danger of whatever you fear the small cell in a bottom balance is in danger of. So, not only are your small cells getting to that point, but all your cells are getting there. Why be afraid of one cell getting there when all of your cells are already doing the same thing? The small cell is the limitation, any extra Ahs in other cells is useless.

My last argument is the idea of whether a .1C charge rate to 95% or a 30x higher discharge rate at 20% will reduce capacity more. As many have said, we do not have the data, but my money is on 3C at 20% SOC is more harmful, therefore, having my batteries as close in SOC % as possible at the bottom is the choice I am making in the long run. The side benefit being if I accidentally drive too far, I will not kill cells. I will have a timer on my A/C outlet for stopping charge in case the charger fails to stop. I feel comfortable with this.


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## EVfun (Mar 14, 2010)

octagondd said:


> You said you do not want the small cell hitting the 3.5V each charge correct? Maybe you are not understanding the process of bottom balance. You measure the total voltage of the system on the first charge when the first cell hits 3.5, and use that total as the cutoff or CV stage for the charger. This cell, regardless of capacity, should reach 3.5V each charge and the charger should shutoff or go CV relatively quickly around that total voltage. So the small cell hits 3.5V which is probably about 95% capacity and the larger cells are at 93 or 94% capacity. As the pack slowly decreases in capacity, the voltages remain. 95% will still be 3.5V if that is the correlation we are assuming here.
> 
> In your top balance situation, all of your cells are reaching the same percentage capacity, let's say 95%, so more of your cells are in danger of whatever you fear the small cell in a bottom balance is in danger of. So, not only are your small cells getting to that point, but all your cells are getting there. Why be afraid of one cell getting there when all of your cells are already doing the same thing? The small cell is the limitation, any extra Ahs in other cells is useless.


Have you tried tried bypassing the timer, with both top and bottom balance, to see what happens? In the real world chargers don't stop hard on a set voltage point because the output wiring and fuse mean the pack voltage point is higher at lower currents (the charger is "seeing" the same voltage inside.) There is a slope to the "fixed" voltage that varies slightly with current. 

Typically (mostly because I've listened to Pete about bottom balancing) the average cell voltage is 3.5 volts at the voltage charger limit. Some will be 3.4 and the smaller ones that are causing the rapid voltage increase will be 3.6 to 3.7 volts. If you leave the charger running at that set voltage the current will fall and the pack voltage will slightly rise. The lower cells (the ones not full) will actually fall in voltage due to decreasing current, while the full one will dramatically rise. With a top balanced pack they all rise, some a few extra hundredths of a volt more than others. I've had a charger run on for about 4 hours. I posted the results at one point in the past, but have forgotten the numbers. I know none when anywhere near 4.0 volts from that charging problem.

On the low end, I've had no problem setting my controller set for the cell count times 2.5 volts. I can get 5C out of the pack with them only sagging to 2.8 vpc, until they get quite low on charge. I have done one hard range test and the car was significantly power impaired for the last few miles of range. That range test I have tacked to my garage wall. It was with 32, 60 amp hour Thunder Sky cells dated February 2010. I went 30 miles without noticing any power limiting from sag, with the current limit set to 300 amps and the pack minimum voltage set to 80 volts. By 34 miles there was noticeable clipping, but it was pretty drivable. I used a 1 amp load to keep the cells from creeping up to quickly to measure. The cells where between 3.20 and 3.22 volts resting. I drove another 2 miles and it was really only suitable on low to medium speed city streets with gentle acceleration. The cells where between 3.06 and 3.16 volts. (actual distances where 86% of indicated due to odometer error)

It's all for my records now. I rebuilt the buggy over the winter and put in a 39 cell pack of the same cells (the 32 cells plus some extras from the same batch as I originally bought 42.) Since I have limited access to the cells I installed the EVworks BMS cell modules I bought back in 2010. Since I didn't hook up the BMS loop they are functioning as 1/2 amp shunt regs with red LEDs to let me know I have hit the shunting voltage. I can check them with a plastic mirror. I have this pack set for a ending voltage of 142 volts, upward to 143 volts at low enough currents (3.66 vpc.) I have the timer set to cut off the charger 13 minutes after the voltage set point is reached. All the red LEDs light within a minute of the point where the voltage set point is reached. Near the point where the charger shuts off all the cells are between 3.64 and 3.71 volts. The next morning the cells are 3.35 to 3.36 volts, one day after than they all agree on 3.34 volts.


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## octagondd (Jan 27, 2010)

I think you are thinking of the method Jack initially proposed which was a simple bottom balance, and then set the CV stage to (cell count x 3.5). This allows for what you are saying here where some cells are 3.6 or 3.7 and some are 3.4. The average pack voltage will make that happen, but what has been proposed lately is a slightly safer method which under charges the bulk of the pack a little more. On the first charge, and maybe a few after to verify, you write down the pack voltage, measured between fuse and shunt, when your first cell hits 3.5. Most of the cells will be 3.4, but for the most part, that cell will get to 3.5 and then the charger will go into CV mode. Set the charger for a pack voltage as close to the measurement made as possible. This may cause longer charge times since the cells will be further down the charge curve when the charger goes into CV mode. During CV mode, the small cell may rise a bit, but not much. So, the overall pack average will be around 3.45/cell. 

A cell or two may hit 3.5 or 3.6 but not many. How many of your top balanced cells hit your HV target? I am assuming all of them. That is what I don't understand about your worries about 1 cell hitting it every time. All of your cells hit it every time, right? If that is not the case, please help me understand what is occurring with the top balance and why you are afraid of a cell or two hitting that same level using a bottom balance. If your concern is the small cell degrading faster then the others because it is finishing higher, I can sort of see that, but at .01C, that degradation percentage is so very minor compared to my next point below.

The next item I have a long term problem with is a small cell in a top balanced situation has the least protection during discharge. It is lower in SOC % than the larger cells at the bottom of the discharge curve and possibly losing capacity at a slightly greater percentage than the larger cells. I think the most protection of capacity loss should be on the small cell, and that is what the bottom balance gives you. It is at a higher SOC% than the larger cells during discharge, when I believe capacity loss is greatest. I don't care that the larger cells are at a lower SOC % and degrading faster, in fact, that is preferred as it may be slowly aligning the pack capacity.

Again, I am not writing this to prove my point to you, in fact, I am just typing myself smart at this point, but I just want it in the general discussion, so others who are reading may be able to weigh the viewpoints. And, thank you for your viewpoints and a good discussion. I know I learned a lot from you and others here early on in my research of this obsessive money pit.


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## IamIan (Mar 29, 2009)

octagondd said:


> A cell or two may hit 3.5 or 3.6 but not many. How many of your top balanced cells hit your HV target? I am assuming all of them.


It should be none of them for the top balance ... that's the point of top balance.

Top balanced pack will have the cells reach the top together ... because they are balanced at the top ... if the HV target is and average of 3.45v per cell than none should be more than 1mV or so above the 3.45v at the top ... they are balanced at the top... there would be none at 3.5v or 3.6v in a top balanced pack when the average reaches 3.45v.

A bottom balanced pack is the one that diverges more at the top from cell to cell voltage when charged ... on every charge ... it is the bottom balanced pack that would be far more likely to see 3.5v or 3.6v individual cells when the pack is at an average of 3.45v.



octagondd said:


> The next item I have a long term problem with is a small cell in a top balanced situation has the least protection during discharge.


Deep discharge is the bread and butter major benefit of the bottom balance.

- - - - - - - 

Top Balance:
Larger risk of over discharging weakest cells --- if you ever do a deep discharge = Big hit less often.

Bottom Balance:
Larger risk of slightly over charging weakest cell --- every time you charge the pack = Small hit more often.

I think either one should have a designed in safety buffer ... like 10-90 or 20-80 % of the battery operational window.


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## EVfun (Mar 14, 2010)

octagondd said:


> The next item I have a long term problem with is a small cell in a top balanced situation has the least protection during discharge. It is lower in SOC % than the larger cells at the bottom of the discharge curve and possibly losing capacity at a slightly greater percentage than the larger cells. I think the most protection of capacity loss should be on the small cell, and that is what the bottom balance gives you. It is at a higher SOC% than the larger cells during discharge, when I believe capacity loss is greatest. I don't care that the larger cells are at a lower SOC % and degrading faster, in fact, that is preferred as it may be slowly aligning the pack capacity.


How often is a pack run near dead? For me that is once in testing. How often is a pack fully charged? Every cycle. I'm not very concerned about my smallest cell going lower than the rest every cycle because most cycles don't go below 40% SOC. If my cells vary 5% (I doubt that) then the cells vary between 37% and 40% SOC. I don't think either is particularly stressful, this is well inside the flat area. I would be concerned about my smallest cell going higher than the rest every cycle, mostly because of the "every cycle" part. 

Oh, and I trust my right foot more than any charger.


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## octagondd (Jan 27, 2010)

IamIan said:


> It should be none of them for the top balance ... that's the point of top balance.
> 
> Top balanced pack will have the cells reach the top together ... because they are balanced at the top ... if the HV target is and average of 3.45v per cell than none should be more than 1mV or so above the 3.45v at the top ... they are balanced at the top... there would be none at 3.5v or 3.6v in a top balanced pack when the average reaches 3.45v.
> 
> A bottom balanced pack is the one that diverges more at the top from cell to cell voltage when charged ... on every charge ... it is the bottom balanced pack that would be far more likely to see 3.5v or 3.6v individual cells when the pack is at an average of 3.45v.


But let's compare apples to apples. If the top balance cutoff is 3.5 or 3.6, then all the cells are reaching that voltage every cycle. If it is 3.45, then they are all getting there. Now if the bottom balance is set at 3.45, then one or two cells is getting to 3.45 or 3.5, at 0.1-0.2C or 10-30 amps current depending on your charger. The rest of the cells are lower, so again, why the concern over a cell or two getting to a higher voltage than the rest of the pack, especially when undercharging and at a very, very minor current, when a top balanced pack has all the cells getting to the same voltage?

I understand the difference between the top balance and the bottom balance and where the voltages are. My concern is some peopleare worried about the top voltage under a 0.1C charge, even though we are all undercharging, and seem to be completely overlooking the 2-5C discharge that actually affects cycle life and the possibility that cells that are lower in SOC, even slightly, may degrade quicker.


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## octagondd (Jan 27, 2010)

EVfun said:


> How often is a pack run near dead? For me that is once in testing. How often is a pack fully charged? Every cycle. I'm not very concerned about my smallest cell going lower than the rest every cycle because most cycles don't go below 40% SOC. If my cells vary 5% (I doubt that) then the cells vary between 37% and 40% SOC. I don't think either is particularly stressful, this is well inside the flat area. I would be concerned about my smallest cell going higher than the rest every cycle, mostly because of the "every cycle" part.
> 
> Oh, and I trust my right foot more than any charger.


Yes, I understand your situation is very specific and you have designed your setup in a way you are comfortable with and give you plenty of cushion, but when talking about the general principles, I think we should make some assumptions that others may want to use the cells down to 20% SOC. I know when I first started looking at the cells I was excited that some people early on were claiming the cells could be used to 0% SOC. The reality hit me later, but still, I think there are many who are using their cells to 30% SOC daily and this is where a top balanced pack has diverging SOC % and possibly greater degradation to cells with a lower SOC. This is fine in that it is using the small cells just as intended, but it is allowing greater protection where none is needed, the larger cells.

Quick question, other than the cells you are experiencing some weirdness with, is there a cell or two in your pack that consistently goes into shunting before the EOC is reached? If so, has the amount of time of the shunting been about the same, decreasing, increasing, seemingly random? I realize you are probably not there at EOC, but just curious if you have noticed anything like that. The theory would be that the larger cells have more energy in them and are further up the curve, so all the cells should reach EOC at just about the same time, every time, but I am curious if you or others have noticed any consistency with a possible smaller cell shunting early.


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## IamIan (Mar 29, 2009)

octagondd said:


> But let's compare apples to apples. If the top balance cutoff is 3.5 or 3.6, then all the cells are reaching that voltage every cycle. If it is 3.45, then they are all getting there. Now if the bottom balance is set at 3.45, then one or two cells is getting to 3.45 or 3.5, at 0.1-0.2C or 10-30 amps current depending on your charger.


A few minor corrections to your apples to apples.

@3.45v for Top balance means all cells are withing ~0.001v of 3.45v when the total pack average is 3.45v ... because they are top balanced.

Under this same apples to apples condition the bottom balanced pack charged up to an average of 3.45v can have individual cells at 3.5v or 3.6v... which is worse for those cells being pushed higher up.

Yes the same ~0.001v difference if your HV cut off is 3.6v.

And for Apples to Apples ... if you took the bottom balanced pack to the same average high voltage of 3.6 it would have pushed individual cells as high as 3.8v or worse ... again ... this is worse for those cells being pushed to higher / potentially over charged ... and the higher voltage of those cells , always happens every charge for the bottom balanced pack.



octagondd said:


> why the concern over a cell or two getting to a higher voltage than the rest of the pack, especially when undercharging and at a very, very minor current, when a top balanced pack has all the cells getting to the same voltage?


The top balanced pack does not have all the cells getting to as high of a voltage as the bottom balanced pack does ... the bottom balance pack always has individual cells that will be charged / pushed to a higher voltage than a top balanced pack ... on every charge... always... it is a con of the bottom balance ... not necessarily a deal breaker ... but it is a con of that method... that users of a bottom balance should be aware of.

I think 'concern' is a bit too strong ... more like advocating being aware of the pros and cons ... that each method has.

One has to decide which is more relevant to their situation ... are you more likely to charge to 90% SoC? ... or dip to 90% DoD? ... are you more likely to charge to 70% SoC ... or dip to 70% DoD? ... etc ... the answer to those is not going to be the same for all applications ... and or all people.

The more likely and more often , one gets to a greater % SoC toward the top ... the more beneficial top balancing is.

The more likely and more often, one gets to a greater % DoD toward the bottom ... the more beneficial bottom balancing is.

The right tool for the job ... is not always just one of the above.



octagondd said:


> I understand the difference between the top balance and the bottom balance and where the voltages are. My concern is some people are worried about the top voltage under a 0.1C charge, even though we are all undercharging, and seem to be completely overlooking the 2-5C discharge that actually affects cycle life and the possibility that cells that are lower in SOC, even slightly, may degrade quicker.


The 2-5C hit you refer to is the big hit ... but only is a hit at all when the pack is discharged low enough in the DoD for the lowest / weakest cell to get low enough to suffer ... if a top balanced pack is only discharged to 50% DoD that 2-5C hit never punishes the weakest cell.

Either one ... driving a cell to too high of voltages ... or driving it to too low of voltages ... either end can shorten the cell life.

higher C rates matter of course ... but they matter on both ends.

2C Regenerative braking going down a hill after a complete charge cycle ... is worse for the bottom balanced battery pack.

2C Discharge at the bottom of DoD ... is worse for a top balanced pack.

Which is more or less likely to happen ... falls back to usage and using the right tool for a given job.

- - - - - - 

It is my opinion and position .... that being honest about both the pros and cons of both methods ... is 100% better than being an advocate only for one option or method... always picking bottom or always picking top ... is worse than the person who fairly and honestly uses the right tool for their situation / context... who uses bottom when it is the best tool ... and who uses top when it is the best tool.


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## octagondd (Jan 27, 2010)

IamIan said:


> A few minor corrections to your apples to apples.
> 
> @3.45v for Top balance means all cells are withing ~0.001v of 3.45v when the total pack average is 3.45v ... because they are top balanced.
> 
> Under this same apples to apples condition the bottom balanced pack charged up to an average of 3.45v can have individual cells at 3.5v or 3.6v... which is worse for those cells being pushed higher up.


This is what I am trying to clear up. It is NOT an average of 3.45v. On the first charge after bottom balancing, when the first cell hits whatever the cutoff voltage is, let's say 3.45, then the pack voltage is measured and that voltage is how the charger is set for the future. That small cell will only get to 3.45 or there abouts. The rest of the pack will be lower. Did you read the first post on the method? We are not talking about setting an average of 3.45v, we are talking about specifically measuring, a small cell and setting the pack voltage based on that. Do the measurement a couple times to verify that cell really is the small cell. What I meant by apples to apples is I was using 3.5 or 3.6 as the HVC because some people have different cells and then you replied by talking about 3.45. Which HVC voltage does not matter, just use the same one when discussing both balancing methods.



IamIan said:


> Yes the same ~0.001v difference if your HV cut off is 3.6v.
> 
> And for Apples to Apples ... if you took the bottom balanced pack to the same average high voltage of 3.6 it would have pushed individual cells as high as 3.8v or worse ... again ... this is worse for those cells being pushed to higher / potentially over charged ... and the higher voltage of those cells , always happens every charge for the bottom balanced pack.


As I just stated, this would not happen





IamIan said:


> The top balanced pack does not have all the cells getting to as high of a voltage as the bottom balanced pack does ... the bottom balance pack always has individual cells that will be charged / pushed to a higher voltage than a top balanced pack ... on every charge... always... it is a con of the bottom balance ... not necessarily a deal breaker ... but it is a con of that method... that users of a bottom balance should be aware of.


Apples to Apples - All top balance cells will reach 3.45 on a full charge, correct? A few bottom balance cells will reach 3.45 and maybe 3.5v on a full charge. Which one do you want on every charge? Every cell reaching a voltage, or a few cells reaching that same voltage?



IamIan said:


> I think 'concern' is a bit too strong ... more like advocating being aware of the pros and cons ... that each method has.
> 
> One has to decide which is more relevant to their situation ... are you more likely to charge to 90% SoC? ... or dip to 90% DoD? ... are you more likely to charge to 70% SoC ... or dip to 70% DoD? ... etc ... the answer to those is not going to be the same for all applications ... and or all people.
> 
> ...


I agree



IamIan said:


> The 2-5C hit you refer to is the big hit ... but only is a hit at all when the pack is discharged low enough in the DoD for the lowest / weakest cell to get low enough to suffer ... if a top balanced pack is only discharged to 50% DoD that 2-5C hit never punishes the weakest cell.


Here is where I think I miscommunicated. I am not talking about cell damage from overcharge or overdischarge. I am talking about normal capacity loss over time. I think a cell that is at a lower SOC% while 2C-5C loads are being demanded of it will lose its capacity at a faster rate than a cell at a slightly larger SOC%. ie. one cell is at 35% SOC and another is at 40% SOC. The cell at 35% may take a bigger capacity loss than the cell at 40%. Over time the smaller cell will continue to get smaller at a greater pace than the larger cell.



IamIan said:


> Either one ... driving a cell to too high of voltages ... or driving it to too low of voltages ... either end can shorten the cell life.
> 
> higher C rates matter of course ... but they matter on both ends.
> 
> ...


I agree, I just would like the one side to be fairly represented, which it has not since many here still are claiming the bottom balance cells will go disastrously into 3.7-3.8V or higher due to the average cell voltage calculation. That is not what is being presented here at all. There is no calculation, it is a hard number based on where the pack voltage is when the smallest cell gets to whatever HVC number you choose. 3.45v in your case.


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## Ampster (Oct 6, 2012)

I agree, and you have done a great job of clarifyng the pros and cons. The manufacturers have to adopt a battery management strategy that has to work for a wide vatiety of people so that implies they will be biased toward a top balanced pack. 

I drive a Smart ED, and a RAV4EV and the information to the driver is significantly different even though those are two actively managed battery systems. I have recently put my VW conversion in storage but I miss the individual cell details and the SOC details about that pack that I got from my BMS. You don't get that in a car made for the masses.


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## EVfun (Mar 14, 2010)

octagondd said:


> Yes, I understand your situation is very specific and you have designed your setup in a way you are comfortable with and give you plenty of cushion, but when talking about the general principles, I think we should make some assumptions that others may want to use the cells down to 20% SOC. I know when I first started looking at the cells I was excited that some people early on were claiming the cells could be used to 0% SOC. The reality hit me later, but still, I think there are many who are using their cells to 30% SOC daily and this is where a top balanced pack has diverging SOC % and possibly greater degradation to cells with a lower SOC. This is fine in that it is using the small cells just as intended, but it is allowing greater protection where none is needed, the larger cells.


I don't know of anyone who has actually cycled a pack to death in real world EV cycles, aside from some of the early offerings that couldn't actually handle 3C discharge rates (at any SOC they got hot and sagged badly.) Because of that I don't think anyone can really say if general charging stress when the cells are in the climbing voltage region is worse or better than 5C at 30% SOC, or even where charging stress starts. I've gone on the assumption that if the cells are staying cool, and if the cells are not being driven under 2.5 volts, or perhaps even 2.0 volts when cold, then the discharge rate effect on cell damage is negligible. At any rate, I think it is shared pretty evenly since the difference in SOC is slight and I don't think we should be designing a pack to regularly go under 30% SOC. Until some of our packs start dying of old age I don't think we'll really know what they like less. We all know cell reversal and gross overcharging can kill them in that cycle.



> Quick question, other than the cells you are experiencing some weirdness with, is there a cell or two in your pack that consistently goes into shunting before the EOC is reached? If so, has the amount of time of the shunting been about the same, decreasing, increasing, seemingly random? I realize you are probably not there at EOC, but just curious if you have noticed anything like that. The theory would be that the larger cells have more energy in them and are further up the curve, so all the cells should reach EOC at just about the same time, every time, but I am curious if you or others have noticed any consistency with a possible smaller cell shunting early.


Oh you seem to know the kind of things I watch.  If I am cycling the pack daily the regs go from all off to all on in well under 1 minute. A cell with slightly higher internal resistance is always first (mid-pack in capacity.) If I don't charge for 2 weeks then there are very noticeably cells that shunt first, by about 10 minutes. I measured the current draw of all my BMS boards, down to a 10 microamp accuracy. They vary from 3.99 milliamps to 4.22 milliamps. The shunt that comes on first is the one that draws 3.99 milliamps. I haven't pulled the reg that shunts 2nd to check it -- I wrote the millivolt reading (divide by 20.5 to get milliamps) on the bottom of each reg. 

My non-BMS observations with the slightly funny acting 7 cells is that their SOC very slowly creeps ahead of the rest, which is a concern. Would that still be happening with a bottom balanced pack while I didn't notice because none of them are the smallest cells? If so it would still be screwing up the bottom balance. Without regs the cells I later added creep up about 0.2 amp hour over 3 months of cycling. That small amount of difference in state of charge results in them being about 0.3 volts higher at the end of a charge (3.8 when the rest of the pack is 3.5.) 3 months of storage did not have that effect. These slightly differently acting cells are from the same batch as the other 32 (from a single batch of Thunder Sky 60 amp hour cells made in late February 2010.) They simply have less cycles on them. It gives me pause to agree with the claim that LiFePO4 cells have 100% Coulombic efficiency.

And no, I don't like taking my cells to 3.63 to 3.72 volts each cycle. It is quick though, I charge at 12 amps and from the time the pack hits 3.62 vpc to the time it hits 3.66 vpc and the charger shuts off is right at 13 minutes. The current tapers back sharply during this short window. The next day the cells are at 3.37 to 3.38 volts, so it might be a slight overcharge (the regs have pulled 80 milliamp hours out of them over 20 hours.) I would like to build my own shut regs that switched the load on at 3.5 to 3.52 volt. I don't have any experience with circuit board layout and sourcing, but have done some experiments with the basic circuit (TL431, 2n2907, red LED, 5 small resistors, one small capacitor, and one load resistor.) I think I could control off-state current better and make the end of charge more gentle.


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## PStechPaul (May 1, 2012)

This discussion has brought up a good point that I had not considered previously, and I had started to lean toward bottom balancing as preferable. But looking at the LiFePO4 charge/discharge curve, the points where the voltage is indicative of SOC occur at about 90% discharged and 95% charged. In normal use, the battery pack generally should not be discharged below 20-30% SOC, and at that point the voltage might not yet accurately show the SOC. If coulomb counting is used, and accurate, this point can be sensed and appropriate action taken, but if voltage under load is used, it might not be so easy, especially when the discharge current may vary widely and abruptly due to driving conditions. And if the pack is nearly depleted, a high current surge is more likely to cause total depletion and even reversal of one or more cells. With a bottom balanced pack, it is more likely that all cells will drop at about the same point and the undervoltage protection should kick in. But if not, there is a greater chance of damaging most of the cells.

OTOH, battery pack charging is usually done under more controlled conditions and lower current, so it is easier to detect the voltage upswing at the 95% SOC point, and the lower current may be less likely to damage the cell by overcharging, although it might be worse than depletion, as long as the discharge current does not cause reversal.

It might be best to keep the battery pack always between 20% and 90% SOC for longest life and least chance of damage. With those target levels, it is not as difficult to determine the end of charge or maximum discharge before shutting down or warning, and the available energy can be estimated by means of Ah in and Ah out. Then perhaps once every six months a top or bottom balance may be performed.


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## onegreenev (May 18, 2012)

> It might be best to keep the battery pack always between 20% and 90% SOC for longest life and least chance of damage.


True, its best to do so but there will always been those that for what ever reason need to ignore whats best and move a vehicle out of a bad situation. In those cases it would be best to have your pack bottom balanced. There have been many cases where near total discharge has occurred from either a leak discharge or driving the car to nearly stopping on the road and all the cells reached the bottom at a safe same level. What is always best is not always possible. Its fine if you choose to sorta top balance your cells during the charge cycle but I prefer to do the bottom routine. All those that I know of who do bottom balance have yet to rebalance their packs unless it was needed to add a cell or two for what ever reason. I have seen posted recently one who did not bottom balance loose a couple cells because of a top balanced pack and the vehicle was driven quite low. 

Ideal is nice but not reality. Some may be able to keep from taking their ride too low but in general life is not that way. 

If I were to build a vehicle for someone I would for sure do a bottom balance and provide instructions on charging and discharging. Knowing people I would be doing this because more likely than not they will discharge too far and if top balanced they would loose some cells. Not good for business. 


Pete


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## IamIan (Mar 29, 2009)

octagondd said:


> Apples to Apples - All top balance cells will reach 3.45 on a full charge, correct? A few bottom balance cells will reach 3.45 and maybe *3.5v* on a full charge. Which one do you want on every charge? Every cell reaching a voltage, or a few cells reaching that same voltage?


Exactly ... and you just described exactly the additional charge wear the bottom balance sees every time ... that I was talking about.

Those bottom balanced cells that went to 3.5v were given more charge wear ... and it will reduce their cycle life ...beating on the weakest cells of the pack every charge ... more so than the cells taken to 3.45v ... which was EVERY cell on the top balance... thus the top balance sees less charge wear.

As I said ... the bottom balance is small damage / wear ... but it happens EVERY charge cycle.



octagondd said:


> Here is where I think I miscommunicated. I am not talking about cell damage from overcharge or overdischarge. I am talking about normal capacity loss over time. I think a cell that is at a lower SOC% while 2C-5C loads are being demanded of it will lose its capacity at a faster rate than a cell at a slightly larger SOC%. ie. one cell is at 35% SOC and another is at 40% SOC. The cell at 35% may take a bigger capacity loss than the cell at 40%. Over time the smaller cell will continue to get smaller at a greater pace than the larger cell.


Yes and ... it happens on charging as well... you can reverse everything in what you described for a charging situation and be just as accurate.

It also gets back to the point I made about the right tool for the job ... not all people will dip down to 35% SoC ... thus damage that would happen there doesn't matter for their application.

As I said ... the bottom balance reduces wear on the bottom DoD ... just as top balance reduces wear on the top of SoC ... if you dip down to 20%SoC or 80%DoD regularly you have a very different situation from the person who never goes bellow 40%SoC.



octagondd said:


> I agree, I just would like the one side to be fairly represented, which it has not since many here still are claiming the bottom balance cells will go disastrously into 3.7-3.8V or higher due to the average cell voltage calculation. That is not what is being presented here at all. There is no calculation, it is a hard number based on where the pack voltage is when the smallest cell gets to whatever HVC number you choose. 3.45v in your case.


The 3.7-3.8v numbers were a different context , agreed ... the same point is still made in you own example of them being pushed to a higher voltage of 3.5v ... you point out the same gap ... either way the bottom balance always gives more charge wear on the weakest cell ... and it does so on every charge.

Weather you care more about that charge wear than the discharge wear at the bottom ... just gets back to usage context... and the right tool for a job.


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## ga2500ev (Apr 20, 2008)

IamIan said:


> octagondd said:
> 
> 
> > Apples to Apples - All top balance cells will reach 3.45 on a full charge, correct? A few bottom balance cells will reach 3.45 and maybe 3.5v on a full charge. Which one do you want on every charge? Every cell reaching a voltage, or a few cells reaching that same voltage?
> ...


This is not correct Ian. With bottom balancing, exactly one cell reaches the top voltage. That's the cell with the smallest capacity, (the runt) . Since all the other cells have more capacity, they will each be at a lower voltage than the 
runt. So at no time does any of the cells get overcharged.


> As I said ... the bottom balance is small damage / wear ... but it happens EVERY charge cycle.


No damage at all. One cell is at 100% SOC, all the others are lower, even if only by a bit.

ga2500ev


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## Roy Von Rogers (Mar 21, 2009)

Not to mention the whole wrong vocabulary about balance, top or bottom.

The only way one can call a pack "In Balance", it to have all cells with the same capacity, voltage has nothing to do with balance.

If you have 100ah cells and you empty and fill with 100ah, the pack is full, and balanced, and the voltage will go exactly where it should go, because your full. And when you empty it, all cells should go empty together, exactly what you want to happen.

The upper and lower voltages are only indicators, to act upon, they are not there to claim balance.

Someone here said he trusted his foot (pedal) more then the charger. 

I trust my TCCH charger a lot more than those aftermarket afterburners.

You should be able to give the keys to your EV to anyone, and tell them just to drive and watch the gas (ev) gauge, and when empty, the vehicle should shut off, your out of gas.

There is also a worst case scenario that one needs to think about, you have 10 to 20 watt resistors on top of your pack to so called balance the pack. The worst case scenario says that all can fail, and you will have xx cells with xx amount of heat going off on top of your pack, something to think about.

So if you have 50 cells, there could be 500 watts of heat or more, going off in your vehicle if all goes wrong.


Roy


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## ga2500ev (Apr 20, 2008)

Roy Von Rogers said:


> Not to mention the whole wrong vocabulary about balance, top or bottom.


I don't think that's the case.



> The only way one can call a pack "In Balance", it to have all cells with the same capacity, voltage has nothing to do with balance.


Not exactly. Cells will not have the same capacity. However, a group of cells can contain the same amount of energy.


> If you have 100ah cells and you empty and fill with 100ah, the pack is full, and balanced, and the voltage will go exactly where it should go, because your full.


The problem is that the cells will not all have a 100 aH capacity. One will be at 100.1, another at 99.8,and the third at 100.5 for example. The ones with 100 Ah or more will have 100 Ah in it. But the one whose capacity ais below 100 Ah will be overcharged. And since each will be at a different state of charge, each will have a different voltage.

So you have two options:

1. Charge each cell to the maximum capacity of the bottom cell. Their voltages and SOC will be different, but their energy will all be the same. This is bottom balancing.

2. Charge each cell to the maximum SOC. Their energy amounts will be different. Their voltages and SOC (100%) will be the same.
This is top balancing.



> And when you empty it, all cells should go empty together, exactly what you want to happen.


Not if you put a fixed amount of energy that overcharges any of the cells. You have to either fully charge one cell, and not fully charge the others, or you have to not fully charge any of the cells in order to store the same energy in each cell.

This is where the voltage becomes important. As a cell overcharges, its voltage starts to rise. The objective is to never have this happen.


> The upper and lower voltages are only indicators, to act upon, they are not there to claim balance.


Agreed that it's not a claim. However, it is a reflection of SOC. Overcharged cells voltages rise while overdischarged cells voltages drop relative to cells that are neither over or under discharged.

ga2500ev


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## EVfun (Mar 14, 2010)

ga2500ev said:


> This is not correct Ian. With bottom balancing, exactly one cell reaches the top voltage. That's the cell with the smallest capacity, (the runt) . Since all the other cells have more capacity, they will each be at a lower voltage than the
> runt. So at no time does any of the cells get overcharged.
> 
> No damage at all. One cell is at 100% SOC, all the others are lower, even if only by a bit.
> ...


I think that is the misunderstanding. From what I gather, even fully charging LiFePO4 cells is somewhat damaging. They last longest if they never see full (or empty.) Then there is the issue of defining "fully charged." It appears to be nothing but a somewhat arbitrarily chosen spot on the Lithium electrode intercalation curve.


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## Roy Von Rogers (Mar 21, 2009)

ga2500ev said:


> I don't think that's the case.
> 
> 
> 
> ...


You know you need to reread what I wrote and the jest of it.

Btw, if you purchase 100ah cell and you get only 98 amps, you didn't get what you payed for, not to mention that Calb and many other cells are over the stated amp hours, so putting in 100ah in a Calb 100ah cell, will never overcharge it

I don't know why I keep getting sucked in to these stupid balancing discussion, just so I can get knit picked over .1 .8 etc.

Over and out. I'm unsubscribing

Roy


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## octagondd (Jan 27, 2010)

EVfun said:


> I don't know of anyone who has actually cycled a pack to death in real world EV cycles, aside from some of the early offerings that couldn't actually handle 3C discharge rates (at any SOC they got hot and sagged badly.) Because of that I don't think anyone can really say if general charging stress when the cells are in the climbing voltage region is worse or better than 5C at 30% SOC, or even where charging stress starts. I've gone on the assumption that if the cells are staying cool, and if the cells are not being driven under 2.5 volts, or perhaps even 2.0 volts when cold, then the discharge rate effect on cell damage is negligible. At any rate, I think it is shared pretty evenly since the difference in SOC is slight and I don't think we should be designing a pack to regularly go under 30% SOC. Until some of our packs start dying of old age I don't think we'll really know what they like less. We all know cell reversal and gross overcharging can kill them in that cycle.


So let me ask the question, since we haven't seen any packs die and we have no specific data about possible greater degradation at the upper and lower SOC%, why are people holding onto the idea to stay away from the bottom 30%? It is unfounded and yet everyone, even me, seem to be very fearful of going into the bottom 20 or 30%. Is it a holdover from Lead? Were the data sheets mistranslated to read DOD when it meant Discharge capacity? You just stated you think people should not go under 30%. I have seen the general lithium ion cell research that shows cycle life is best when recharging between 50% and 30% SOC. Is it the same for these cells? Not sure. Jack's small data sample from CALB seemed to show a fairly linear .01% degradation in 500 full cycles from 2.0v to 3.6v, although it started to look like it was flattening out a bit closer to 500. If there is no greater degradation of capacity at the top or bottom, then my idea of harming smaller cells with large loads goes out the door, but if there is some truth that pulling a lot of power, from a cell that is less full than others, may harm its cycle life more, then I will stick with bottom balance.





EVfun said:


> Oh you seem to know the kind of things I watch.  If I am cycling the pack daily the regs go from all off to all on in well under 1 minute. A cell with slightly higher internal resistance is always first (mid-pack in capacity.) If I don't charge for 2 weeks then there are very noticeably cells that shunt first, by about 10 minutes. I measured the current draw of all my BMS boards, down to a 10 microamp accuracy. They vary from 3.99 milliamps to 4.22 milliamps. The shunt that comes on first is the one that draws 3.99 milliamps. I haven't pulled the reg that shunts 2nd to check it -- I wrote the millivolt reading (divide by 20.5 to get milliamps) on the bottom of each reg.
> 
> My non-BMS observations with the slightly funny acting 7 cells is that their SOC very slowly creeps ahead of the rest, which is a concern. Would that still be happening with a bottom balanced pack while I didn't notice because none of them are the smallest cells? If so it would still be screwing up the bottom balance. Without regs the cells I later added creep up about 0.2 amp hour over 3 months of cycling. That small amount of difference in state of charge results in them being about 0.3 volts higher at the end of a charge (3.8 when the rest of the pack is 3.5.) 3 months of storage did not have that effect. These slightly differently acting cells are from the same batch as the other 32 (from a single batch of Thunder Sky 60 amp hour cells made in late February 2010.) They simply have less cycles on them. It gives me pause to agree with the claim that LiFePO4 cells have 100% Coulombic efficiency.
> 
> And no, I don't like taking my cells to 3.63 to 3.72 volts each cycle. It is quick though, I charge at 12 amps and from the time the pack hits 3.62 vpc to the time it hits 3.66 vpc and the charger shuts off is right at 13 minutes. The current tapers back sharply during this short window. The next day the cells are at 3.37 to 3.38 volts, so it might be a slight overcharge (the regs have pulled 80 milliamp hours out of them over 20 hours.) I would like to build my own shut regs that switched the load on at 3.5 to 3.52 volt. I don't have any experience with circuit board layout and sourcing, but have done some experiments with the basic circuit (TL431, 2n2907, red LED, 5 small resistors, one small capacitor, and one load resistor.) I think I could control off-state current better and make the end of charge more gentle.


Excellent info, thank you for your very detailed data gathering and reporting.


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## dougingraham (Jul 26, 2011)

octagondd said:


> So let me ask the question, since we haven't seen any packs die and we have no specific data about possible greater degradation at the upper and lower SOC%, why are people holding onto the idea to stay away from the bottom 30%? It is unfounded and yet everyone, even me, seem to be very fearful of going into the bottom 20 or 30%


I did a 6 month test with three of my 100AH cells. I cycled them several times to get a baseline for the capacity. One cell I charged up and tucked away in a closet. Another I charged to 3.30 volt resting voltage and put it in the closet. The third I drained to 2.5 volts. After a couple of days this settled in at about 2.75volts. That went into the closet too. After 6 months I checked the resting voltage to see if there was any perceived self discharge and there wasn't. I then cycled the cells to determine if there was any change in the capacity. The cell stored at 2.75 volts did suffer close to a half a percent loss of capacity. The others showed no change.

Does this mean that deep discharges are bad for the cells? No, we can't make that conclusion. But I do believe that long term storage at a low state of charge is probably not the best idea. But even at 6 months I would not call this a catastrophic loss of capacity. This is less than what you would get with normal daily cycling.

There have been a couple of studies done that Jack mentioned that imply deep discharges are hard on the cells. And long term overcharging has been shown to be detrimental which is probably why the charging algorithm has dropped the CC/CV point from the original 4.2V down to around 3.6V. This same can probably be extended to the long term use of shunt balancers where some cells are daily held at the CV point at current levels far below the 0.05C termination point the charge algorithm specifies.


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## IamIan (Mar 29, 2009)

EVfun said:


> I think that is the misunderstanding. From what I gather, even fully charging LiFePO4 cells is somewhat damaging. They last longest if they never see full (or empty.) Then there is the issue of defining "fully charged." It appears to be nothing but a somewhat arbitrarily chosen spot on the Lithium electrode intercalation curve.


That is what I was trying to describe.
thanks.



ga2500ev said:


> This is not correct Ian. With bottom balancing, exactly one cell reaches the top voltage. That's the cell with the smallest capacity, (the runt) . Since all the other cells have more capacity, they will each be at a lower voltage than the
> runt. So at no time does any of the cells get overcharged.
> 
> No damage at all. One cell is at 100% SOC, all the others are lower, even if only by a bit.
> ...


I did not use the term 'overcharged' ... because 'overcharging' is different from what I was describing ... I called it charge wear ... or the amount of battery wear and tear that comes from charging the cells.

It does work exactly as I described.

Your 'No damage' claim is incorrect in the context of what I was describing.

It is well tested and established that running a cell full cycle 0% to 100% SoC or DoD that cell will wear out faster and give you fewer cycles ... than a cell that was run from 10% ot 90% ... or 20% to 80% ... even though the 100% is not 'overcharged' ... it still does put more wear on the battery... no overcharge is needed ... the additional wear happens anyway.

That Runt you refer to is exactly what I was talking about ... that runt is being pushed to a higher SoC during charge than all the other cells in the pack ... If the others get to hang out at 95% it's pushed higher to like 96%+ ... and it is the weakest cell that is being beaten on the most every charge.

I'll grant as I did many posts ago to bottom balance advocates that the penalty at the top is a small hit ... but it does happen on every charge.

The Top Balance major issue is the potential for a bigger hit at the bottom ... but unlike charging ... going that low into the SoC ... does not happen every time.

So it becomes a personal choice between ... rare ( if ever ) big hit on discharge ... or small hit every charge.


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## PStechPaul (May 1, 2012)

You can easily choose to charge only to 90% or 95% which should avoid the degradation at the very top, just as you can set your discharge alarm/shutdown to 20% or whatever you consider safe.

Something else to consider is that using the cells from 90% down to 30% corresponds to 60% of their capacity, while 95% to 15% is 80% of capacity. So if the more gently used cells get 2000 cycles, that may be a total of 120,000 Wh, and the ones used harder get 1600 cycles, they will actually have delivered 128,000 Wh. Thus, although the cycle life appears better at less extreme usage, it is not really a fair comparison.


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## EVfun (Mar 14, 2010)

octagondd said:


> So let me ask the question, since we haven't seen any packs die and we have no specific data about possible greater degradation at the upper and lower SOC%, why are people holding onto the idea to stay away from the bottom 30%? It is unfounded and yet everyone, even me, seem to be very fearful of going into the bottom 20 or 30%. Is it a holdover from Lead? Were the data sheets mistranslated to read DOD when it meant Discharge capacity? You just stated you think people should not go under 30%. I have seen the general lithium ion cell research that shows cycle life is best when recharging between 50% and 30% SOC. Is it the same for these cells? Not sure. Jack's small data sample from CALB seemed to show a fairly linear .01% degradation in 500 full cycles from 2.0v to 3.6v, although it started to look like it was flattening out a bit closer to 500. If there is no greater degradation of capacity at the top or bottom, then my idea of harming smaller cells with large loads goes out the door, but if there is some truth that pulling a lot of power, from a cell that is less full than others, may harm its cycle life more, then I will stick with bottom balance.


The cell manufacturers' data sheets generally show a shorter cycle life when cycled to 20% SOC, compared to 30% SOC. I don't know how they do their testing, but it is almost certainly some type of "accelerated" test which generally means the discharges and charges are done faster than typical use. How that will compare to cells in EV service, where the peak discharge rates are often 3C to 10C but the average discharge rates generally between 0.5C and 1.5C is an unknown. In addition, you can determine by driving your EV that cell sag at 30% SOC during 5C discharges is slightly greater than it is at 90% SOC. This sag under load gets worse at ever lower states of charge. In addition, a number of testers have reported increased heat from high discharge rates when the cells are at a lower SOC, with this heat increase roughly in proportion to the increased sag. 

On the discharge side, I'm running on the rough assumption that low SOC accelerated aging isn't a simple line, but most likely an increasing slope that seems to start somewhere around 30% SOC. Since these cells have been cycled to quite low levels by some EVers and testers and seem to be fine I'm guessing it is both gradual and fairly small. That is why I want my differences there -- it becomes a small difference in a small effect.

On the charge side, I'm running on the rough assumption that charging accelerated wear is more about the actual SOC than the voltage seen, provided the voltage never exceeds 4 and the cells are not being charged into conditions that cause heating. Since the amps for all cells in a pack is the same, effectively the difference in wear on the pack will be voltage based, because that speaks to their difference in SOC. (or higher internal resistance, which also likely accelerates charging wear some by increasing charging heat.) I'm guessing this starts above 3.4 volts, unless the charging rate exceeds recommendations or they start heating. Charging also must slow at lower temperatures because plating damage becomes an issue. 

I have considered a bottom balance of my pack over the winter so I can drive next year for a while and then re-check the series discharge voltages. It would be a test to see if they actually stay balanced. I can easily check only 13 of my 39 cells with a DMM. I have collected all the potential troublemakers in that row: The 7 added cells that show "creep," The smallest and second smallest cell, The 2 cells with slightly higher than typical internal resistance. I also have 2 "normal" cells completing that row. I can mirror check the shut regulators on all the cells, which is why I like them (and would like a lower voltage version more.)


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## octagondd (Jan 27, 2010)

EVfun said:


> The cell manufacturers' data sheets generally show a shorter cycle life when cycled to 20% SOC, compared to 30% SOC. I don't know how they do their testing, but it is almost certainly some type of "accelerated" test which generally means the discharges and charges are done faster than typical use. How that will compare to cells in EV service, where the peak discharge rates are often 3C to 10C but the average discharge rates generally between 0.5C and 1.5C is an unknown. In addition, you can determine by driving your EV that cell sag at 30% SOC during 5C discharges is slightly greater than it is at 90% SOC. This sag under load gets worse at ever lower states of charge. In addition, a number of testers have reported increased heat from high discharge rates when the cells are at a lower SOC, with this heat increase roughly in proportion to the increased sag.


I believe this is where some of the false assumptions started. The data sheets do not show discharges to various SOC%. They show the capacity remaining after FULL 100% Charge/Discharge cycles at .5C Look at the upper right graph on the second page of this sheet. Most people, myself included, thought that this graph represented 4 different cells that were cycled to a certain SOC% and that was how many cycles they got, but that was incorrect. The lines on the graph represent the actual cycle number of the cell being tested to full Charge and Discharge. This is good news for us, since it shows that at .5C these cells can do a lot of FULL cycles. It does not state that the cycle life gets worse if you discharge deeper. It states that under full charge/discharge cycles at a constant .5C, these cells can give you 3000+ cycles before the capacity is degraded to 80% of its original size (actually closer to 90%). Also, the chinese character next to each cycle number can be translated to mean sequence or number which, to me, is like the th we add to numbers. The first line is a discharge curve for the 1000th cycle the last line is the 8000th cycle.

Couple all this with the data Jack received from Sky Energy for 500 cycles and the fact that the .01% degradation rate for FULL cycles projected to be 2000 cycles down to 80% of original capacity and it is clear, in my mind, that the data we have is about full cycles and not to any particular SOC%.

I also find it very interesting that every box on the first page of the TS spec sheet that requires words to explain what the number is, has chinese characters there, except the cycle life DOD box. Wouldn't DOD need to be written in chinese for someone to translate into English? I think this could be a mistranslation and what was intended here was "To 80% of original capacity."

I realize the train has derailed here a bit from the original topic. Apologies to Skooler.


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## IamIan (Mar 29, 2009)

octagondd said:


> It does not state that the cycle life gets worse if you discharge deeper.
> 
> in my mind, that the data we have is about full cycles and not to any particular SOC%.


Maybe not in that one ... but it is true , anyway.

We do have data about SoC% effects on cycle life ... feel free to read the 150 page attached pdf ... it explains the mechanisms ... and if you like you could go on to read the 88 references they site about the data this one is based on.

The short version as they summed it up as:


> the longest lifetime is observed for cells cycled with low peak currents and a narrow SOC range and they also show the lowest late of lithium loss.


- - - - - 

Although at the opposite end ... instead of at the bottom A123 does tell you in their battery use literature about the negative effects to cell life is a cell is held to higher SoC.

From A123 usage guidelines ... attached.


> A small amount of capacity loss occurs over time (calendar aging), and it varies with battery state of charge (SOC) and storage temperature. Batteries stored at high SOC or high temperatures will lose capacity faster due to internal impedance growth


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## octagondd (Jan 27, 2010)

IamIan said:


> Maybe not in that one ... but it is true , anyway.
> 
> We do have data about SoC% effects on cycle life ... feel free to read the 150 page attached pdf ... it explains the mechanisms ... and if you like you could go on to read the 88 references they site about the data this one is based on.
> 
> ...


Yes I have looked through the Jens Groot study. It is specifically for HEVs and the only cycle that could come close to our usage is cycle E. The other cycles were from 30% to 50% SOC. So, when comparing total amp hours delivered, cycle C which was constant current discharge for 90% of the cell for 2000 cycles delivered 18000 AHs(assuming the cells were 10AHs). Cycles A,B, and D which only used 20% of the cell capacity for 9000 cycles delivered 18000 AHs. Cycle E which is still not how we use our cells, but the closest of the cycles, used 90% of the capacity for over 3000 cycles and delivered 27000 AHs.

So, they say more cycles, but they are only using 20% of the cell. Can't drive too far on 20% of a cell. The deeper discharge cycle E produced less cycles but more Ahs delivered over its life. I would like to see a study like this, but for pure electric vehicles, with and without regen and using various SOC ranges.


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## IamIan (Mar 29, 2009)

octagondd said:


> So, they say more cycles, but they are only using 20% of the cell. Can't drive too far on 20% of a cell. The deeper discharge cycle E produced less cycles but more Ahs delivered over its life.


The part it shows is that there are less cycles from the wider / deeper cycles DoD or SoC ... which AFAIK , is the part earlier in question ... if you go deeper down or taller up in DoD or SoC you will get fewer cycles ... we do have solid data about it.

I understand in some applications total life time battery available wh is a more useful metric ... but not always ... some applications the cycle number is more useful ... like many things ... it depends on context.

Example #1 : More Cycle Better:
About ~50% of Americans drive on average ~25 Miles or less per day... someone with a ~10 Mile Commute each way ... will drive that commute every day... Weather it is 20% SoC/DoD window from a ~100 Mile BEV .. or 80%SoC/DoD window from a ~25 Mile BEV ... but they will get more commuting days ... and the battery will last more years ... if it is the smaller window ... they may or may not get more total available wh ... and in some cases ... the smaller window would mean less total wh ... but that doesn't change more days and more years is still , more days and more years ... making the battery use a larger/wider SoC/DoD window , even if it gives more life time wh ... would still reduce the days / years that BEV could do that ~10 Mile Commute... ie the number of cycles... the years the BEV lasts is a marketing selling point .. people will pay more for 10 years than they will for 5 years ... as is the total max distance one can drive ... even if someone only drives ~10Miles each way to commute people will pay more for the ~100 Mile BEV that they don't use the extra distance of.

Example #2 : More life time available Wh Better:
A carpenter who heavily uses a combination cordless drill / screw gun ... He uses the full available battery charge regularly , sometimes more than 1 cycle a day , depending on how many screws or holes ... if some kind of BMS in the battery pack or gun were to limit the SoC/DoD window in order to try and get more cycles out of the gun ... it would get the more cycles ... but if it resulted in less total life time available wh ... than it means fewer holes drilled , screws driven , etc ... His task is not a daily cycle that can still be accomplished on more or less SoC/DoD window ... his is a task that will consume __wh to drill a hole , or drive the screw... less total life time available wh would mean he drives fewer screws and drills fewer holes... weather he used 5 cycles to get __wh or 3 cycles to get the same __wh ... he will do as many cycles as he needs to get the __wh he needs to get the job done... thus more cycles at less wh doesn't benefit him.


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## octagondd (Jan 27, 2010)

IamIan said:


> The part it shows is that there are less cycles from the wider / deeper cycles DoD or SoC ... which AFAIK , is the part earlier in question ... if you go deeper down or taller up in DoD or SoC you will get fewer cycles ... we do have solid data about it.


Yes, you are correct, a little nipicky, but correct. When most people here are talking about cycles, they are really talking about wh, or miles driven. I am in the DIY electric car community and am specifically talking about the LiFePo4 cells we use and the potential wh delivered by the cells used in this community. As I said in my previous reply, the study you gave is specific to HEVs and does not show any data that is very relevant to the DIYEC community regarding the types of cells we use, or the manner in which most of us use them. Maybe you could glean some info if you have a new AC system with regen, but as I said, only 1 of their cycles is even close to how most people here use their vehicles. Some people, myself included, designed their system to have double the range they need or more in case a charge failure occurs, or they want to go somewhere further on the weekend. I could have purchased 100AH cells and gotten to work with plenty to spare, but decided to go with 160s, so I could get home if it failed to charge at work. If the CALB CA series was out when I purchased, I would have done it since they can take more current and have less voltage sag. I am now actually considering going both directions and only charging once a day. 



IamIan said:


> I understand in some applications total life time battery available wh is a more useful metric ... but not always ... some applications the cycle number is more useful ... like many things ... it depends on context.
> 
> Example #1 : More Cycle Better:
> About ~50% of Americans drive on average ~25 Miles or less per day... someone with a ~10 Mile Commute each way ... will drive that commute every day... Weather it is 20% SoC/DoD window from a ~100 Mile BEV .. or 80%SoC/DoD window from a ~25 Mile BEV ... but they will get more commuting days ... and the battery will last more years ... if it is the smaller window ... they may or may not get more total available wh ... and in some cases ... the smaller window would mean less total wh ... but that doesn't change more days and more years is still , more days and more years ... making the battery use a larger/wider SoC/DoD window , even if it gives more life time wh ... would still reduce the days / years that BEV could do that ~10 Mile Commute... ie the number of cycles... the years the BEV lasts is a marketing selling point .. people will pay more for 10 years than they will for 5 years ... as is the total max distance one can drive ... even if someone only drives ~10Miles each way to commute people will pay more for the ~100 Mile BEV that they don't use the extra distance of.


It is all about wh. What are you talking about? Based on the study you presented, people would be better off and get more miles and commute days (wh) out of their BEV, if they only charged it every 3 or 4 days if their total commute was only 20 miles and their range was 100 miles. You either charge every day for 10 years and get X wh or you charge every 3 days for 15+ years and get X(1.5) wh. Which one gets you to work for more work days? Ask Jack Rickard how often he charges his vehicles. His packs will last for 20+ years at the rate he is using them. Also, you are very concerned about the charger getting cells to the top of the charge curve, so I am guessing you would be only charging every 3 days in this scenario. So, no, a shallow range does not provide you with more commute days. People buy a 100 mile BEV because they may want to drive it 100 miles from time to time, not because of their commute or thinking they will get more commute days if they only use 20% of it and charge it every day.



IamIan said:


> Example #2 : More life time available Wh Better:
> A carpenter who heavily uses a combination cordless drill / screw gun ... He uses the full available battery charge regularly , sometimes more than 1 cycle a day , depending on how many screws or holes ... if some kind of BMS in the battery pack or gun were to limit the SoC/DoD window in order to try and get more cycles out of the gun ... it would get the more cycles ... but if it resulted in less total life time available wh ... than it means fewer holes drilled , screws driven , etc ... His task is not a daily cycle that can still be accomplished on more or less SoC/DoD window ... his is a task that will consume __wh to drill a hole , or drive the screw... less total life time available wh would mean he drives fewer screws and drills fewer holes... weather he used 5 cycles to get __wh or 3 cycles to get the same __wh ... he will do as many cycles as he needs to get the __wh he needs to get the job done... thus more cycles at less wh doesn't benefit him.


You are correct, more wh is more wh. More miles is more miles and more screws is more screws.


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## IamIan (Mar 29, 2009)

octagondd said:


> It is all about wh.
> What are you talking about?


I disagree.

As I wrote before... Years of service is a selling feature.

If charged once per day ... than that means ... that more cycles = more days... more days = more years ... which is a selling feature.

Weather it is less total net wh ... does not change , that it is , more days or more years... That is why vehicle warranties are not ... after __ Wh of service... the general public does not think in terms of Wh , thus is has very little marketing value.



octagondd said:


> Based on the study you presented, people would be better off and get more miles and commute days (wh) out of their BEV, if they only charged it every 3 or 4 days if their total commute was only 20 miles and their range was 100 miles. You either charge every day for 10 years and get X wh or you charge every 3 days for 15+ years and get X(1.5) wh.


Commute days is not = wh in the context I gave.
Commute days = cycles in the context I gave... and in that context charged once per day ... more cycles = more days.

You absolutely can change the context ... but that has no impact on the context I described.

Most people are creatures of habit ... charge every day ... or every week ... etc ... asking people to manually monitor the wh usage and charge only as needed , between 3 or 4 commutes per charge ... is asking for many unsatisfied and unhappy customers who will give you bad PR... when they 'forgot' ... when they falsely estimated ... when traffic or weather was worse than usual that day ... etc.

While talking about different contexts ... such commuting People would also get more total net miles , whs, and days from 4 packs of 25 miles each , swapped out as they wear out ... ( than they would from 1 pack of 100 Miles , even if charged as you suggest ) ... each of those four being used for a ~80% SoC/DoD Window each day ... the smaller and lighter pack means less total net energy to move the , otherwise same , vehicle ... but it is also important to remember about this context ... is that such a vehicle would have a lower market value ... even though it used less wh per mile ... even though it gave more total miles , more total wh , etc... people would not be willing to pay as much for it... because it does not sell nor is it valued and bought by the general consumer for how many wh , or miles they get from it.



octagondd said:


> Which one gets you to work for more work days? Ask Jack Rickard how often he charges his vehicles.


If charged once per day ... it the one with the smaller SoC/DoD window... that is the only requirement needed for it to last more days... and I think the general consumer is realistically going to do ... charging once per day is the more likely scenario... than some daily wh math exercise.

What % of the millions of vehicle drivers do you think it is reasonable to think will be equal to or better than Jack? ... Do you think what he does and how much he knows , is a fair comparison / example of the average driver in the US?

I think he is in the very small minority % of vehicle drivers... that vast majority doesn't even know what a wh is... much less asking them to tack and accurately predict their wh usage.



octagondd said:


> Also, you are very concerned about the charger getting cells to the top of the charge curve, so I am guessing you would be only charging every 3 days in this scenario.


Concerned is not an accurate description ... very is even less accurate ... if you charge to a higher SoC ... you will get fewer cycles ... that's how it works ... weather that fewer cycles is or is not a concern ... will depend on the context of usage ... that , is my position ... as I tried to explain previously.



octagondd said:


> People buy a 100 mile BEV because they may want to drive it 100 miles from time to time, not because of their commute or thinking they will get more commute days if they only use 20% of it and charge it every day.


100% agree... but not for the reasons you give.

Even though they would get more from 4 packs of 25 miles being replaces as worn out ... they don't care it wold get them more wh ... thus it is not about wh to the consumer ... they don't care that they only do one 80 mile trip every 2 years ... they want to haul around the 4x heavier battery on every trip of ~2 miles to the corner store.

I also agree they ( the consumer ) is not thinking about 20% usage ... but the OEM who gives it a 5year warranty or a 10 year warranty ... they very much are thinking about what the likely usage will be ... and they know that the average consumer is not going to do wh counting and carefully calculating daily wh needs and such ... the average consumer will have a daily habit ... they will plug it in ... and that average consumer wants years of warranty ... they don't understand ... and don't care at all about wh ... they don't understand or care if they get less wh and how that might effect them ... as long as the warranty is for 10years instead of 5 years ... it is worth more to the consumer.


----------



## octagondd (Jan 27, 2010)

IamIan said:


> I disagree.
> 
> As I wrote before... Years of service is a selling feature.
> 
> ...


Dude, you are seriously reaching and backpedaling to save face. Really, more days of charges = better? What science brought you to that conclusion? Do you think adding and removing energy 3650 times to get X wh is the same as adding and removing energy 2703 times to get X(1.5) wh? Do people with gas gauges watch it and then fill up when it is low, or do they fill up every day? The gas car industry didn't seem to have an issue with people monitoring their energy consumption and then re-filling when necessary. Incredible bad PR when someone is stuck calling for a tow truck or walking to the gas station? No, just some idiot who didn't watch his gas gauge. If a certain treatment of cells is going to give you more wh and thus more miles, then people will treat them that way and manufacturers will make them function that way. End of story.



IamIan said:


> While talking about different contexts ... such commuting People would also get more total net miles , whs, and days from 4 packs of 25 miles each , swapped out as they wear out ... ( than they would from 1 pack of 100 Miles , even if charged as you suggest ) ... each of those four being used for a ~80% SoC/DoD Window each day ... the smaller and lighter pack means less total net energy to move the , otherwise same , vehicle ... but it is also important to remember about this context ... is that such a vehicle would have a lower market value ... even though it used less wh per mile ... even though it gave more total miles , more total wh , etc... people would not be willing to pay as much for it... because it does not sell nor is it valued and bought by the general consumer for how many wh , or miles they get from it.


Of Course they would get more miles, because they are not using a small SOC range and are swapping the pack once they don't have enough range for their commute. Again, people aren't buying 100 mile range vehicles because they are going to use a small SOC, they are buying them to go 100 miles on occasion. Once someone has an electric car for a period of time, they are only going to charge when they think they need it, just like a gas car. Again, ask Jack how often he charges his most used vehicle. Also, range in BEV's is absolutely critical. You cannot create a vehicle that will take you the exact range you need, because batteries degrade, so you cannot get there after 100 trips. You must have more range than you regularly need, but if the batteries last longer by discharging them deeper, like cell phones and computers, then you will only charge every couple days to get the maximum wh out of the cells.



IamIan said:


> If charged once per day ... it the one with the smaller SoC/DoD window... that is the only requirement needed for it to last more days... and I think the general consumer is realistically going to do ... charging once per day is the more likely scenario... than some daily wh math exercise.


Wrong. It doesn't last more days. 15 years is more days than 10 years. Simple math, I think.



IamIan said:


> What % of the millions of vehicle drivers do you think it is reasonable to think will be equal to or better than Jack? ... Do you think what he does and how much he knows , is a fair comparison / example of the average driver in the US?
> 
> I think he is in the very small minority % of vehicle drivers... that vast majority doesn't even know what a wh is... much less asking them to tack and accurately predict their wh usage.


Again, I am talking about this community. Did you read that part, or just shrug it off as me typing myself smart?



IamIan said:


> Concerned is not an accurate description ... very is even less accurate ... if you charge to a higher SoC ... you will get fewer cycles ... that's how it works ... weather that fewer cycles is or is not a concern ... will depend on the context of usage ... that , is my position ... as I tried to explain previously.


Show me the money. How does it work, with actual data, show me. I want max wh out of the life of my cells, show me how a small SOC range is going to give me that? You cannot use the previous study you posted, because it clearly disproves that theory for HEVs. I have killed the data you have presented so far and you just want to side step it by shifting the conversation to the general public or whatever.




IamIan said:


> 100% agree... but not for the reasons you give.
> 
> Even though they would get more from 4 packs of 25 miles being replaces as worn out ... they don't care it wold get them more wh ... thus it is not about wh to the consumer ... they don't care that they only do one 80 mile trip every 2 years ... they want to haul around the 4x heavier battery on every trip of ~2 miles to the corner store.
> 
> I also agree they ( the consumer ) is not thinking about 20% usage ... but the OEM who gives it a 5year warranty or a 10 year warranty ... they very much are thinking about what the likely usage will be ... and they know that the average consumer is not going to do wh counting and carefully calculating daily wh needs and such ... the average consumer will have a daily habit ... they will plug it in ... and that average consumer wants years of warranty ... they don't understand ... and don't care at all about wh ... they don't understand or care if they get less wh and how that might effect them ... as long as the warranty is for 10years instead of 5 years ... it is worth more to the consumer.


The consumer doesn't have to do wh calculations, the manufacturer, or in our case, the ah meter already does it. In fact the manufacturers have taken it even further by then doing complex calculations to create a range meter. Which does vary on usage, but people are fairly competant and figure out that gunning it, or hauling a boat, or driving uphill will lower their range just as it does in a gas vehicle.

If you decide to or are currently doing a small SOC range, good luck. We will see what happens down the road.


----------



## IamIan (Mar 29, 2009)

octagondd said:


> Dude, you are seriously reaching and backpedaling to save face. Really, more days of charges = better? What science brought you to that conclusion?


Incorrect... ZERO back pedaling ... I've explained the basis for it several times... I'll try again.

I have repeatedly not claimed more days of charges = better... My position has been the same ... which is better A or B will depend on the context of usage.

Context A = Charged ONE time EVERY DAY

Context B = Charged as few or as many times as is needed to get x Wh out.

A & B are not the same.

For the person who WILL use it in Context A ... and also ... who values more years / commutes from the vehicle in that context ... for that person A is better... as much as you seem to be against that... that is 100% accurate ... and it is not a far fetched situation.

That is not everyone ... that is not every situation.

I have been 100% consistent in this position of mine.



octagondd said:


> Do you think adding and removing energy 3650 times to get X wh is the same as adding and removing energy 2703 times to get X(1.5) wh?


I've already said previously it is not the same... why you would wonder if I think that confuses me?

I know you love to focus on this context B ... but that doesn't change what will happen in Context A.



octagondd said:


> Do people with gas gauges watch it and then fill up when it is low, or do they fill up every day?


Like most things ... it depends.
I know several people who get gas as part of a regular routine ... once per week ... etc... they think of the gas in their car as getting them __ Days , they do not think in terms of MPG , %of Tank , wh of chemical energy used, etc.

And As I have already written several times ... if something like Context B is the situation ... than they will more benefit from treating it accordingly ... I agree with that ... but that doesn't change anything for the person who is in Context A.



octagondd said:


> If a certain treatment of cells is going to give you more wh and thus more miles, then people will treat them that way and manufacturers will make them function that way. End of story.


You are welcome to your opinion.

But in case you haven't noticed ... the OEMs are not treating the cells as you would advocate them to do ... they ARE using small % SoC/DoD operating windows.

As much as you might like to think the average person will act that way ... I have my doubts ... under your logic , everyone on the road would be a hypermiler because they would get better MPG ... the VAST majority of people ... do NOT care ... do NOT understand ... and have absolutely no interest in every understanding.



octagondd said:


> IamIan said:
> 
> 
> > *If charged once per day* ... it the one with the smaller SoC/DoD window... that is the only requirement needed for it to last more days... and I think the general consumer is realistically going to do ... charging once per day is the more likely scenario... than some daily wh math exercise.
> ...


You forget the stipulation I included : ... See bold above.

And yes the studies are clear ... is it simple ... as soon as that condition applies ... than ... what I wrote is 100% correct... as much as you keep fighting against it ... and seem to want to avoid the possibility of that ever happening.



octagondd said:


> Again, I am talking about this community. Did you read that part, or just shrug it off as me typing myself smart?


I did read it... I did not shrug it off as you typing yourself smart.

I was pointing out that what Jack does and is willing to do ... is not a valid comparison to what the average consumer does and is willing to do... Given how several of the community of this site clashed with him ... it is questionable even what % of this community would be valid to make that comparison to Jack.



octagondd said:


> IamIan said:
> 
> 
> > Concerned is not an accurate description ... very is even less accurate ... if you charge to a higher SoC ... you will get fewer cycles ... that's how it works ... weather that fewer cycles is or is not a concern ... will depend on the context of usage ... that , is my position ... as I tried to explain previously.
> ...


I have ... you already agreed with the findings in Jen Groot.



JenGroot said:


> the longest lifetime is observed for cells cycled with low peak currents and a narrow SOC range and they also show the lowest late of lithium loss.





octagondd said:


> I want max wh out of the life of my cells, show me how a small SOC range is going to give me that?


I have never once made that claim.

Max wh is not the same as max Cycles.
If you want max Wh ... than treat them for that.
If you want max Cycles... than treat them for that.



octagondd said:


> You cannot use the previous study you posted, because it clearly disproves that theory for HEVs. I have killed the data you have presented so far and you just want to side step it by shifting the conversation to the general public or whatever.


I agree the above JenGroot Study does disprove that ... I never claimed it proved that ... I have only claimed what it has proven... you have not killed this data ... that data remains 100% intact.

It is not a side step or shift at all to talk about the context of usage ...how it is most likely to be used ... that question of context of usage ... is 100% the center of my position I have been 100% consistent about.



octagondd said:


> The consumer doesn't have to do wh calculations, the manufacturer, or in our case, the ah meter already does it. In fact the manufacturers have taken it even further by then doing complex calculations to create a range meter. Which does vary on usage, but people are fairly competant and figure out that gunning it, or hauling a boat, or driving uphill will lower their range just as it does in a gas vehicle.
> 
> If you decide to or are currently doing a small SOC range, good luck. We will see what happens down the road.


And the OEMs are not following your advise to get the most Wh from the pack ... they ARE using smaller % SoC/DoD window ... What does that tell you?

My pack will have a total of less Wh over it's life ... I am aware of the cons of my choice ... I am also aware of the pros ... and in my context the pros out weigh the cons , so that's what I am doing... which option is better depends on the context of usage.


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## octagondd (Jan 27, 2010)

Well, I am done with this now.

Good luck everyone, as they say, Your mileage may vary!


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## elanmel (May 3, 2010)

I'm planning to send my elcon charger back to set a new charge curve for a lithium upgrade. I'm trying to figure out what charge cutoff setting to tell them to use.

I intend to install 40 100ah cells, which I will bottom balance as per message #1 in this thread ( including adding 24-48 hours to allow cells to diffuse). I'm not planning to use a bms at this time.

From reading this thread it seems the best practice would be to charge the bottom balanced pack so that the charger begins cutting current when the weakest cell reaches 3.5v (could be higher, I'm assuming a relatively conservative top charge as my goal). One would then set the charger so it stops it's charge cycle at the amperage required to push the weakest cell to this target voltage. The Elcon tech specifically says "the charger does not shut down at 3.5v. It stays at 3.5v until the current drops down to I3 which is AH/60, then it shuts down".

So what should I tell the tech to use as an initial setting? I know I can send it back to change this after I measure actual cell performance, but I'd like to get the setting close the first time, and maybe avoid having to do this twice.


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## alvin (Jul 26, 2008)

elanmel said:


> I'm planning to send my elcon charger back to set a new charge curve for a lithium upgrade. I'm trying to figure out what charge cutoff setting to tell them to use.
> 
> I intend to install 40 100ah cells, which I will bottom balance as per message #1 in this thread ( including adding 24-48 hours to allow cells to diffuse). I'm not planning to use a bms at this time.
> 
> ...


 
To me that sounds like it is trying to top balance.


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## GizmoEV (Nov 28, 2009)

elanmel said:


> I'm planning to send my elcon charger back to set a new charge curve for a lithium upgrade. I'm trying to figure out what charge cutoff setting to tell them to use.
> 
> I intend to install 40 100ah cells, which I will bottom balance as per message #1 in this thread ( including adding 24-48 hours to allow cells to diffuse). I'm not planning to use a bms at this time....


I would do the bottom balance as you indicate then charge your 40 cells to 40x3.5V=140V. The charger should then hold this voltage, the CV stage, until the current drops to 100*0.05C=5A.

If your charger has the ability I would also have them set up the 10 charge "profiles" to have the ending voltage in either 3.5V increments above and below 140V or maybe 1V increments so you can go up or down by 5V in 1V increments to fine tune things.

FWIW, I have been running several cycles on an 8 cell 20Ah GBS pack using a Powerlab8. It charges until a cell reaches the cell target voltage and then cuts back current to until the termination current is reached. I've noticed that sometimes the first cell to reach the target voltage is not always the one at that voltage when the target current is reached. I'll try to remember to post one of the parts of the graph showing this when the cycling is finished. My point is, that as the charger cuts back current you may have the "top" cell trade places with another cell.

What is you charger currently set to terminate at? Can it or do you have something else which can charge your pack at 5A or more while you watch it carefully at the end of charge so you get an idea of how your pack will behave?


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## onegreenev (May 18, 2012)

> FWIW, I have been running several cycles on an 8 cell 20Ah GBS pack using a Powerlab8. It charges until a cell reaches the cell target voltage and then cuts back current to until the termination current is reached. I've noticed that sometimes the first cell to reach the target voltage is not always the one at that voltage when the target current is reached. I'll try to remember to post one of the parts of the graph showing this when the cycling is finished. My point is, that as the charger cuts back current you may have the "top" cell trade places with another cell.


This is why you can't really top balance with a BMS. It is trying to balance a charge current which changes all the time. Bottom balancing is the best bet. No real need to use a device to watch your low cell. Its good to know your low cells and watch them manually every few months until your tired of seeing that all is fine. Then maybe once per year. Don't forget those nordlock washers in all you HV connections including the contactors. Get a loose connection and you can have a disaster on your hands. 

Check out Jehu's latest video and it shows what happens when you get loose connections. http://www.youtube.com/watch?v=4W_ty3DEaMA

Pete


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## Ampster (Oct 6, 2012)

onegreenev said:


> This is why you can't really top balance with a BMS. It is trying to balance a charge current which changes all the time. Bottom balancing is the best bet. No real need to use a device to watch your low cell. Its good to know your low cells and watch them manually every few months until your tired of seeing that all is fine. Then maybe once per year. Don't forget those nordlock washers in all you HV connections including the contactors. Get a loose connection and you can have a disaster on your hands.
> 
> Check out Jehu's latest video and it shows what happens when you get loose connections. http://www.youtube.com/watch?v=4W_ty3DEaMA
> 
> Pete


Good advice about tight connections. I would like to add some clarification to your statement about top balancing and how a I believe s BMS top balances the pack. The BMS is not trying to balance the current going into each cell. It is actually using the shunts to stop current from going into the cell or cells that have reached the target voltage. This allows the slower cells, which have greater capacity, to reach target voltage. 
I am NOT trying to advocate for top or bottom balancing here, but just clarify what happens, so readers can make their own decision. 

For what it is worth, one of Jack Rickard's cars top balances the way I have described.


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## onegreenev (May 18, 2012)

Not any that HE built. Not even sure the Tesla does that.


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## Ampster (Oct 6, 2012)

onegreenev said:


> Not any that HE built. Not even sure the Tesla does that.


Of course the BMS isn't in the cars he built, the BMS is in his Tesla. I respect your opinion and your choice to bottom balance exactly as Jack R advocates. My point was that if you don't understand how top balancing works, don't make misleading statements about what it does or does not do to the cells when it is active. 

FYI and for other readers benefit, the Tesla BMS is an active battery management system that includes thermal management as well as management at the cell level. When I say cell level, I mean blocks of paralleled cells that the BMS sees as one cell. For all I know it may manage not only at the top but at the bottom and in the middle. Those details Tesla has chosen to keep proprietary. We know more about the thermal management from information that is public.


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## onegreenev (May 18, 2012)

Top balancing is not all that hard to understand and I do understand that even shunting off current while charging is not a stable affair. Trying to balance active current into the cells is not stable. It is chasing the voltage and currents for what? a few amp hours? Not my cup of tea and if one chooses it is of course their choice. Just want them to understand the stable bottom balancing that is MUCH safer for their cells. Why all the complexity for something that is not needed? I do know not all BMS systems are safe for your cells. Even ones that come from the manufacturer of the cells. 

Once your car is charged and you hop in and go how long are you at full?


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## Ampster (Oct 6, 2012)

onegreenev said:


> ........... I do understand that even shunting off current while charging is not a stable affair. Trying to balance active current into the cells is not stable. .............


What do you mean by "shunting.......is not stable"? Where is the data to support that statement? Let us forget a BMS is even involved. 

If I am charging my pack and one cell is getting to the target but the rest are 10 millevolts lower, and I take that cell out of the series string and continue to charge the other cells, where is the affair not stable? Or are you saying that shunting at the top is less stable than shunting at the bottom?


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## onegreenev (May 18, 2012)

Put your volt meter on the cells during shunting and graph each cell during the shunting portion of the charge cycle with active shunting. No BMS involved. Just shut resistors. 

Anyone have a graph of their cells during the charge cycle where you are shunting to keep a cell from going above the rest? 

What is interesting is if you truly top balance you should never ever have to have any shunting to get any cell back in balance. EVER. If any cell shunts it is not in balance. Nor is the pack.


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## onegreenev (May 18, 2012)

> Or are you saying that shunting at the top is less stable than shunting at the bottom?


No one shunts on the bottom. If your pack is bottom balanced it would never need to be done. EVER. Thats the beauty of bottom balancing.


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## Ampster (Oct 6, 2012)

onegreenev said:


> Put your volt meter on the cells during shunting and graph each cell during the shunting portion of the charge cycle with active shunting. No BMS involved. Just shut resistors.
> 
> Anyone have a graph of their cells during the charge cycle where you are shunting to keep a cell from going above the rest?
> 
> What is interesting is if you truly top balance you should never ever have to have any shunting to get any cell back in balance. EVER. If any cell shunts it is not in balance. Nor is the pack.


Pete, 
I am not trying to argue about top vs bottom balancing. I am trying to understand the instability that you are talking about when shunting the current around a cell that is higher than the others. Is there a difference between taking a cell out of the string or shunting the current around that cell so that all the other cells continue to get current? You have not shown me where that instability that you refer to takes place. Yes, when I take that high cell (3.5 volts) out of the string it begins to drift down until it stabilizes at 3.32 while the other cells continue to charge at 3.4 on their way to my target of 3.5. That is what I observe with my voltmeter. Is that what you call unstable?


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## GizmoEV (Nov 28, 2009)

Ampster said:


> Pete,
> I am not trying to argue about top vs bottom balancing. I am trying to understand the instability that you are talking about when shunting the current around a cell that is higher than the others. Is there a difference between taking a cell out of the string or shunting the current around that cell so that all the other cells continue to get current? You have not shown me where that instability that you refer to takes place. Yes, when I take that high cell (3.5 volts) out of the string it begins to drift down until it stabilizes at 3.32 while the other cells continue to charge at 3.4 on their way to my target of 3.5. That is what I observe with my voltmeter. Is that what you call unstable?


Maybe the attached pictures will point out a little of what Pete is trying to say. It isn't exactly the same thing as shunt balancing since the balancing was turned off for this 10 cycle test. This is using a PowerLab8V2 on 20Ah GBS cells (http://www.kta-ev.com/Battery_4_GBS_3_2V_20AH_cells_p/bat-gbs-lfmp20ahx4.htm). These apparently have a little Mn in them so they are not pure LiFePO4 cells. The cycle setup was to charge to 3.600V at 3.5A then hold until the current dropped to 350mA, rest for 3 hours then discharge at 3.5A down to 2.800V until the current dropped to about 250mA, rest for 3 hours then repeat. The thing is that the PowerLab8 used individual cell voltages rather than average pack voltage to start to cut back current so this is where it changes the results a little from what it would look like if an average cell voltage were used. I presume that any load on the cells from monitoring is the same for each cell but I have no support for that presumption. I've included the first peak after a full discharge and the last complete peak since monitoring stops as soon as the charging stops on the last cycle. That is why I didn't include peak 10. The cells were bottom balanced prior to the cycle test however I did not let all the cells sit for 24 hours prior to starting the test so they may not have been completely bottom balanced. I've also attached the first and last "trough" of the cycles. The calculated IR for the first "peak" is also included. Note the IR between hours 10-11 is different from one cell to the next. This will cause the cell to have a different voltage for a given current. This is what will "fool" a top balancer into thinking that the cell is full when it may not be. For example, in the first peak cell 8 never reaches 3.600V but it and cell 7, which does reach 3.600V, have the same voltage after a 3 hour rest period.

I think the thing to note is that the current began decreasing after the first cell hit the target voltage and yet some cells continued to rise in voltage and some started to drop in voltage immediately.

I'm going to rebottom balance the pack, let it sit for 24 hours, then use the PowerLab8 to only monitor the pack and charge the cells with a separate power supply to see what the top of charge voltages actually looks like. Since a proper bottom balance lets the cells rest for several hours before declaring the cells balanced the IR of the cell is essentially taken out of the balancing equation. Top balancing doesn't have this luxury.

Since these cells are for my push mower I may just do a top balance and see what the resting voltages are to get some more data on the issue. I just have to be done by the time my grass starts growing again.


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## Red Neck (Feb 1, 2013)

Incredible how people manage to turn every single simple option into a religion..

Useless artificial ideologies aside:

Bottom balancing is perfect for those who have a charger which allows setting the charging end voltage to the user, especially if they do not have access to their drive inverter settings and are unable to set the cutoff voltage for the pack.

Top balancing is perfect for people who have a charger with a fixed ending voltage and access to their inverter, where they can set the cutoff voltage
for the car at a voltage which prevents the weakest cell to over deplete.

Simply as that. Everything else is ideological garbage which this species appears to be programed to develop on top of any insignificant topic or simple option. Can't believe that people in EV communities can become fanboys of any such given available choice and guard them emotionally. 

I use both methods, depending on the car and which of those things above can be set.

Note on Ricard. The man has some knowledge and it is great that he experiments and lets results known. He is also a jerk, cannot be trusted completely because he is an ideolog and as such will defend his strong statements even if they later turn out to not be the best or are wrong, etc..

He has issues with coming to terms with him not knowing some things and becomes insulting when it happens. So follow the experiments, try them out on your own when you can but do check what works for you and try not to become reduced to becoming something so intellectually limited as a proponent of a charging method?!? This goes to extremists on the top and
bottom part of the charging spectrum


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## kennybobby (Aug 10, 2012)

*What kind of charging is this?*



GizmoEV said:


> ...for this 10 cycle test. This is using .. 20Ah GBS cells ... The cycle setup was to charge to 3.600V at 3.5A then hold until the current dropped to 350mA...


typically i've seen spec sheets call out [C/20] amps as the cutoff current to end the charging during the CV stage. Are you monitoring temperature--are they getting warm or swelling? 

The data is confusing--how come the voltage is not held constant during the CV stage? What's the current doing--any current measurements? The message here seems to be that you have an unbalanced pack that is getting more so after 10 cycles of some sort of non-standard charging scheme. 

Did the first charging cycle consist of 10 hours at 3.5 amps?


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## Ampster (Oct 6, 2012)

I couldn't read the chart captions on my phone but those look like some of the charts I see on my powerlab. When I get back to my laptop later today I will check out in more detail. I completely understand that the fluctuations we see at the top are because of the different internal resistance of the cells. On the powerlab when we are charging and hit the target voltage the powerlab is in contant voltage mode and the current begins to fluctuate. If that is what Pete was saying, then I must have misunderstood him. Is the reason that we don't see any fluctuation at the bottom because we are discharging at a constant current and when we hit the target voltage the load shuts off?


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## Hippie Djohn (Oct 24, 2011)

Red Neck said:


> Note on Ricard. The man has some knowledge and it is great that he experiments and lets results known. He is also a jerk, cannot be trusted completely because he is an ideolog and as such will defend his strong statements even if they later turn out to not be the best or are wrong, etc..
> 
> He has issues with coming to terms with him not knowing some things and becomes insulting when it happens.


Haha  So true! 
He's really a jerk and offending when you ask him critical questions that he can't answer or if you slightly disagree with him.
Big time JERK !


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## EVfun (Mar 14, 2010)

GizmoEV said:


> I'm going to rebottom balance the pack, let it sit for 24 hours, then use the PowerLab8 to only monitor the pack and charge the cells with a separate power supply to see what the top of charge voltages actually looks like. Since a proper bottom balance lets the cells rest for several hours before declaring the cells balanced the IR of the cell is essentially taken out of the balancing equation. Top balancing doesn't have this luxury.


You might want to wait longer than that. I just did a capacity test on my buggy pack. My typical is 0.2C load until the first cell hits 3.000 volts. I have left the cells since completing that and noticed that the lower cells continued to rise in voltage for 4 days. The first cell to 3.000 was 3.161 the next day and 3.185 four days later. Now 6 days later it is still 3.185 volts.


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## GizmoEV (Nov 28, 2009)

Red Neck said:


> Incredible how people manage to turn every single simple option into a religion..


That is not what is happening with the posts just prior to your message. It sounds like you are trying to make it that way, however. I simply provided some data to try to help us all understand what Pete was saying about the differences between bottom balancing vs top balancing with shunt balancers. Regardless of whether it matters to you or not, getting information helps people make informed decisions. I think that is what you are trying to say but it got lost in your presumption of this being a political/religious war.


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## onegreenev (May 18, 2012)

Its funny. As Jack would say, the cells could not give a damn about political differences. They could care less about what you think. They could care less if it is an inconvenience to you. Tough. Irregardless of the intricacies of how things work, if you bottom balance your cells you will have a very stable setup and won't need to have shunt resistors or any other energy wasting things connected. If you decide to do that its your choice. That is not the issue. 

No amount of arguing will make it different. So far after 4 solid years of driving electric cars the bottom balance issue has proven to work perfectly. My self and many many others not even part of this list will concur. We did not take Jacks word as gospel. We tested it and proved it and confirmed it and guess what? It friggin works. Cool, simple and stable and safe. Also the issue with solid connections has been a big issue and he found a source that he tested as well and proved would be an excellent choice for your connections. His idea, no but he did test it rather than just tell you. Same with the bottom balance issue. Still it is argued. Still it stands strong. It always will at least for the LiFePO4 cells and most likely for many others as well. 

Pete 

Religion, hardly. I think the religion is those trying to break the bottom balance issue, not those standing fast to it.


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## GizmoEV (Nov 28, 2009)

*Re: What kind of charging is this?*



kennybobby said:


> typically i've seen spec sheets call out [C/20] amps as the cutoff current to end the charging during the CV stage. Are you monitoring temperature--are they getting warm or swelling?
> 
> The data is confusing--how come the voltage is not held constant during the CV stage? What's the current doing--any current measurements? The message here seems to be that you have an unbalanced pack that is getting more so after 10 cycles of some sort of non-standard charging scheme.


What you describe is the standard charging procedure. What I learned from previous charge cycles and also from these cycles is that within three hours of rest the OCV of the cells dropped below 3.40V which means that they were not over charged. That is if these LiMnFePO4 (or is it LiFeMnPO4) cells act substantially similar to LiFePO4 cells. GBS said that the standard charge/discharge parameters for a capacity test on a single cell was charge at 0.3C to 4.00V and hold until the current drops to 0.05C. Rest 30 min then discharge at 0.3C to 2.50V. Charging to 3.6V and discharging to 2.8V is well within that range.

The way that the PowerLab8 works is that it uses the individual cell voltages to determine when 3.600V was reached rather than the pack average. Similarly on the discharge cycle. This is why I said that after a better bottom balance I would use the PowerLab8 for monitoring and not for charging so I could see what happens when the total pack voltage is used instead of individual cell voltages to determine current cut back. The only problem is that I will have to manually control the charging since my PS doesn't have separate voltage sense wires to determine when to switch into CV mode. Again, this was a (mostly) bottom balanced pack so yes, the top was not in balance.



kennybobby said:


> Did the first charging cycle consist of 10 hours at 3.5 amps?


No, that was just the first full peak after a discharge. I've attached some more pics. Remember, these are 20Ah cells so the charge time should not be much more than 20Ah/3.5A=5.7h. I attached the PowerLab8 file along with a delimited text file in the .zip if you want the raw data.

[EDIT: I did a pack discharge at 1A until the first cell hit 2.40V. Attached is the monitoring I did just after the discharge process ended. Clearly the pack was not well bottom balanced. Just keep this in mind when looking at the other attached graphs.]


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## GizmoEV (Nov 28, 2009)

EVfun said:


> You might want to wait longer than that. I just did a capacity test on my buggy pack. My typical is 0.2C load until the first cell hits 3.000 volts. I have left the cells since completing that and noticed that the lower cells continued to rise in voltage for 4 days. The first cell to 3.000 was 3.161 the next day and 3.185 four days later. Now 6 days later it is still 3.185 volts.


That is a good idea. I wonder if the PowerLab8 while monitoring puts a differential load on the cells being monitored or if it is exactly the same on each one. Anyone know?

I did a pack discharge to 2.40V at 1A and it finished this morning so I put the PowerLab8 into monitor mode. It has been over 11 hours and the low cell (#1) is at 2.611V and has been at that voltage for the last 5 hours whereas the high cell (#8) just bumped up to 2.809V. Cell 7 and 8 were the last two to get bottom balanced and I didn't wait nearly as long as cell 1 sat before hooking everything up for the cycling. It looks like it will be a week or more before I'm ready to hook them up again.


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## GizmoEV (Nov 28, 2009)

The only reason I top balanced my Gizmo pack is because of the limited control I have over my Zivan charger. I haven't touched it for 27 months and it isn't going out of balance like several told me it would. I figured a 20 cell pack would be easy enough to monitor by hand to provide a little more data about whether I needed to have cell level balancers on it. So far I'm not seeing the evidence saying I should. This 8 cell pack for my push mower is best if it is bottom balanced. I'll be charging it with a bench PS and will know to shut it off when I reach 20V under load and know no cell will kill off any other cells.



onegreenev said:


> Religion, hardly. I think the religion is those trying to break the bottom balance issue, not those standing fast to it.


Fear of what the data says?


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## kennybobby (Aug 10, 2012)

Thanks for the explanation and the data/graphs--now i understand what you were doing in this experiment.


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## onegreenev (May 18, 2012)

GizmoEV said:


> The only reason I top balanced my Gizmo pack is because of the limited control I have over my Zivan charger. I haven't touched it for 27 months and it isn't going out of balance like several told me it would. I figured a 20 cell pack would be easy enough to monitor by hand to provide a little more data about whether I needed to have cell level balancers on it. So far I'm not seeing the evidence saying I should. This 8 cell pack for my push mower is best if it is bottom balanced. I'll be charging it with a bench PS and will know to shut it off when I reach 20V under load and know no cell will kill off any other cells.
> 
> 
> 
> Fear of what the data says?


You don't shunt balance your cells either do you? If a top balance is done properly you should never need to balance at the top. Seems to me to be a bit tougher to balance the top at least by hand than doing it on the bottom first by hand. Still don't like my cells being ragged on the bottom. Too damn easy to over discharge and ruin cells but if you are real good at staying off the bottom and know that no one else will drive your beast then balancing top without shunting should be just fine and still no BMS would be needed. Monitoring the low voltage and have a cut back would still be good but most smart chargers can do that. Or a simple JLD meter that can shut down the controller and or charger as needed at a specific voltage.


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## GizmoEV (Nov 28, 2009)

I used my BMS modules to top balance and then immediately removed them so it wasn't too hard to do. Because it is so hard to get a Zivan programmed for what the user really wants I had to tell them that I was wanting to maximize my voltage without going over what my components could handle, which is true. They sent me some charge curves to choose from and I told them to program it for 19 LFP cells. I run 20 cells and used the voltage calibration pot to lower the voltage to 69.1V so that even though the current drops to ~100mA and holds that until the timer times out (Which varies between 45min and 2 hours depending on how long the charging has been going on. I think this is the lead acid equalizing phase really.) my pack doesn't over charge. I verified it this summer with a no parasitic loads after charge test and the highest cell was resting at 3.36xV and the lowest was something like 3.358V.

With a 70 mile pack and a typical 6 mile commute it is just a matter of what day should I charge rather than will I over discharge things. I rarely use my 240V NG3 since I like to meter my energy use with a Kill-a-Watt meter on my 115V NG1. The timer on the NG1 will only allow it to put in about 120Ah before timing out so I usually charge soon after going past 100Ah on the 200Ah pack.

When I start seeing a large variation in voltage between the two halves of the pack or I am no longer using it every day I'll probably go to a bottom balanced scenario and install a relay in the AC line and something like a JLD volt meter to turn off the charger when it reaches the target voltage. I'd replace the charger but it is hard to get a charger as light as the Zivan NG1. I can't get away with more weight in the nose of my Gizmo otherwise I won't be able to get in and out.


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## IamIan (Mar 29, 2009)

onegreenev said:


> Religion, hardly. I think the religion is those trying to break the bottom balance issue, not those standing fast to it.


I'm curious.

Why then do you think the 'Big boys' ... do not use that bottom balance and no active BMS method as Jack advocates?

Tesla doesn't , Nissan doesn't, GM doesn't ... etc ... They have lots and lots of testing data too ... and lots and lots of highly educated engineers ... etc.

If the data supports it soo strongly ... why then ... do you think they do not do it that way?


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## IamIan (Mar 29, 2009)

GizmoEV said:


> I wonder if the PowerLab8 while monitoring puts a differential load on the cells being monitored or if it is exactly the same on each one. Anyone know?


AFAIK every Volt meter has to put at least some minimum load on what it is testing.

But for the PL8 ... and most volt meters ... the load while just monitoring... is EXTREMELY low ... and the PL8 in particular does give the ability to account for any load it might have during monitoring.

If you use a very sensitive and reliable other meter to determine the very small load the PL8 uses during monitoring ... and see if there is any noticeable variation among the various tap point sensors ... you can adjust for that in a spread sheet in just a few seconds.

Or you could also ask PL8 OEM to adjust the the factor built in offsets they apply to the internal software to compensate for it ... the worst they will do is say no... or charge you a fee of some kind.

Although I think the effect is very tiny ... well below the PL8 published resolution specs.

The Whole main +/- voltage points are factory specs to have a resolution of +/- 6mV

the balancing sensors ribbon 1-8 cells ... for A123 cells have a spec of +/- 78uV


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## dedlast (Aug 17, 2013)

IamIan said:


> I'm curious.
> 
> Why then do you think the 'Big boys' ... do not use that bottom balance and no active BMS method as Jack advocates?
> 
> ...


I think it's for the same reason that you don't find Linux on mass-market computers. The masses don't want to mess with figuring it out, even if it is arguably better to use. 

Another thing is they have a ton of liability issues to worry about. Even if the data did/does actually strongly support the no-BMS/bottom balance method, the mindset of the masses, including legislators and even some (all?) of the racing series, says BMS is required, so that's what they have to do. 

Bill


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## jddcircuit (Mar 18, 2010)

I am also cycling some cells. I found this data interesting in a couple of ways. The x axis is time sampled every 2 seconds. There is some error in the voltage measurements perhaps +/- .03 is my guess right now. Pack current is on the right side secondary y axis in the graph.







After bottom balancing a group of 8 CALB 40AH SE (blue) cells, I charged them at a constant current .5C until the first cell hit 3.7V. I was surprised to see that one of the cells was so far out ahead of the rest.

I then tried a 1.5C discharge with my Low voltage cutoff set at 2.7 volts. It appears that cell 2 has a lot more voltage sag starting around 50% SOC. I then let the cells continue to discharge at lower amperage until 2.7 volts was reached. At lower currents the cells appear to be well grouped at the bottom.

So cell 6 and Cell 2 seem to have very different characteristics.

In my other group of cells the same cell limited my high side and low side cutoff which is what I wanted out of my weakest cell.

I just got some CALB 40AH CA (grey) cells. My goal is to do some fast charge/discharge demonstrations. I am looking forward to the comparison.

I haven't written my Constant Voltage control code yet but that might help me see how my weakest cell or highest IR cell limits my operational SOC region.


Jeff


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## Red Neck (Feb 1, 2013)

1. The big boys aren't end customers which need to make life work with one pack of batteries but have plenty of cells, so they can match them by capacity and use BMS systems (Tesla's probably scews things up because it meddles both at top and bottom instead just at top, which would result in far less unbalance over time which currently requires a full range charge every few thousand kms to balance again).

2. Big boys don't always know best but they have full access to their inverters, chargers, etc, so they employ over meddling rather than simplicity. Furthermore, they need to sell something they can make seem as complex and worth their premium and budgets, even though it does not improve the product you get.

So their reasons aren't tied to what you face as a single end user.

Read my previous post if you want to know what is optimal for which kind of individual end user.


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## Ampster (Oct 6, 2012)

dedlast said:


> I think it's for the same reason that you don't find Linux on mass-market computers. The masses don't want to mess with figuring it out, even if it is arguably better to use.
> 
> Another thing is they have a ton of liability issues to worry about. Even if the data did/does actually strongly support the no-BMS/bottom balance method, the mindset of the masses, including legislators and even some (all?) of the racing series, says BMS is required, so that's what they have to do.
> 
> Bill


Well then, let's talk about the RC guys and the electric bike guys. Many of them use BMS's and chargers that top balance. Many also use a much more volitile chemistry so they have their own reasons. 

Much earlier in this thread someone said that the choice to top or bottom balance was up to each person based on their circumstances.


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## GizmoEV (Nov 28, 2009)

IamIan said:


> I'm curious.
> 
> Why then do you think the 'Big boys' ... do not use that bottom balance and no active BMS method as Jack advocates?
> 
> ...


In addition to what others posted the "Big Boys" you mentioned don't, AFAIK, use LiFePO4 cells. I am not about to propose that my testing/experience on LiFePO4 cells applies to other chemistries let alone other lithium chemistries.


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## GizmoEV (Nov 28, 2009)

IamIan said:


> AFAIK every Volt meter has to put at least some minimum load on what it is testing.
> 
> But for the PL8 ... and most volt meters ... the load while just monitoring... is EXTREMELY low ... and the PL8 in particular does give the ability to account for any load it might have during monitoring.
> 
> ...


Thank you for that info. Hopefully the load is the same across the cells. I am, however, questioning the precision of the PowerLab8 when monitoring individual cells. Note the graph of the cell voltages as I was manually loading individual cells. Only one cell at a time had a resistor across it but it is clearly evident that the voltage measurement on 1 or 2 other cells changed. This makes me question the reliability of the measurements.

[EDIT: FWIW, I added a new graph of the after pack discharge voltages to the original post with my PowerLab8 results. I clearly didn't have a bottom balanced pack.]


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## Ampster (Oct 6, 2012)

GizmoEV said:


> In addition to what others posted the "Big Boys" you mentioned don't, AFAIK, use LiFePO4 cells. I am not about to propose that my testing/experience on LiFePO4 cells applies to other chemistries let alone other lithium chemistries.


Yes that is correct, I believe the Smart ED uses LiNiMnCoO2 and the Tesla uses a modified 18650 cell with similar chemistry. I believe those cells have higher energy density than LiFePO4 and a higher nominal voltage.


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## GizmoEV (Nov 28, 2009)

jddcircuit said:


> I am also cycling some cells. I found this data interesting in a couple of ways. The x axis is time sampled every 2 seconds. There is some error in the voltage measurements perhaps +/- .03 is my guess right now. Pack current is on the right side secondary y axis in the graph.
> View attachment 17302
> 
> After bottom balancing a group of 8 CALB 40AH SE (blue) cells, I charged them at a constant current .5C until the first cell hit 3.7V. I was surprised to see that one of the cells was so far out ahead of the rest.
> ...


I've wondered about what the different IR really does to a cell internally. For example, take a bottom balanced LiFePO4 pack that was bottom balanced at say 2.50V and left to sit for a week to be sure all cells were truly balanced at the same charge level. What would happen if the pack were discharged until a cell like your cell 2 went to a terminal voltage of 0V or possibly negative even though it wasn't truly at 0% SOC. Would it really be ok any way? What about on the charge end? We really only can measure terminal voltage so maybe a top balanced pack with a cell like your cell 6 aren't a problem for spikes of higher voltage when at a high charge rate like might happen in a regen situation.


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## Ampster (Oct 6, 2012)

jddcircuit said:


> I am also cycling some cells. ...............
> 
> I then tried a 1.5C discharge with my Low voltage cutoff set at 2.7 volts. It appears that cell 2 has a lot more voltage sag starting around 50% SOC. I then let the cells continue to discharge at lower amperage until 2.7 volts was reached. At lower currents the cells appear to be well grouped at the bottom.
> 
> ...


Thanks for that fascinating analysis. It appears the bottom moves around depending on the c rate. Does that mean that if I bottom balanced at 0.5c and then hit my pack at 1.5c when I was at a low SOC I might have one weak cell go below my target?

EDIT
I hadn't seen Gizmo's post before this post, but he is getting at the same issue.


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## jddcircuit (Mar 18, 2010)

GizmoEV said:


> I've wondered about what the different IR really does to a cell internally. For example, take a bottom balanced LiFePO4 pack that was bottom balanced at say 2.50V and left to sit for a week to be sure all cells were truly balanced at the same charge level. What would happen if the pack were discharged until a cell like your cell 2 went to a terminal voltage of 0V or possibly negative even though it wasn't truly at 0% SOC. Would it really be ok any way? What about on the charge end? We really only can measure terminal voltage so maybe a top balanced pack with a cell like your cell 6 aren't a problem for spikes of higher voltage when at a high charge rate like might happen in a regen situation.


Those are good questions but sorry I don't know. I don't know what the bottle neck is within the cell that is restricting the potential balancing or what type of stress it is causing the cell.

Like you indicate, I would be inclined to top balance if I had a cell like cell 6 along with cell 2. In that way I could maximize my region of lower IR across the pack for normal operation.

I am pretty green when it comes to batteries but as a test engineer I will measure and compare and ask questions when things look funny.


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## onegreenev (May 18, 2012)

I can charge a cell to 3.4 volts and it will rest at 3.35 volts. I can take the same cell and charge it to 3.5 volts and it will rest at 3.35 volts. I can take that same cell again and charge it to 3.6 volts and it will rest at 3.35 volts. 

Balance that!


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## EVfun (Mar 14, 2010)

onegreenev said:


> I can charge a cell to 3.4 volts and it will rest at 3.35 volts. I can take the same cell and charge it to 3.5 volts and it will rest at 3.35 volts. I can take that same cell again and charge it to 3.6 volts and it will rest at 3.35 volts.
> 
> Balance that!


WTF? I run a top balanced pack and I know that if the charging current is equal that will not be the result. Those charge numbers would be about 3.32 volts, 3.35 volts. and 3.40 volts, respectively, the next day.


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## onegreenev (May 18, 2012)

Like I said. I do with my cells.


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## onegreenev (May 18, 2012)

I'll continue to show and push the safe bottom balance methods no matter what the masses say. I believed in the beginning that it was paramount that I have a BMS system or my hard earned investment would be ruined. I was told I would ruin them if I did not have a BMS and if I did not TOP balance with active balancing on every charge. After investigating and testing I know that is just poppy cock. Just like I was told when I first got into building an electric car that I could build one with lead acid that would go 60 plus miles per charge. Ooops, they forgot one thing. They failed to mention that you must be doing 25mph and no stops. Lead acid cells and 60 miles on the freeway is total bull shit. Just like Top balancing and BMS systems and the need to actively shunt balance every charge. Bull. 

I do however know you need a way to stop the charge reliably at a specified charge level like you get with CV/CC or if you like to a specific voltage then terminate. I recipe. Same with bottom balancing them statically which you really don't get at the top. But you can get close. I choose the better way. Your cells. You don't have to believe the masses you know. They don't always know the answers. They think they do and they take from a totally different type of battery. Lithium batteries are not lead acid. Never have been never will be. 

I guarantee you that there are very many who figured out that the bottom was the best and safest and not by blind belief. But by actual doing and clarifying. Top without a BMS does work as long as you know for sure your ride will never be driven below a specified level. 

I have my charger control the top charge voltage and my controller the bottom. I can't drive below 2 volts per cell. My controller will never allow the voltage to drop below 2 volts per cell average per the pack. Very reliable. That is my management system.


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## BentMike (Oct 20, 2013)

EVFun,

It should be easy to check a cell for self-discharge.

Charge one up and check it. But be careful, there are often diffusion processes going on when you create disequilibrium. I expect that is what we see when we put 60V on a nominal 48V pack.

If you go watch Rickard having his intern do a BB in a recent video, he is not just driving the cells down once, he is settling all the cells in. So when they drift back up he pull a little more out, or they put a little more in if they overshoot. They meter this carefully, and insist the care is necessary. The cells in a stable condition are on the curve where battery function is continuous, that's where the self discharge will be negligible.

When you push the cells up at the top there are non continuous things going on hence the weird blip at high voltage. There is a disequilibrium the farther you go above nominal. So you see the cells drift down.

I bet you can stabilize them at 48V and they don't drift. 

Anyway, LFP clearly don't have self discharge like LAB. If Rickard says you can check them for variations after 100 cycles or so, that is pretty good. No two cells are just the same, so repeated charge cycles won't drive them all to exactly the same state.

I can't substantiate this, but it makes sense to me. You could probably talk to or email Rickard and he will give you an earful. If he answered this a lot, he might not be all that forthcoming.

*On what basis do you doubt this? Can you propose a phenomena by which a stabilized cell self discharges? 
*


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## GizmoEV (Nov 28, 2009)

onegreenev said:


> I can charge a cell to 3.4 volts and it will rest at 3.35 volts. I can take the same cell and charge it to 3.5 volts and it will rest at 3.35 volts. I can take that same cell again and charge it to 3.6 volts and it will rest at 3.35 volts.


Without ending currents given you really haven't said anything. I can do what you said and get different results. It all depends on what you use for ending current. I charged a cell one time to 3.6-3.8V and it rested at about 3.55V for days before I finally used it to test what would happen if I connected it with a cell at around 20% SOC. How did I get it to rest that high? Simple, put it on a power supply which holds the target voltage for a day or two. It definitely over charged the cell!


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## BentMike (Oct 20, 2013)

onegreenev said:


> I can charge a cell to 3.4 volts and it will rest at 3.35 volts. I can take the same cell and charge it to 3.5 volts and it will rest at 3.35 volts. I can take that same cell again and charge it to 3.6 volts and it will rest at 3.35 volts.
> 
> Balance that!


I will venture that you are describing "stray capacitance." The battery is charged, charges build up elsewhere could even be electrostatic, They will make no real power, but show as potential. The stray capacitance bleeds off and you are back to your fully charged voltage.


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## Ampster (Oct 6, 2012)

GizmoEV said:


> Without ending currents given you really haven't said anything. I can do what you said and get different results. It all depends on what you use for ending current. I charged a cell one time to 3.6-3.8V and it rested at about 3.55V for days before I finally used it to test what would happen if I connected it with a cell at around 20% SOC. How did I get it to rest that high? Simple, put it on a power supply which holds the target voltage for a day or two. It definitely over charged the cell!


That is all I was asking Pete, a few pages back, about clarifying what you meant when you said the cells were not stable at the top. You offered no hard evidence to support that statement. Yes, there are steep curves at either end. GizmoEV and Jddcircuit have offered charts to allow other readers to reach their own conclusions based on that evidence. I see the same evidence when I cycle my cells with my PL8.


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## onegreenev (May 18, 2012)

My controller charges with CC/CV and terminates at .2C. It is a nominal 120 volt charger. I charged to 3.65 volts for an average pack voltage then hold the volts until the amperage reaches .2C the terminates. I have 10 voltage settings I can use in 1 volt increments. Algorithm remains the same for all voltage settings. 100ah cells. 

Quite boring but I can show you a cell that was charged to 3.65 volts and show it rests at 3.33 volts? I can show you a cell charged to 3.5 volts and show you it rests at 3.33 volts. Is that what you want to really see? A boring White Box and a volt meter connected to show 3.33 volts and a graph showing I actually did charge it to 3.65 volts? 

One, two or three decimal places for the readings? Usually two is touted. At three there is some variances. Not always. Maybe I'll do an A-123 cell. Its resting voltage is a little different but not by much.


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## Ampster (Oct 6, 2012)

onegreenev said:


> My controller charges with CC/CV and terminates at .2C. It is a nominal 120 volt charger. I charged to 3.65 volts for an average pack voltage then hold the volts until the amperage reaches .2C the terminates. I have 10 voltage settings I can use in 1 volt increments. Algorithm remains the same for all voltage settings. 100ah cells.
> 
> Quite boring but I can show you a cell that was charged to 3.65 volts and show it rests at 3.33 volts? I can show you a cell charged to 3.5 volts and show you it rests at 3.33 volts. Is that what you want to really see? A boring White Box and a volt meter connected to show 3.33 volts and a graph showing I actually did charge it to 3.65 volts?
> 
> One, two or three decimal places for the readings? Usually two is touted. At three there is some variances. Not always. Maybe I'll do an A-123 cell. Its resting voltage is a little different but not by much.


No, I want to see the instability that you claimed at the top. Now it is quite boring, and still no evidence of unstable cells at the top.

Ampster, over and out.


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## onegreenev (May 18, 2012)

Do you monitor each cell in your pack while charging? While charging the the cells voltages will muck around. They are not stable during charge or discharge. Only at rest are batteries stable. The mucking around of the cells while charging or discharging is an ACTIVE process. Want to see paint dry then watch them while you charge. They do move around. Even from one charge to the next. Easiest to see on a graph. While charging you can't truly balance them. If I were to balance on the bottom while driving the vehicle I'd never be able to do it. I'd have to stop and let them sit. Add or take away some more until I got the to stabilize at a specific voltage. 

You know, this process of balancing is to be done ONCE. Not every charge. It is far easier to balance on the bottom where the voltages are quite stable and nearly empty. When full you can have a cell at 3.4 volts while charging even at 1 amp and you can't balance it. Once you remove the charge the cell WILL drop. That there is no doubt. It will come to rest at a specific voltage. Charge it up to 3.5 volt and it will still rest at that resting voltage. It is the resting voltage of the cell but you can have differing states of charge if you do this on the top. It may be an amp hour or less but its still not balanced. 

You know. most of this balancing arguing is pretty nit picky and usually over a couple amp hours of capacity. About 1/2 mile at best. 

Top balancing will leave the bottom ragged. Bottom balancing will leave the top ragged. Capacity wise. Not voltage wise. This there is NO DOUBT. This is known. But if you top balance and either have a BMS or Not you leave the bottom vulnerable to ruining a battery. At the top if you top balance and charge to the utter most ends you gain very little and stress your cells. If you top balance and charge to 3.5 or 3.6 volts then your fine except the vulnerability of the cells on the bottom. 

That is all this is about. It is KNOWN that while charging that voltages rise, fall and dance around. What proof do you need for that. That happens with any battery. Put on a volt meter and watch. Graph it. 

This balancing issue is a capacity issue. 

Take 10 cups of water. Each holds 10 cups except one holds 9 1/2 cups. Fill each cup till full. Typical of a top balance with shunting. On each cup is a valve. Have all valves open at once and drain the cups. Close the valve when the one cup is empty. That is your usable capacity. Keep the valves open you now run on 9 cups, but with the battery those other cells will drive current through and kill the one cell making it useless and maybe even burn down your ride. 

Now do the same with the cups but only fill each with 9 1/2 cups of water. This is the usable capacity. Now if you let the valve stay open to the end they all empty at the same time. I still have the same capacity. 

But what most do is only fill them to partial full anyway. So what then is the purpose of TOP balancing if you don't squeeze out every ounce of juice? 

If you only charge to 3.5 volts anyway why not do this from the bottom. Balance on the bottom then charge to 3.55 volts. Your top will at rest look to be balanced anyway and your bottom WILL be balanced and in the event you happen to drive beyond a normal discharge you will know your cells are safe. 

Here is one more thing. When a cell is at rest at 2 volts I can put in some current and raise that voltage and it will move the voltage up. Noticeably up. If the cell is FULL and I put current to it it goes up then crashing back down. It won't stay up. The bottom is more stable. I'll do a video of that, how about that.


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## onegreenev (May 18, 2012)

Ampster said:


> No, I want to see the instability that you claimed at the top. Now it is quite boring, and still no evidence of unstable cells at the top.
> 
> Ampster, over and out.


Put voltage meters on your cells and connect them to your laptop and graph your charge and discharge of all cells together and the put them side by side and you will SEE the varying voltages. It is not rock stable. I see no reason you keep arguing this when it has been shown in the past by folks testing cells. 


Ahaaaa, nevermind this is all to boring anyway.


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## onegreenev (May 18, 2012)

Time to go bottom balance my CALB's anyway.


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## jddcircuit (Mar 18, 2010)

onegreenev said:


> I'll continue to show and push the safe bottom balance methods no matter what the masses say. I believed in the beginning that it was paramount that I have a BMS system or my hard earned investment would be ruined. I was told I would ruin them if I did not have a BMS and if I did not TOP balance with active balancing on every charge. After investigating and testing I know that is just poppy cock. Just like I was told when I first got into building an electric car that I could build one with lead acid that would go 60 plus miles per charge. Ooops, they forgot one thing. They failed to mention that you must be doing 25mph and no stops. Lead acid cells and 60 miles on the freeway is total bull shit. Just like Top balancing and BMS systems and the need to actively shunt balance every charge. Bull.
> 
> I do however know you need a way to stop the charge reliably at a specified charge level like you get with CV/CC or if you like to a specific voltage then terminate. I recipe. Same with bottom balancing them statically which you really don't get at the top. But you can get close. I choose the better way. Your cells. You don't have to believe the masses you know. They don't always know the answers. They think they do and they take from a totally different type of battery. Lithium batteries are not lead acid. Never have been never will be.
> 
> ...



You seem to have a very narrow view of the situation and seem to broadly use the term BMS for a circuit that shunts charge current based on an upper cell voltage threshold. This charge current regulation method is only a single type of BMS.

There are several more BMS configurations that you seem to be discounting that may actually have merit depending on the given pack situation.

I just posted a graph of a bottom balanced pack that showed a cell that reached full well before the others thus limiting the upper end of the charge cycle prematurely. In the same pack there is another cell that has excessive sag at lower SOC region. By realigning the SOC of these two cells with respect to the others may improve the operational region of the pack.

I think we all agree that the lower end SOC voltage will hold more stable after removing current than the upper SOC voltage. This is very convenient when initially balancing the cells when you do not have other means of pack management or optimization.

One might say just remove those cells with limiting characteristics from the pack and I would agree and thank my BMS for identifying the limitation and opportunity for pack improvement.

Regards
Jeff


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## onegreenev (May 18, 2012)

Actually I am very much aware of the different types of BMS systems available. I also know the limits of my cells and it did not take a BMS to tell me.


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## IamIan (Mar 29, 2009)

GizmoEV said:


> I am, however, questioning the precision of the PowerLab8 when monitoring individual cells.


You can always contact PL8 about getting your unit recalibrated by them ... or you could run a series of tests to determine the offset to account for it in the spread sheets ... or if it's close enough for you as is , use it as is.


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## racunniff (Jan 14, 2009)

GizmoEV said:


> I am, however, questioning the precision of the PowerLab8 when monitoring individual cells.


Are you questioning the precision, or the accuracy? http://en.wikipedia.org/wiki/Accuracy_and_precision


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## GizmoEV (Nov 28, 2009)

racunniff said:


> Are you questioning the precision, or the accuracy? http://en.wikipedia.org/wiki/Accuracy_and_precision


I'm questioning the precision. Once an instrument is precise, accuracy is easy to correct for with mathematics. I didn't look at your link but precision is basically the repeatability of a measurement. When I discharged one cell in the series the voltage reading on another cell changed. This means the instrument isn't precise.


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## dtbaker (Jan 5, 2008)

onegreenev said:


> Top balancing will leave the bottom ragged. Bottom balancing will leave the top ragged. Capacity wise. Not voltage wise. This there is NO DOUBT. This is known. But if you top balance and either have a BMS or Not you leave the bottom vulnerable to ruining a battery. At the top if you top balance and charge to the utter most ends you gain very little and stress your cells. If you top balance and charge to 3.5 or 3.6 volts then your fine except the vulnerability of the cells on the bottom.



this really does sum it up very well.... Still up to the individual to choose. 

The only thing I would add is that if bottom balancing you have to be able to set your end of charge voltage such that the charger can catch the total pack voltage when the highest cell first starts climbing. This precludes using the inexpensive fixed curve chargers like Elcon and the ilk UNLESS you have it programmed with a #613 or #620v curve that allows some user end voltage adjustment.

I find it easier to top balance to a benign top voltage than to EVER bring cells down to 3.0 or less. The charger can then be pre-programmed to end at average for 3.55-3.60vpc without concerns, and I have faith in myself and driving habits to never take the pack to the ragged bottom. 

I will add my data points of having observed basically no relative drift between cells after top balancing, after hundreds of charges, without a BMS or any other partial pack loads.


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## Zak650 (Sep 20, 2008)

Actually Jack at EVTV has put together a package that let's you set the voltage, charging amps, finish amps with a simple serial port instruction. It's 4 kw TCCH charger (different voltages) and a nice robust module that controls the 4kw canbus TCCH charger it requires 12V power, has three instructions you send as ascii text from a computer over the serial port via usb cable, an example below:
V116.4
A28.6
E4.6
sending this info to the control box sets it all up, unplug the usb cable.

go to EVTV, it works, I'm charging a 16 cell pack with a 168v 4kw charger at this moment.
V56.0
A28.0
E6.0

Gone are the days of inflexible charging


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## onegreenev (May 18, 2012)

If your elcon happens to have the can bus feature when you purchase it. Mine does not have that feature. Great little item. Soon to be a big seller. Full control for your charging needs.


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## dtbaker (Jan 5, 2008)

Zak650 said:


> Actually Jack at EVTV has put together a package that let's you set the voltage, charging amps, finish amps with a simple serial port instruction. It's a nice robust module that controls a 4kw canbus TCCH charger it requires 12V power, has three instructions you send as ascii text from a computer over the serial port via usb cable,


sounds very interesting.... should work with the Elcons, in any size that has a CANbus? A question I would have is whether you can also change the curve type... i.e. from an older lead-acid floodie curve to a simple CA->CV for LiFePO4 like factory curve #613 ?

...I poked around on the evtv site for a while, but couldn't find this.... do you happen to have the URL, or remember which page of stuff it is on?


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## BentMike (Oct 20, 2013)

I saw EVTV talking about a couple things. One is the GEM EV product they have developed. Think I saw another less developed gizmo in a more recent video.

I wish I could remember which video and where in it. There's hours and hours of them. The recent videos about the GEM EV are labeled that way in the video archive.

The easiest thing would be to call them and ask.


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## Zak650 (Sep 20, 2008)

dtbaker said:


> sounds very interesting.... should work with the Elcons, in any size that has a CANbus? A question I would have is whether you can also change the curve type... i.e. from an older lead-acid floodie curve to a simple CA->CV for LiFePO4 like factory curve #613 ?
> 
> ...I poked around on the evtv site for a while, but couldn't find this.... do you happen to have the URL, or remember which page of stuff it is on?


EVTV is selling a 4kw charger and control unit together, it's under battery chargers. You would have to check with them if they are selling the control separately or if they have any plans to. 
From EVTV
TCChargerModel:	
168v 30A
208v 22A
258v 18A
290v 16A
389v 12A
Price: $1,895.00


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## DavidDymaxion (Dec 1, 2008)

I read that DeWalt tool battery packs bottom balance, and they have a five year warranty. I would guess tool batteries get a more severe workout than car batteries, and that there are more tool batteries than car batteries.

It is a good question as to what algorithms the big guys use.



IamIan said:


> I'm curious.
> 
> Why then do you think the 'Big boys' ... do not use that bottom balance and no active BMS method as Jack advocates?
> 
> ...


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## BentMike (Oct 20, 2013)

IamIan said:


> I'm curious.
> 
> Why then do you think the 'Big boys' ... do not use that bottom balance and no active BMS method as Jack advocates?
> 
> ...


I believe it is easier to keep the batteries balanced on the fly with top balancing. You have to tear the pack down and set each cell aright manually, and the building of the pack is more laborious. But, I think it is the better choice for the DIY guy, not for production. One downside of top balancing is the care , feeding and quality of BMS. If you can skip them altogether and get better results for a little elbow grease, then it is appealing. Also keep in mind the the OEM EV's are using a fraction of their packs capacity to prevent the over charge/discharge issues. We on the either hand like to push farther, at least with LFP.

EVTV will tell you that as time goes on, battery capacities drop, and drift away from each other, that the chance of over discharging arises even though they are balanced at the top. Also LFP are all Jack is messing with, and though I hear Tesla is putting 18650s in the S model now, and could do it, there may be reasons not to do so with other chemistries (I am too new to comment on anything but LFP).

I see that Jack Rickard has a semiautomated, single cell, bottom balancing gizmo. I think you could do it with the JLD404. I am waiting for mine. If it has a PID function (as the JLD612's I use do) then it might do a very good job indeed - unattended. But I will still have to hook up each cell individually. Jack seems to think this is a one time operation. I might be a "show me" guy on that.

For my larger format 48V pack of 40Ah's, I will probably go with 17 cells instead of 16. This seems like cheap insurance against over charging. If you only have 60V (3.53V/cell x 17 cells) then you can't mess up a pack of 17. I will use a JLD404 to monitor V, A, and Ah, and to cutoff at 34V. Even if something goes amiss, bottom balancing may be good protection against over discharge.

No BMS needed.


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## pdove (Jan 9, 2012)

This thread should be deleted. It is full of false information. If any of you want to know about Lithium batteries then read some actual scientific papers on it and then you will understand. 

Here's a little know fact the failure mechanism in Lithium batteries is electrolyte breakdown and the forming of the SEI layer. This plating prevents lithium Ions from _insertion_ (or _intercalation_) into the electrode. Lithium ions accumulate on the surface of the anode where they are deposited as metallic Lithium. 

If the charging voltage is increased beyond the recommended upper cell voltage, excessive current flows giving rise to two problems. Plating and over heating.

The consequence is a reduction in the free Lithium ions and hence an irreversible capacity loss and since the plating is not necessarily homogeneous, but dendritic in form, it can ultimately result in a short circuit between the electrodes (i.e. cell reversal). Lithium plating can also be caused by low temperature operation. Allowing the cell voltage to fall below about 2 Volts by over-discharging or storage for extended periods results in progressive breakdown of the electrode materials.

On the Anode side -First the anode copper current collector is dissolved into the electrolyte. This increases the self discharge rate of the cell however, as the voltage is increased again above 2 volts, the copper ions which are dispersed throughout the electrolyte are precipitated as metallic copper wherever they happen to be, not necessarily back on the current collector foil. This is a dangerous situation which can ultimately cause a short circuit between the electrodes. 

On the Cathode side- keeping the cells for prolonged periods at voltages below 2 Volts results in the gradual breakdown of the cathode over many cycles and a consequent permanent capacity loss. With Lithium Iron Phosphate cells this can happen over a few cycles.

Excessive current also causes increased Joule heating of the cell, accompanied by an increase in temperature. Operating at high temperatures brings on a different set of problems which can result in the destruction of the cell. In this case, the Arrhenius effect helps to get higher power out of the cell by increasing the reaction rate, but higher currents give rise to higher I2R heat dissipation and thus even higher temperatures. This can be the start of positive temperature feedback and unless heat is removed faster than it is generated the result will be thermal runaway.

The cycle life quoted in manufacturers' specification sheets normally assumes operating at room temperature. This would be totally unrealistic for automotive applications. Graphs showing how cycle life varies with tempeature like the one above are seldom provided by cell manufacturers.


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## IamIan (Mar 29, 2009)

pdove said:


> This thread should be deleted. It is full of false information. If any of you want to know about Lithium batteries then read some actual scientific papers on it and then you will understand.


I do wonder about the resurrection / reviving of a 4 month old already dead thread to ask for that thread to be killed? ...  ... but anyway...



pdove said:


> it can ultimately result in a short circuit between the electrodes (i.e. cell reversal).


A short circuit is NOT cell reversal ... two very different things.



pdove said:


> Excessive current also causes increased Joule heating of the cell, accompanied by an increase in temperature.


Well excessive current ... is Excessive after all.

Worth noting , as the temperature of the cell increases you can get a decrease in the ohms ... less ohms = less joules of heat at the same amp rate.



pdove said:


> but higher currents give rise to higher I2R heat dissipation and thus even higher temperatures.


Higher heat dissipation , by definition is more heat joules removed (dissipated) from the cell... which would not itself lead to higher temperatures.


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## pdove (Jan 9, 2012)

Here's a black and decker patent for bottom balancing 
[28]In prior art methods of battery charging, each cell is charged to a predetermined voltage, regardless of the resulting capacity in the cell. According to the principles of the present disclosure, each of the battery cells is discharged to a predetermined low capacity. Once the cells are balanced at this low capacity, an equal amount of capacity is added to all of the cells. To accomplish this, the cells may be charged from the predetermined low capacity until any of the cells reaches a maximum voltage. At this point, charging of all the cells can be stopped. FIGS. 4A-B and 5 graphically depict charging according to these principles. 

Read more: http://www.faqs.org/patents/app/20090096419#ixzz2wHOmT4JI


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## onegreenev (May 18, 2012)

Discharge each cell using a coarse method of discharge and stop at a predetermined minimum voltage. The use device like a the PowerLab 6 or 8 and then do a fine discharge using the cc/cv method until you reach a pre-determined voltage. Once all cells are within that predetermined voltage you connect them in series and charge the pack to a predetermined voltage using the CC/CV until the predetermined amperage is reached. That is your full capacity. It is simple and effective. You may have a cell or two that wiggle above or below the predetermined level but usually not by much. Once the charge is done the cells will settle to a natural balance and will usually be around 3.38 max. My old Hi-Power cells would always settle in at 3.33 volts. Even if I charged to 3.65 or 3.55 they would always settle to a natural state. Each type will be a bit different. There is no need to charge the LiFePO4 above 3.55 volts.

Open source. Works and has a proven track record.


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## jddcircuit (Mar 18, 2010)

pdove said:


> Here's a black and decker patent for bottom balancing
> [28]In prior art methods of battery charging, each cell is charged to a predetermined voltage, regardless of the resulting capacity in the cell. According to the principles of the present disclosure, each of the battery cells is discharged to a predetermined low capacity. Once the cells are balanced at this low capacity, an equal amount of capacity is added to all of the cells. To accomplish this, the cells may be charged from the predetermined low capacity until any of the cells reaches a maximum voltage. At this point, charging of all the cells can be stopped. FIGS. 4A-B and 5 graphically depict charging according to these principles.
> 
> Read more: http://www.faqs.org/patents/app/20090096419#ixzz2wHOmT4JI


Thank you for that link.
I noticed that US patent application US20090096419 also linked to China patent grant of CN201466159 in google patents. Different inventors but same priority date, same basic invention. The US patent application does not appear to have been granted yet.

edit: As I look closer it might be the same patent just a Translation confusion on my part

I thought I heard that Dewalt had a patent on this maybe it was Black & Decker.

I am becoming a fan of this bottom balancing procedure as I test more cells. The voltage of the cells near the top of the SOC region while current is flowing does not seem to correlate to SOC as well as it does at the bottom. However both end voltages are influenced by rate of current.

I agree with the patent when it recommends stopping the charge when any single cell reaches a full condition (definition of full may vary). The single cell fault resolution is a function of the number of cells in series and some packs are getting pretty tall (high voltage).

Many of us only use pack voltage to determine full SOC but with the understanding that a rouge cell may get over charged if it's capacity is very different from the rest.

Using pack voltage as opposed to individual cell voltages to stop charging is an acceptable compromise for many of us DIYers because of the cost associated with continuous monitoring of each cell voltage and the relatively similar capacity of these cells not requiring it.

Some are clever enough to split the pack into two groups and compare the voltages which helps to add measurement resolution and increases the chances of detecting a premature single cell full condition.

Regards
Jeff


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## glyndwr1998 (Apr 27, 2013)

Hi Jeff,

I use simple cell loggers to monitor each cell voltage and use the alarm port out trigger to switch a relay that cuts the charger.
In addition I use a jld 404 that also cuts the charger on pack voltage setting.

I am also in the process of modding the zivan charger to manually adjust the output voltage to the pack voltage as another safet precaution.

I have 76 cells in series and 11 cell loggers.

Anthony.


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## onegreenev (May 18, 2012)

Loggers add an inconsistent load on each cell and will cause an imbalance over time. Not a good thing. Use a setup that will allow you to turn on briefly a volt meter to check your voltages but to turn off as soon as you release the button. A Lee Hart Battery Bridge is a good idea to monitor without having to connect to single cells. If one cell goes out the bridge will show that and which side the bad cell is located. Then all you need to do is check the half pack and find the offending cell. A simple procedure.


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## glyndwr1998 (Apr 27, 2013)

Yes the loggers do place an uneven load on the cells, I use the resistor method on cell 7 to drain about 1ah per month running.

This is my first project using lifepo4 cells, I've got them in a plug in prius mod , I didn't want to pay an extra £1500 for a bms, especially when some are not that reliable and end up damaging the cells.

After reading I opted for the cell loggers as it was affordable, and it was the only system I could find that allows individual cell monitoring, albeit not perfectly as they do imbalance.

If you know of any other more effective solution that doesn't cost as much as my cells cost, then please advice, I would to find an alternative, although this does work very well, it is by no means a plug and play system, it does involve frequent intervention.

Many thanks, anthony.


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## onegreenev (May 18, 2012)

Just wire them up so that you turn them on via a single momentary switch. Push the switch then check your voltages then release the switch effectively turning them off. Keeps the power off except when you need to check. I am quite sure that you don't need to check voltages at every moment they are in use. Just check once and awhile and have a bat bridge installed to monitor a split pack all the time. That is a better way.


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## evmetro (Apr 9, 2012)

I can't imagine what it would be like to drive any of my conversions around without having the real time gauges that display highest cell and lowest cell. Towards the end of the range, I can keep my eye on the lowest one, and when using regen, I can watch the highest one. It is nice to watch all 45 cells at the same time, but having an instant lowest cell reading is easier since it is only one number to watch. I keep reading about bottom balancing, but I think the Orion has me spoiled forever. I have been watching my charge from my recliner with my iPad while I have been reading threads on this site....


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## onegreenev (May 18, 2012)

I can't imagine watching all 60 cells every time I go for a drive either. I did check my first pack of Lithium to be sure which cells were above or below after a solid bottom balance. On the bottom there was never an issue and when the pack was charged there was no cell above 3.7 during the charge process. All cells settled to 3.33. So with that I knew and know that no cell will ever go above a danger point and at the bottom no cell will ever go below a threshold before any other so all the cells are safe to the point where the car really won't even move. If your regen is set correctly it won't ever over charge your pack. Since the cells don't drift around there is never any reason to continue to watch each cell. I actually got quite bored watching and seeing no change from charge to charge. I do have a volt meter so I can see if the pack is below what it should be and I also never had or will have a source to discharge each cell. I don't want to introduce any cause of an imbalance. I don't want to have my pack out of balance on the bottom. Main reason is, you charge your pack so its full and as soon as your down the street the pack is OFF the top anyway. It is so briefly on the top that it is kinda odd to have that part of the charge balanced. I prefer to have the bottom in the unlikely event that someone over drives the pack. I actually have the pack parameters set to never go below a particular voltage no matter what. It works flawless but if for any reason it did go below that threshold the cells would be matched and not in any danger of being killed by the other cells. Ive been there before and you can trust me when I say you can kill cells in seconds buy driving high current through a cell that discharges before the others. It ain't a pretty sight. 

With my current pack I will bottom balance and install a bat bridge and monitor the two halves. I will have a volt meter and I will have an amp meter. 

The only reason I'd have a BMS is to have a dashboard via an android or something to watch my speed and such. Not to have cell level monitoring. At this point I will stick with bottom balancing. I won't charge above 3.55 volts and will not discharge below 2.4 volts static. So at full charge I will have 213 volts before they settle. On the bottom I will have 144 volts. They will settle to about 3.35 volts after charge. Even if I went to 3.65 they would settle to 3.35 volts. I will check to be sure no cell ever goes above 3.7 volts during the charge cycle. If they do I will replace them. Simple. 

Pete


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## jddcircuit (Mar 18, 2010)

evmetro said:


> I can't imagine what it would be like to drive any of my conversions around without having the real time gauges that display highest cell and lowest cell. Towards the end of the range, I can keep my eye on the lowest one, and when using regen, I can watch the highest one. It is nice to watch all 45 cells at the same time, but having an instant lowest cell reading is easier since it is only one number to watch. I keep reading about bottom balancing, but I think the Orion has me spoiled forever. I have been watching my charge from my recliner with my iPad while I have been reading threads on this site....


That is very cool.

Too bad it is not very cheap.


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## glyndwr1998 (Apr 27, 2013)

The Orion is a nice piece of technical kit, but it is expensive, but for that expense it does do a lot and gives a lot of info.

I am hoping that someone starts an open source project of similar functions that we can build ourselves for a reasonable cost.

I would love an Orion, but couldn't justify the expense for a hobby project, I'd rather more cells for more range.

Anthony.

[lQUOTE=evmetro;383029]I can't imagine what it would be like to drive any of my conversions around without having the real time gauges that display highest cell and lowest cell. Towards the end of the range, I can keep my eye on the lowest one, and when using regen, I can watch the highest one. It is nice to watch all 45 cells at the same time, but having an instant lowest cell reading is easier since it is only one number to watch. I keep reading about bottom balancing, but I think the Orion has me spoiled forever. I have been watching my charge from my recliner with my iPad while I have been reading threads on this site....[/QUOTE]


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## dougingraham (Jul 26, 2011)

evmetro said:


> I can't imagine what it would be like to drive any of my conversions around without having the real time gauges that display highest cell and lowest cell. Towards the end of the range, I can keep my eye on the lowest one, and when using regen, I can watch the highest one. It is nice to watch all 45 cells at the same time, but having an instant lowest cell reading is easier since it is only one number to watch. I keep reading about bottom balancing, but I think the Orion has me spoiled forever. I have been watching my charge from my recliner with my iPad while I have been reading threads on this site....


And I can't imagine wanting to watch this stuff anymore. I don't even have a voltmeter or ammeter anymore. After satisfying my curiosity I took out the voltmeter and ammeter. I kept only the stuff that is necessary for driving a car. I have the original tach hooked up but don't really need it. Speedo works. I use the trip odometer as my range gauge. The only other instrument is the 12v battery meter and that only because it just happened to work after the conversion. I found that the voltmeter and ammeter are a distraction and when I offered to let people drive it they were more reluctant than now without it. It is just a car now that happens to be really fun to drive and makes no noise or odd smells.


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## evmetro (Apr 9, 2012)

Dougingraham, You make a very good point about not watching the instrumentation all the time. I think that a good quality conversion should be able to operate with nothing other than a gauge for SOC, and a speedometer. My dash display wakes up to a default screen of nothing but speedometer and SOC, so if somebody else drives one of my conversions there is nothing to be intimidated by. Since I like a lot more info, a swipe from my finger across the screen brings me into my personal favorite with the big speedo in the middle and all the important data, particularly the instant highest cell and instant lowest cell. As much as I love all the data, bells, and whistles though, I think that a good build should be able to withstand a strange driver who has never driven an EV. My hat is off to the DIYers who use duct tape, deck screws and coat hangers for having the perseverance to convert their cars, but a lot of those cars are in a different league where they can only be driven by the builder. OK, end thread jacking. I will stick with top balancing from the recliner.


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## onegreenev (May 18, 2012)

evmetro, got a question about the orion BMS. Can it be setup to do exactly what you want but the ability to turn off top balancing?


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## evmetro (Apr 9, 2012)

Yea, you could run it with no balancing. You can set the voltage for the Orion to begin balancing while charging at whatever voltage you like, and you can also tell it to stop pulling down cells after the charger shuts off at whatever voltage you like as well. If you set the begin balancing and stop balancing voltages to the same as the charger cut off voltage it will never balance the cells. For my top balancing, my cells don't start balancing until they have reached 3.4 volts, and then my cut off is set for 3.55. After the charger cuts off, I have the Orion set up to pull the high cells down until any of them hit 3.4 again which is not really very long. You could very easily leave the tops ragged, and you could witness all of your cells as they approach discharge. You can even have a real time graph of all your cells from your trip to review at the end of your drive. You would know immediately if any of your cells was not discharging as low as the rest.


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## Ampster (Oct 6, 2012)

jddcircuit said:


> ...
> ... with the understanding that a rouge cell may get over charged if it's capacity is very different from the rest.
> ......


LOL What does one do if their cells are yellow instead of rouge?


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## Vhclbldr (Feb 8, 2014)

onegreenev said:


> Loggers add an inconsistent load on each cell and will cause an imbalance over time. Not a good thing. Use a setup that will allow you to turn on briefly a volt meter to check your voltages but to turn off as soon as you release the button. A Lee Hart Battery Bridge is a good idea to monitor without having to connect to single cells. If one cell goes out the bridge will show that and which side the bad cell is located. Then all you need to do is check the half pack and find the offending cell. A simple procedure.


Thanks for that recommendation! I just looked it up and the battery bridge looks like a simple solution to pack monitoring. (http://www.evdl.org/pages/battbridge.html)


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## onegreenev (May 18, 2012)

So vehicle builder, what are you building? Yes the batt bridge is a quite simple and elegant device.


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## Old.DSMer (May 18, 2012)

*Bottom Balance Problem*

I'm still on the fence regarding either a passive battery "MONITORING" system or an active "MANAGEMENT" system. Recent events have me heavily considering a BMS.

I tested every CALB CA100 cell before building the 86s1p pack. 4.8% range in capacity. 104 lowest, 109 highest (from 3.50 to 2.70).

Bottom balance was within 2.2% (from 2.692 to 2.752).

I stopped initial charge when first cell reached 3.50V. Re-adjusted my charger cut-off as per bottom balance procedure. Since then, I've had about 12 partial cycles.

I noticed the resting voltage creeping higher on the last 3 charges. And last night, the charger ran longer than it should have based on my Ah usage. So I stopped the charge and measured some voltages. *Note the high voltage cells were not all my lowest capacity ones.*

Below is my chart showing the initial bottom balance and the wide charge variance with some cells measured last night. After my initial charge, they were all sitting around 3.35-3.40 so I was very happy and was confident in my bottom balance procedure. From what I understand, the LiFePO4 cells have nearly 99.9% Ah efficiency. So what would cause this huge variance in just a short period of time?

I don't want to start another BMS debate - I would just like to hear some opinions from other bottom balancers.

*1. Have you seen this with a new pack before?
2. Do I need to re-bottom-balance again? I bled off the highest cell to get it back down to about 3.5.*

Thanks for any input!


----------



## onegreenev (May 18, 2012)

*Re: Bottom Balance Problem*

That graph looks pretty good. Being bottom balanced I'd expect to see the top looking like that. The rest of those cells at between 3.2 and 3.3 are not full. The cells that are closer to the 3.5 range are most likely your lowest capacity cells as those will usually fill first. Looks like you have about 10 of them that are lower in capacity than the others. 

Now that you have them charged go take your car for a spin up the street and back. Then re do the voltage readings and compare them to the graph. Is the graph now more equal across the voltage range? Should be. 

Then drive it to your cut off point and do another reading. 

Post all the graphs. 

I see no reason not to monitor the pack cells but be sure you can do so using your DC DC so you don't imbalance your pack.


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## pdove (Jan 9, 2012)

I agree with onegreenev.

Sometimes after charge the cells show a higher voltage but as soon as you put a load on them they drop down to 3.38 or so. I don't think you can damage them charging to 3.5 unless you hold a trickle charge on them.

The charger should be set to cut off either when it reaches your set point voltage or if using CC CV mode when the voltage and current limit is reached.

You have to wait at least 24 hours to get a true reading after charging otherwise. The voltage drops for quite a while after the charger cuts off in my experience.


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## Old.DSMer (May 18, 2012)

*Re: Bottom Balance Problem*

Thanks for the input guys - I'm still learning!!!




onegreenev said:


> The cells that are closer to the 3.5 range are most likely your lowest capacity cells as those will usually fill first.


Thats the funny thing...they are not. In fact, some of them tested in the higher range of my pack. So maybe there were some measurement errors with the PL6 due to a bad connection or something?




pdove said:


> Sometimes after charge the cells show a higher voltage but as soon as you put a load on them they drop down to 3.38 or so. I don't think you can damage them charging to 3.5 unless you hold a trickle charge on them.
> 
> The charger should be set to cut off either when it reaches your set point voltage or if using CC CV mode when the voltage and current limit is reached.


My concern was definitely damage due to over-charging. I was not happy to see that 3.634 cell up at 3.8 when charging 

I'm going to reduce my cut-off and re-measure and re-post the chart. I'll also try get the measurements after driving. I'm just using a DVM and measuring each cell...so it takes some time.

Is it normal for a pack to be within 3% after the first charge. And then drift away this far after only a few cycles? Or is this re-adjustment "normal" for bottom balancing? I have not seen anyone post about this much variance. Especially considering they were so similar after the first charge?


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## skooler (Mar 26, 2011)

Did you allow the cells to settle after discharge on the initial balance?

Fwiw. I always go to within 5 thousandths of a volt and have never lost a cell using this method.


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## Old.DSMer (May 18, 2012)

Yep, some settled for almost 3 weeks (takes a while to bottom balance/data log 86 cells!). There were a couple with some self discharge and fell below 2.700. I brought them back up before the first charge. They were not my lowest capacity ones either - I'll be keeping an eye on them though.

My maximum range was 0.060V which equates to 2.2%. So you get the entire pack to within 0.005V? Thats 0.19% ????!!!!

I have no intentions of ever testing my bottom balance in the real world. I have other options in case my charge gets too low. I can live with 2.2%. In fact, I thought that was pretty good over 86 cells . Perhaps not...?


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## onegreenev (May 18, 2012)

Did you do a proper CC/CV charge based on your first cell going to 3.5 or did you just stop the charge? 

Just curious to see the results. Mostly interested in the voltages after a couple minutes of driving. Mine were the same voltage. 

I did my charging based on the assumption that I will cut off at 3.5 volts average of the pack and not when the first cell reached 3.5. So I had some cells that had a voltage at 3.7 volts and one at 3.8 volts when my charger went into the CV stage. At that point no cell increased voltage but filled to the point where the amperage dropped to 2 amps then terminated the charge. At no time did any cell go above 3.8 volts but when I let it sit for 24 hours all my cells were at 3.333 or 3.334 volts. After a quick drive all were exactly the same. Even if I charged then did a quick drive up the street and back. It caused all the cells to drop to an equal state across all cells. 

When I mentioned checking your voltages after you discharge I only mean when you are ready to re-charge due to your set level you set to be your cut off point. Just wanting to see the voltages after you drive for even a minute and then after you drive to the point you must re-charge. Not your lowest voltage but your re-charge voltage that you set.


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## onegreenev (May 18, 2012)

Yes, be careful of those cells that exhibit a self discharge. Took mine 3 weeks to drop below 1 volt yet when charged and then discharged it still holds 107ah. Cell is not bloated and will work but if left any time it will self discharge to nothing. Strange thing that is.


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## IamIan (Mar 29, 2009)

*Re: Bottom Balance Problem*



Old.DSMer said:


> I noticed the resting voltage creeping higher on the last 3 charges. And last night, the charger ran longer than it should have based on my Ah usage. So I stopped the charge and measured some voltages. *Note the high voltage cells were not all my lowest capacity ones.*


I would first keep in mind which ever side you balance on the opposite is likely to be very unbalanced ... bottom balanced = poor top balance ... top balance = poor bottom balance ... etc... so the voltage variation at the top when you bottom balanced the pack is not in itself all that surprising.

- - - - - 

I would second keep in mind the different types of 'balance'... although it might seem like insignificant detail to many.

Terminal voltage correlates to SoC and SoE ... but it is not a 100% perfect Correlation... thus voltage balanced is not 100% identical to SoC or SoE balance... this correlation get better if you give the cells enough time to 'settle' first before taking the voltage readings... and the correlation gets better if all the cells are tested and compared at the same temperatures , etc.

It is never going to be a perfect 100% correlation ... even as good as a 99% correlation still means 1% not correlated ... or ~0.035v out of 3.50v reading... it doesn't really need to be perfectly 100% .. but it is (I think) useful to keep in mind that it isn't.



Old.DSMer said:


> From what I understand, the LiFePO4 cells have nearly 99.9% Ah efficiency. So what would cause this huge variance in just a short period of time?


As for cause this huge variation ... I would first point back to the above distinction between types of balance and the effects of balancing on one side (bottom or top) with what one will see on the opposite side.

- - - - - -

Like any chemical battery the performance changes under different conditions... yes around ~99% is often sited by many as a generalization... and generally the variations are usually ignored ... but it can be useful to keep some of those variation trends in mind.

Bellow are some different example conditions that cause that ~99% to vary... of course not all cells are 100% identical so the exactly numbers should be expected to vary ...I more site the trends of that ~99% variation seen bellow , not so much any specific number.

It varies with the rate Link

The % of stored capacity a given set 1Ah removes varies with temperature of that one cell... Link.

The voltage reading changes as the cell rests ... even with no change in SoC or SoE... Link

The rate of Self Discharge (as tiny as it is) of the cells varies with Temperature ... Link

The resistance of the cell changes with temperature and SoC... Link

The Total Cycle (Charge+Discharge) efficiency changes with the depth ... Ah Link ... similar trend in Wh Link

And of course the exact details of the above also will vary some from cell to cell... but differences from cell to cell can cause balance differences ... although some of those variations are 'balance' correcting ... and actually help the cells stay in balance and or move back into balance.


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## Old.DSMer (May 18, 2012)

onegreenev said:


> Just curious to see the results. Mostly interested in the voltages after a couple minutes of driving. Mine were the same voltage.


Below is my graph after 52km driving - 40% freeway, 30% city. Cells are all hovering near 3.298. Looks pretty level which makes me happy to see (in green).



onegreenev said:


> I did my charging based on the assumption that I will cut off at 3.5 volts average of the pack and not when the first cell reached 3.5. So I had some cells that had a voltage at 3.7 volts and one at 3.8 volts when my charger went into the CV stage. At that point no cell increased voltage but filled to the point where the amperage dropped to 2 amps then terminated the charge. At no time did any cell go above 3.8 volts but when I let it sit for 24 hours all my cells were at 3.333 or 3.334 volts. After a quick drive all were exactly the same. Even if I charged then did a quick drive up the street and back. It caused all the cells to drop to an equal state across all cells.


My first charge was till the first cell hit 3.500. I then adjusted my charger cut-off. Originally, all the cells were near 4.420-3.480 with the highest at 3.500. They were all so close on the initial charge. So you can imagine my surprise when I noticed a cell as high as 3.800 and the charger was still running!!! I have since reduced my cut-off voltage to prevent that cell from going so high. I'll have to wait till the weekend and re-measure after a full charge and after letting them relax overnight.

It sounds like you guys are telling me things look normal, so I'll reduce my cut-off slightly and continue running and monitoring. Thanks for all the feedback!!!


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## Old.DSMer (May 18, 2012)

*Re: Bottom Balance Problem*



IamIan said:


> I would first keep in mind which ever side you balance on the opposite is likely to be very unbalanced ... bottom balanced = poor top balance ... top balance = poor bottom balance ... etc... so the voltage variation at the top when you bottom balanced the pack is not in itself all that surprising.


Yes, I do understand this. I was just surprised by the increasing variance since my initial charge. Plus, my charger was running an exceptionally long time and was not reaching cut-off. The highest cell was above 3.8 and that got me worried.



IamIan said:


> Terminal voltage correlates to SoC and SoE ... but it is not a 100% perfect Correlation... thus voltage balanced is not 100% identical to SoC or SoE balance... this correlation get better if you give the cells enough time to 'settle' first before taking the voltage readings... and the correlation gets better if all the cells are tested and compared at the same temperatures , etc.


I agree - especially for LiFePO4, which is why we count Ah to determine true SoC.




IamIan said:


> And of course the exact details of the above also will vary some from cell to cell... but differences from cell to cell can cause balance differences ... although some of those variations are 'balance' correcting ... and actually help the cells stay in balance and or move back into balance.


Thanks for all the info in the links - I will definitely have to study them in more detail when I have some spare time!


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## kennybobby (Aug 10, 2012)

Is that second graph 'after' driving the fifty km, or after charging following the drive? i would estimate that little drive might have pulled at least 50 A-Hrs out of your pack, and i don't see how there can be any cells over 3.33 following a 30-mile drive unless they were seriously over-charged to begin. 

Also looks like two cells are no good or else weren't charged at the start.


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## onegreenev (May 18, 2012)

I believe the second graph is overlaid on the first one. If you look at the first one you will see those same high cells. So the second is the one that looks pretty much even across the board. Something I'd expect to see.


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## Old.DSMer (May 18, 2012)

onegreenev said:


> I believe the second graph is overlaid on the first one. If you look at the first one you will see those same high cells. So the second is the one that looks pretty much even across the board. Something I'd expect to see.


Correct. Sorry guys, I should have included a legend.

Red: Initial bottom balance voltages
Blue: Voltages right after charging
Green: Voltages after 50 km drive (used about 40 Ah)

I reduced my cut-off from 299 down to 295. After that 50 km drive, I recharged and that highest cell only hit 3.542 instead of 3.8. I probably have a little room to go higher, but I'll leave it as is for now and re-check all voltages on the weekend after charging and letting the cells rest overnight.


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## IamIan (Mar 29, 2009)

*Re: Bottom Balance Problem*



Old.DSMer said:


> I agree - especially for LiFePO4, which is why we count Ah to determine true SoC.


Disclaimer:
For probably more than 99% of the people out there that's perfectly fine... close enough... and the difference I point out bellow will not matter at all to those 99+% of people... which is fine.

- - - - - 

Technically Ah counting is not determining true SoC... there is a correlation between them , but that correlation is not 100% and it changes with different conditions.

SoC is a % of total... that changes with conditions.
Ah is a specific quantified thing and doesn't change.

Temperature is probably the easy one to see ... but it is not the only factor that can effect the correlation between SoC and Ah.

1 Ah charge to a +30 degree Celsius cell showing 20Ah of available discharge capacity is a ~5% change in SoC.

That same 1 Ah charge to the same cell now at -20 degree Celsius might only show around ~12Ah of available discharge capacity and is a ~8% change in SoC.

Of course that's an extreme example +30 to -20 ... and even then it only changes from about ~5% to ~8% change in SoC from the same 1Ah.


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## GizmoEV (Nov 28, 2009)

*Re: Bottom Balance Problem*



IamIan said:


> That same 1 Ah charge to the same cell now at -20 degree Celsius might only show around ~12Ah of available discharge capacity and is a ~8% change in SoC.
> 
> Of course that's an extreme example +30 to -20 ... and even then it only changes from about ~5% to ~8% change in SoC from the same 1Ah.


That is for a given discharge rate. If you lower the rate then the difference is even less between the two temperatures. Of course that lower rate might not allow you to drive your car very fast.


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## GizmoEV (Nov 28, 2009)

When I first installed my TS LiFePO4 pack I used some top balancing BMS boards and noticed that it wasn't always the same cells which started shunting first and also that the time a given cell shunted before the charger cutoff was different. I have since quit using top balancing and it has been 3 years and I haven't seen any drift in the cells. The ones at the top at the end of charge do trade around a bit, however.


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## IamIan (Mar 29, 2009)

*Re: Bottom Balance Problem*



GizmoEV said:


> That is for a given discharge rate. If you lower the rate then the difference is even less between the two temperatures. Of course that lower rate might not allow you to drive your car very fast.


Yup... slower or faster rates of charge or discharge will also make the difference larger or smaller... If in that cold winter you pull power at a higher / faster rate ... more rolling resistance in cold , more wind resistance in cold , using the cabin heater, headlights because it's dark earlier, etc ... than you can change the correlation , and make the difference even larger.

The specific SoC is another one ... even with all other variables the same .. 1Ah into a battery at 10% SoC does not give exactly the same correlation to SoC as it would at 90% SoC (even if the same 1Ah, same rate, same temperature, etc.)

Although , I still don't want to give anyone the wrong idea ... these are pretty small effects ... even before some factors make any small difference even smaller.


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## IamIan (Mar 29, 2009)

GizmoEV said:


> When I first installed my TS LiFePO4 pack I used some top balancing BMS boards and noticed that it wasn't always the same cells which started shunting first and also that the time a given cell shunted before the charger cutoff was different. I have since quit using top balancing and it has been 3 years and I haven't seen any drift in the cells. The ones at the top at the end of charge do trade around a bit, however.


Some of the less than 100% efficiency details of LiFePO4 act as minor / weak self balancing effects , which can counteract some of the other remaining effects that continue to contribute to pulling a pack out of balance.

That 'trading around a bit' you saw before with BMS and then later without one ... that is itself you seeing a small drift of the balance of the cells in the pack ... It's just that the small drifting you saw was not producing the net effect of greater and greater loss of overall pack balance.


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## GizmoEV (Nov 28, 2009)

IamIan said:


> The specific SoC is another one ... even with all other variables the same .. 1Ah into a battery at 10% SoC does not give exactly the same correlation to SoC as it would at 90% SoC (even if the same 1Ah, same rate, same temperature, etc.)


That one I have a hard time believing given the definition of current. SOE effects yes but not SOC.



IamIan said:


> Some of the less than 100% efficiency details of LiFePO4 act as minor / weak self balancing effects , which can counteract some of the other remaining effects that continue to contribute to pulling a pack out of balance.
> 
> That 'trading around a bit' you saw before with BMS and then later without one ... that is itself you seeing a small drift of the balance of the cells in the pack ... It's just that the small drifting you saw was not producing the net effect of greater and greater loss of overall pack balance.


I'm not sure it was a difference in SOC as much as voltage under a given set of conditions. Each time a charge was done each battery had aged a bit, was at slightly different temperature than the previous time and the thermal energy was distributed differently than the previous time, and a host of other factors. All of these have an impact on the effective internal resistance of the cell and different parts within the cell so they don't charge identically each time. The same number of electron/ion pairs move through the circuit/between plates but they aren't likely to go in the exact same places or in the same order each time. It is merely terminal voltage that is "trading around" not that the relative SOC is different each time. That is a problem with top balancing since it is typically (always?) done while charging where as bottom balancing is (should be) done with no current flow with extended periods of letting the cell rest between "adjustments".


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## IamIan (Mar 29, 2009)

GizmoEV said:


> IamIan said:
> 
> 
> > The specific SoC is another one ... even with all other variables the same .. 1Ah into a battery at 10% SoC does not give exactly the same correlation to SoC as it would at 90% SoC (even if the same 1Ah, same rate, same temperature, etc.)
> ...


I don't see how the definition of current has such an effect ??
Please explain ??

I've measured changes in the Ah cycle efficiency at different SoC levels in a batch of A123 LiFePO4 cells... there was variation but the trend / change in Ah cycle efficiency at different SoC levels seemed to be fairly clear trend ... given what I've seen thus far , I'm over ~90% certain of this behavior ... even if I will be the first to agree it is a small effect.

Link



GizmoEV said:


> I'm not sure it was a difference in SOC as much as voltage under a given set of conditions.


Fair enough.

But .. given that resistance goes down as SoC goes up ... and the resistance goes down as the temperature goes up ... with all the cells in series seeing the same amps of current flow ... and you seeing a voltage rise of specific cells changing in order of which cell happens first ... that still seems to be very likely to be an indication of relative cell drift.

The only exception that comes to mind ... would be if the order of cell changes you saw always were toward cells that were on the outside of the pack (which might have been a bit colder).



GizmoEV said:


> so they don't charge identically each time.


I'm confused ??

If they don't charge identically... That in itself (is very likely) = drift.

You can't have them charge differently (even slightly differently) and still have them charge the same ... the two are mutually exclusive.

It seems unlikely to me differences in charging would 100% exactly match 100% every time the relative differences in real time cell capacity ... in order to have the same 1Ah for example give the __% to SoC of every cell (even those with different real time Ah capacity).


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## sabahtom (Mar 1, 2011)

*Use of RC charger to mid-balance*

I understand that the goal of balancing is to have them all respond to charge/discharge at the same rate.

I've got tw decent Rc chargers. I set them at 4A max current, they only do 5A, and I run them with a desktop fan so they go fine for hours on end.

I've spent some hours watching how they react to the cells, and they seem to be consistent at keeping the charge rate below the 4A and below 3.6v until the cell stabilises, which I understand to mean the V changes at a rate the charger thinks is safe, or stable...something like that. After that, it goes to CV and CC mode.

So I've got 97 100AH cells. My target V is 3.32 to 3.55. Arbitrary, but I don't want to spend the rest of my life doing this and that's close to where they're at.

My question: is it reasonable to call the cells sort-of balanced if they are all charging at CC and CV, and within 3.32 to 3.55V? I'm looking for 80% max charge and 30% max discharge so I think with this range I'd be safe. The reason is that they now all respond to charge by rising at about 10mv per mAH. 
Otherwise I'll just have to discharge them all to 2.8v, somewhat time-consuming.


----------



## onegreenev (May 18, 2012)

*Re: Use of RC charger to mid-balance*



sabahtom said:


> I understand that the goal of balancing is to have them all respond to charge/discharge at the same rate.
> 
> I've got tw decent Rc chargers. I set them at 4A max current, they only do 5A, and I run them with a desktop fan so they go fine for hours on end.
> 
> ...



Take them all down to 2.6 volts. They will bounce up a bit higher. That is well into the cliff of the discharge and never let them go into that range (static) afterword. Yes, its time consuming but a required event if you bottom balance. I twice did a mediocre balance job and lost cells. It must be done right or don't bother and expect to loose cells. 

Or if its too inconvenient for you to do the bottom balance then just go buy a BMS and let it do the work and just keep them top balanced. 

You can't do a MID balance. Ranging from 3.32 to 3.55 can be a very wide gap. Don't DO MID balancing.


----------



## sabahtom (Mar 1, 2011)

*Re: Use of RC charger to mid-balance*

Hi Pete

Sorry that was a late-night post. The range I'm aiming at is 3.23 to 3.255, but I'll take your note and push them down to 2.6. 

I've got a BMS from hipower - a MONITORING system only, no shunting. 

But according to their engineer it has no internal fusing, which is a question I didn't know to ask when I bought it. I might see if I can get to know the weak cells and put them all in a corner, so I can keep the wires tidy and just monitor the weak ones with a single module. Someone said that adding a fuse on the cell terminal may throw off the sensing from the BMS though.

The Hipower manual says to charge them to full before first use, but bottom balancing wouldn't be feasible then so I guess nobody does it that way?



onegreenev said:


> Take them all down to 2.6 volts. They will bounce up a bit higher. That is well into the cliff of the discharge and never let them go into that range (static) afterword. Yes, its time consuming but a required event if you bottom balance. I twice did a mediocre balance job and lost cells. It must be done right or don't bother and expect to loose cells.
> 
> Or if its too inconvenient for you to do the bottom balance then just go buy a BMS and let it do the work and just keep them top balanced.
> 
> You can't do a MID balance. Ranging from 3.32 to 3.55 can be a very wide gap. Don't DO MID balancing.


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## psron (Jun 19, 2012)

Sorry for butting-in... and for not scouring the whole topic, BUT... ;-)

Has there been any discussions here about Full-time Active Charge Shuttling Cell Balancing? (whew!) (and yes, that's "FACSCB")

I've seen a design that has the ability to "shuttle" at around 10 Amps at relatively low cost... and scaled-up could do 50 Amps or more... _and_ with extremely high efficiency. (based on high freq. flyback switching power supply technology, utilizing a single common large-ish inductor)

No matter the pack voltage, it constantly monitors all cells, and determines "rich" and "poor" cells - and "takes from the rich and gives to the poor"... sts.

Topic here on DIYEC...

Since it takes some electronics "per cell" (or per parallel cell-group), it can get cumbersome... but relative to the whole project - not terribly expensive. (but yes, more than shunt balancing)

IMHO... if I build a car for someone (even if it's for my own family members), I want it to be a "done deal" when I hand over the keys. I don't want to ever "have" to work on it again... where bottom balancing sounds like visiting the doctor every 6 months "just because".

Electric cars should require LESS maintenance, not more.

Again... that's "IMHO"... and though everyone has opinions, I like mine most ;-)

(OK, well I've got 30+ years in electronics engineering design work to justify my opinions... lol)


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## dcb (Dec 5, 2009)

Probably overcomplicated, and possibly less efficient than a passive approach.
http://liionbms.com/php/wp_passive_active_balancing.php

Why you think passive bottom balancing can't be automated (requires a trip to the doctor)? 

If it is only needed every 6 months (probably a lot longer), how do you justify the additional expense/complexity of active (when active may still be less efficient)?


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## skooler (Mar 26, 2011)

@psron Suggest reading the thread fully. There is no requirement to revisit cells. 

We have dozens of packs using this method with many thousands of miles, and have discovered ine key fact with Prismatic LiFePO4- there is no such thing as cell drift with prismatic LiFePO4.

I recently took down a 74cell pack which was bottom balanced to 2.700 vpc two years ago and now has almost 20,000 miles on it.

Guess what, when I hit 199.8v ALL cells wer withing a few thousandths of a volt.


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## psron (Jun 19, 2012)

By the terminology at the URL you listed, the proper term for what I was talking about is "Redistribution"... except the method I pointed to uses "cell to cell", with any cell the source, and any cell the target.

Full-time redistribution protects every cell, all the time. One of the biggest benefits is when cells near EOL... which happens to every single cell ever made. 

Aging (LONG term, not just a couple years!!!) reveals differences in cells, and if you only bottom-or-top balance part-time... you will never get the benefits of "real-time" redistribution. 

Stop and think about this for a minute...

Our hypothetical "what if" case is an EV 6-8 years down the road. We have a pack size of 100s, 320V, 100Ah cells, 33kWh. (serious install)

Let's consider worst-case scenario... we have a very weak 100Ah cell (maybe a mfg defect) that is now down to 30Ah capacity. Most of the other cells are at averaging about 70Ah capacity. 

We need to "make up" about 40Ah capacity in that one weak cell. Since we have 100 cells, each "donor" only has to donate 0.404Ah in order to keep the "bad cell" up to par with all the others... insignificant.

So, instead of only getting 30% of your original possible range due to the one bad cell, you are now able to get 99.4% of the range that all the other cells have to offer... you get the summed effect of a pack with every cell at 69.56Ah capacity.

You will never get cell reversal, because the system will pump as much capacity into it as needed to average out the bad cell. (unless that cell opens up)

For DIYers... sure... we can monitor the system, replace or removed bad cells as needed... and it's cheaper... but for a 100% "hand-off" project, where the customer is willing to pay a bit more for the ultimate reliability, this _*seems* _to be _*the*_ answer.

MOSFETs in this range are cheap... we're only dealing with switching a single cell voltage into an inductor... maybe 20-50 Amp peaks, but only at 4V maximum. Same goes for the Schottky diodes... very low voltage requirements. 
In production quantities (1k/part price breaks):
NTMFS4937NT1G MOSFET, N CHANNEL, 30V, 0.0032OHM, 70A, SOIC-8 $0.20 or less (need 2/cell)
Schottky diodes: about $0.20 to $0.35 each, need 2/cell

So the overhead for semiconductor components for each cell are maybe as high as $1.00/cell. Add another $1 for related components (MOSFET drivers, passive, etc) and we're at $2/cell, $200 for a 100-cell pack. 

Microcontrollers and associated parts to manage/measure groups of maybe 16 cells: $3-4/group, about $28 for 7 groups, now the total is $228. 

Main microcontroller as "overseer": about $10 (1 needed). total now $238. 

Mechanical housings, connectors, cables, etc.: $25; for a total parts of $263. Overhead and profit margin for DIY market: 100% ???

Cost to user $526 for a complete 100-cell system.

Original cost of 100 cells of CALB 100Ah LiFePO4 - $12,500
Percent of battery cost: 4.2% 

Got a 50-cell car? $326 cost to user; or about 5.2% of battery cost.

Even if it's twice of these costs, it's a bargain considering the benefit.

Again... *IMHO*.

BTW... this same technology is being considered for next-gen PV Solar panels, to eliminate the massive power losses (and HOT SPOT cells) during partial shading. 

By redistributing power from energized cells to the shaded cells (each PV cell still has significant capacitance, which can store energy temporarily), you can (again) "average out" those shaded cells, and still get significant power from the array/panel... essentially equal to the amount of power generated by the illuminated cells. 

As it is now, when shading occurs, the entire panel is effectively only putting out as much power as the LEAST illuminated cell in the series-string can output... which causes over-heating. (the old "only as good as the weakest link" syndrome)


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## PStechPaul (May 1, 2012)

I think this does have some merit, especially if implemented on a large enough scale to bring the costs down. The DIY market may be too small, though, and might not grow very much (if at all) as more and better EVs and batteries are manufactured, so this might only find practical application in commercial EVs. But it would be interesting to design and build such a system.


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## dcb (Dec 5, 2009)

my guess is cell to cell is like the bubblesort of active balancers (not good). And this is a bottom balance thread... 

there are EVs with over 100k miles on them, and the pack has dropped ~ %20, you would need to post-mortem one of them to see how far their individual cell capacities have spread to see if this has any hope of ever being useful. Battery manufacturing is constantly improving and liquid cooling are factors in keeping the pack consistent (and probably the degree of paralleling).

If this is for your own car, you could always add it later. Though I think you are being optimistic about the price and value proposal. The data so far indicates it doesnt add any value, what are you speculating?

It is like buying something up front, that your car *might* find useful 10 years from now, when the pack is down to %40 of its original capacity, but new replacement batteries are twice the capacity and half the cost. I'd need GOOD data (including bms catastrophic failure rate and emi effects) before subjecting friends and family to that experiment.

If one cell capacity is far below all the others, it is time to go to the doctor.


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## Hollie Maea (Dec 9, 2009)

I've used active cell balancing. It's great for systems where the end user needs to be completely shielded from ever thinking about the batteries. In a couple of years, I'm guessing all of the OEMs will use it.


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## dcb (Dec 5, 2009)

Sorry I keep hounding on this, any system that can balance the pack (or just monitor it, i.e. idiot light) will do for worry free operation. The biggest concerns are oversizing the pack enough so that it doesn't routinely overcharge or overdischarge for the expected range.

Limiting to %86 charge can double the life (quite a bargain!), limiting to %72 can quaduple it. That seems to be where the real payback is. Use 80ah instead of 60ah, limit charge to %76, and your pack goes 4x as many miles over its lifespan. 

Most "active balance" advocates tend to think in terms of wringing out the most they can, 0-100% charge of each and every cell. That is absolutely the wrong way to think about it if you are trying to extend the pack life for worry free miles.

the balance current needed is on the order of milliamps, an i/o pin can probably handle it after initial balance.

http://www.cse.anl.gov/us-china-wor...rmity of Lithium-ion battery_lu Langguang.pdf

Not saying manufactureres won't adopt active balancing (marketing for uneducated masses), but they are better served with cell matching and paralleling (like tesla, or even chevy/nissan). There are numerous technical hurdles to even get to that point, and it will change as batteries change, and in the mean time a lot of cells are going to be charged unnecessarily and simply waste energy, as errors in SOC and CE and SD and etc compound.

I'd like to know what engineering data they made their decision on if they do go with it.

Either way, I'm sure the shuttle charge approach is an efficiency disaster.


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## dougingraham (Jul 26, 2011)

Hollie Maea said:


> I've used active cell balancing. It's great for systems where the end user needs to be completely shielded from ever thinking about the batteries. In a couple of years, I'm guessing all of the OEMs will use it.


I expect that when the OEM's take a close look at the data they have collected will go without anything more than a cell voltage monitor so they can see when something bad is going to happen. On the front end they will come up with a way to weed out the bad cells and maybe even match for capacity. With enough volume this becomes practical. All the end user ever needs to see is a little red light that says

Service
battery
now!


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## wb9k (Apr 9, 2015)

I confess I haven't read this entire thread, but I wanted to put a few things out there that don't seem to be clear to most here based on the parts of the thread I have read.

*The consequences of overdischarge: *Letting an LFP cell discharge to a level so that it comes to rest below 0.3 Volts (call it 0.5 to give yourself some safety margin) causes permanent damage to the cell. The copper anode material begins to dissolve and go into solution in the electrolyte. This becomes dangerous when you try to recharge the cell. The copper grows dendrites that puncture the separator layer, shorting the cell internally. This is why cells exposed to this condition get warm and don't hold a charge well. Keep charging the cell anyway, and you are inviting a catastrophic event. Apart from gross overcharge, this is the most dangerous thing you can do with a LiFePO cell. Driving the cell negative will destroy the cell very quickly. If it just gets very low without going negative, it takes a little longer--you may be able to recover the cells if you charge them before they have had a few hours to linger there. 

*The consequences of overcharge: *Charging LFP to a point that it comes to rest above 3.60 Volts will cause Li to plate permanently onto the cathodes, making that Li unavailable for future energy transfer. IOW, you get capacity loss and some elevation of cell impedance (because the plated areas of the cathodes are also no longer usable). The higher the overcharge and longer the time spent there, the worse the problem is. High SOC is somewhat stressful for the cell compared to lower SOC, but using the bottom 20% of the capacity for driving is even more stressful than charging to 100%. Oversize the pack to the fullest extent possible and avoid both. Not charging to 100% is fine (good, in fact), but you cannot top balance below 3.45, maybe 3.4 Volts. You're not far enough into the "upper tail" of the charge curve to accurately balance below that. Gross overcharge, of course, can lead to cell venting and possibly fire. 

*Cell "drift" is inevitable: *Several here have reported months, or even a year or so of trouble-free operation using bottom balancing and no BMS. With well-matched brand new cells, this is not hard to believe. If you can go years like this though, you are very lucky indeed. No two cells are exactly alike, and the older they get, the less similar they become. Temperature gradients in the pack cause cells to age at different rates and in different ways. Moisture ingress, corrosion, loose or dirty connections will all compound this phenomenon. As cell impedance begins to diverge (and it absolutely will over time) the losses across those cells (during both charge and discharge) also diverge, and the result is uneven apparent rates of charge/discharge. Investigate *Peukert loss* if this is a foreign idea to you. The lack of "parasitic loads" or fact that "all cells are in series" will NOT protect you from this phenomenon. By the time the pack is a few years old, there's a good chance you'll have a pack that requires so much manual intervention to keep in line that nobody would want to deal with it. I suspect many of these experiments end this way...the car gets driven for maybe a few years until keeping the pack in line just becomes unmanageable or there is a cell failure. Then the car gets sold and/or parted out. In the case of cell failure, you can of course replace the cell, but the new one will be even further out of whack from the rest of the pack than the old cell likely was. Your troubles will not stop there, they will continue to compound for reasons that should be becoming clear by now. All of this (and safety) are the reasons that no OEM uses this approach. They're not ignoring the cost savings possible, they simply cannot take advantage of them because this approach is unreliable and potentially unsafe. 

IMO, nobody should be selling cars set up like this to anybody but other hobbyists who know the risks well and are willing to deal with them. Telling "lay" buyers this will work fine forever is just wrong, because that is highly unlikely. The ONLY proper way to protect from overdischarge is to have cell level monitoring that will warn the driver when a cell gets low and shut the car off altogether when it gets dangerously low. Without this, you are running on blind faith and will always have the risk of cell damage and even fire upon recharge. Putting cars like this on the open market is, to me, totally unacceptable.


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## EVfun (Mar 14, 2010)

"Peukert loss" has nothing to do with Lithium batteries. It is a somewhat crude mathematical description of the change in available capacity of lead acid batteries at different discharge rates. It is not even a loss in capacity with lead acid batteries, it just represents that part of the capacity that isn't available at faster discharge rates without the cell voltage dropping below 1.75 volts.


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## wb9k (Apr 9, 2015)

Um...actually it applies to all types of batteries. Li batteries are less affected than most, and lead is the worst, but impedance variations across cells cause there to be different amounts of loss across the cell during energy transfer. This is one of the main things that balancers are intended to cope with. Actual capacity does not change, but usable capacity does. And it's not the same amount in every cell. This means some cells are wasting more energy as heat than others--in both directions. This stuff adds up over time. 

Let's use a really crude example to make this simple to understand. You've got 4 1 Volt cells in series, each with exactly 1 Ah of capacity. Three of them have an IR of 1 Ohms, the fourth has an IR of 2 Ohms. When you discharge this pack, the high IR cell will dissipate twice as much power as heat than the other threee cells. All cells delivered the same amount of power to the load, but they did not discharge at the same rate because their internal losses are not equal. The same thing happens on the charge cycle. Even if you start off perfectly balanced, it should be easy to see that after just one cycle you are no longer in balance. If you don't reset balance every charge cycle, this will get worse and worse until you are eventually in a scenario where it will take hours to get the pack back in line manually--assuming you catch it before a cell is destroyed.

When things are shiny and new, this isn't likely to be a big issue. But as time goes on, it will become more and more of a problem. And if you replace a failed cell with a new one, the difference in cell impedance is going to be amplified even further. This can get real hairy real fast by the time it comes to that.

(Edited a couple times to correct some bad numbers....coffee still taking effect here...


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## psron (Jun 19, 2012)

wb9k said:


> Um...actually it applies to all types of batteries. Li batteries are less affected than most, and lead is the worst, but impedance variations across cells cause there to be different amounts of loss across the cell during energy transfer. This is one of the main things that balancers are intended to cope with. Actual capacity does not change, but usable capacity does. And it's not the same amount in every cell. This means some cells are wasting more energy as heat than others--in both directions. This stuff adds up over time.


I agree with your statements.

...but good luck trying to get non-technical people to understand this. I worked with the engineering aspect of secondary cells for years... dealing directly with manufacturers, so I know what I've seen personally, read in their lab tests, and discussed with manufacture engineers. 

Rechargeable cells are just far more complex than a cursory discussion can do justice... it involves many non-linear, non-predictable chemical and environmental interactions. If the all of the materials used were 100% pure... and the operating environment were far more controlled, then it becomes far more predictable... but they are FAR from it... these are consumer products we're discussing, not aerospace spec.

BTW... in the full-context reading you would see that nobody is condoning either over-discharge, over-charge, or even 100% SOC or DOD. My entire premise was that with proper cell management and charge redistribution, a "battery" can be expected to deliver far more of its rated capacity, without harm. Leaving 40% or more capacity unused is a waste of energy, space and money. 

There's no arguing that the charge redistribution system that I described will cost far more than manual bottom-balancing, but the idea is increased battery life, more useable driving range from a given pack capacity, and minimizing required maintenance.


Don't forge to give your wives a hug and "thanks" today for being a great Mother to your kids!

(...and your own Mother, if she's still with us!)


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## EVfun (Mar 14, 2010)

wb9k said:


> Let's use a really crude example to make this simple to understand. You've got 4 1 Volt cells in series, each with exactly 1 Ah of capacity. Three of them have an IR of 1 Ohms, the fourth has an IR of 2 Ohms. When you discharge this pack, the high IR cell will dissipate twice as much power as heat than the other threee cells. All cells delivered the same amount of power to the load, but they did not discharge at the same rate because their internal losses are not equal. The same thing happens on the charge cycle. Even if you start off perfectly balanced, it should be easy to see that after just one cycle you are no longer in balance. If you don't reset balance every charge cycle, this will get worse and worse until you are eventually in a scenario where it will take hours to get the pack back in line manually--assuming you catch it before a cell is destroyed.


Uh... no.

With the case you have outlined above they 4 cells have NOT delivered the same amount of power to the load, they have delivered the same number of amp hours to the load, but at different voltages. The cells have discharged the same amount, but one is hotter. 

Cells store amp hours. By that I mean they do something (different things for different chemistries) that store potential electrons over on the negative active material. In the case of LiFePO4 cells they store Lithium atoms inside graphite when charged, but would be at a lower potential if the Lithium was stored in the Iron phosphate on the positive plates.

Peukert's losses are not even losses. I've done the experiment with Optima batteries. You can discharge a fully charged yellow top at 100 amps for 20 minutes until it drops to 10.5 volts (33 amp hours.) You can immediately disconnect that load and connect a 10 amp load and the battery voltage will spring back up, allowing you to safety extract more amp hours before the battery gets back down to 10.5 volts. Furthermore, when charging the Optima you put in about 5% more amp hours than where removed (Lead acid is less efficient at charging compared to Lithium.) This was true if I stopped after the 20 minutes at 100 amps, or after further discharging the battery at 10 amps.


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## IamIan (Mar 29, 2009)

EVfun said:


> Peukert's losses are not even losses. I've done the experiment with Optima batteries. You can discharge a fully charged yellow top at 100 amps for 20 minutes until it drops to 10.5 volts (33 amp hours.) You can immediately disconnect that load and connect a 10 amp load and the battery voltage will spring back up, allowing you to safety extract more amp hours before the battery gets back down to 10.5 volts.


The same Peukert effect happens with LiFePO4 cells as well .. they will hit 2.80 v per cell faster (less Ah or Wh discharged) under a 100+ Amp constant discharge load than they will under a 10 amp constant discharge load.

I'll agree they aren't really directly "losses" ... but the Peukert effect is still there .. and as with any real world battery the Peukert k value will change and the real world values fit to what the equation gives will have variation... but that's also true for any real world battery.


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## EVfun (Mar 14, 2010)

Think about what you just posted Ian. Amp hour are stored, electrons are not stored. The electrons go from the negative terminal, through the load, to the positive terminal while the Lithium ion goes from the negative plate to the positive plate.

As far as Peukert's losses, it is a formula written to approximate the net effect of different discharge rates on AVAILABLE capacity Lead acid batteries. It is not an actual loss in the sense of effecting future charges. A new and quite likely different formula should be devised for Li (and it still isn't an actual loss in the sense of effecting the next charge.)


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## samwichse (Jan 28, 2012)

EVfun said:


> Think about what you just posted Ian. Amp hour are stored, electrons are not stored, they go from the negative terminal to the positive terminal while the Lithium ion goes from the negative plate to the positive plate.
> 
> As far as Peukert's losses, it is a formula written to approximate the net effect of different discharge rates on AVAILABLE capacity Lead acid batteries. It is not an actual loss in the sense of effecting future charges. A new and quite likely different formula should be devised for Li (and it still isn't an actual loss in the sense of effecting the next charge.)


Meh. Same formula with different constants.

It's still Peukert's law, I don't know where you're getting all this lead-acid-only idea.



> Peukert's law, presented by the German scientist*Wilhelm Peukert*in 1897, expresses the*capacity*of a battery in terms of the rate at which it is discharged.


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## EVfun (Mar 14, 2010)

samwichse said:


> Meh. Same formula with different constants.
> 
> It's still Peukert's law, I don't know where you're getting all this lead-acid-only idea.


Wilhelm Peukert had no access to secondary Lithium ion cells. Transferring Lead acid knowledge over to Li chemistries has led to plenty of battery damage. It should not be done without new study and experiments. I cannot rely on his study of Lead Acid batteries to provide a formula I would use on LiFePO4 cells (or LiFeYPO4 cells like I have been running for 5 years.) I cannot suggest others do either, since the apparent Peukert's effect is so slight. For most EV suitable Lithium cells "k" seems to be below that of even good AGM Lead acid batteries. 

Anyway, my problem lies with this (posted by *wb9k*):


> Cell "drift" is inevitable: Several here have reported months, or even a year or so of trouble-free operation using bottom balancing and no BMS. With well-matched brand new cells, this is not hard to believe. If you can go years like this though, you are very lucky indeed. No two cells are exactly alike, and the older they get, the less similar they become. Temperature gradients in the pack cause cells to age at different rates and in different ways. Moisture ingress, corrosion, loose or dirty connections will all compound this phenomenon. As cell impedance begins to diverge (and it absolutely will over time) the losses across those cells (during both charge and discharge) also diverge, and the result is uneven apparent rates of charge/discharge. Investigate Peukert loss if this is a foreign idea to you. The lack of "parasitic loads" or fact that "all cells are in series" will NOT protect you from this phenomenon.


I can agree, to a point, with the first part. I've had trouble free years (my cells are now 5 years old) without using a BMS (I have one in my garage, even used it for a little bit.) I do wonder how running naked will work out as the cells get into their golden years, but I can check every charge if I want (I run top balanced.) However, the suggestion that Peukert has ANYTHING to do with developing cell imbalance is simply a misuse of what he was measuring. There is no actual capacity loss measured by it, only what capacity is not available _unless you discharge more slowly_.


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## IamIan (Mar 29, 2009)

EVfun said:


> Think about what you just posted Ian. Amp hour are stored, electrons are not stored. The electrons go from the negative terminal, through the load, to the positive terminal while the Lithium ion goes from the negative plate to the positive plate.


Fine .. well go down that rabbit hole.


A battery does not store Ah = correct.

The battery + terminal and - terminal are in series .. the same Ah go in to the + terminal as go out the - terminal .. no net change in Ah = no stored Ah .. That is how a series electrical circuit work.

It isn't just about electrons.
+1 Li-Ion -1 Li-Ion = +1-1 = 0 Ah change



EVfun said:


> As far as Peukert's losses, it is a formula written to approximate the net effect of different discharge rates on AVAILABLE capacity Lead acid batteries.


Perukert k value changes for different values.

The Peukert effect is not only for PbA batteries... As I wrote before .. the same Peukert effect you described for PbA also happens (with a different k value) for LiFePO4.



EVfun said:


> It is not an actual loss in the sense of effecting future charges.


Already agreed with you on this point.



EVfun said:


> A new and quite likely different formula should be devised for Li


Only have to change the k value of the equation.

Just like I don't need a new F=MA formula just because you changed the M ... I just change the value of M and it still works ... change the 'k' value to match the battery and the Peukert formula still works.

Example of Peukert effect on LiFePO4 shown in attached graph.



EVfun said:


> However, the suggestion that Peukert has ANYTHING to do with developing cell imbalance is simply a misuse of what he was measuring. There is no actual capacity loss measured by it, only what capacity is not available _unless you discharge more slowly_.


Cells are in series .. you can't discharge 1 or 2 out of the whole pack more slowly (less amps) than any other... thus the relevance of Peukert.


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## EVfun (Mar 14, 2010)

IamIan said:


> Cells are in series .. you can't discharge 1 or 2 out of the whole pack more slowly (less amps) than any other... thus the relevance of Peukert.


I would say "thus the irrelevance of Peukert," but whatever. Peukert's equation is NOT about any change in capacity. It is only about the need to discharge more slowly if you want to use more of the capacity. Since it is NOT about any change in capacity it does not effect the needed amp hours to recharge any of the cells in series. It speaks in no way about cell aging either.


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## wb9k (Apr 9, 2015)

Just to clarify my own positions.....

I never thought anyone here was "advocating" overcharging or overdischarging. I do think the calendar life perils of 100% SOC are grossly overstated by some, and the dangers of overdischarge are perhaps a bit understated by some as well. My post spelling out the dominant trends associated with these types of abuse was simply an attempt to bring that aspect of the discussion into clearer focus. 

The Peukert effect, to me, is nothing more than an attempt to explain electrochemical losses in a cell--any type of cell. That Peukert was working with lead is, in the end, nothing more than happenstance. The chemistry makes it very easy to see, granted, but every type of cell exhibits electrochemical losses (temporary loss, not permanent loss, but loss nonetheless) which tend to increase with current, all else being equal. Because Peukert did nothing more than extrapolate data found empirically into a crude formula that worked only with the battery he had on hand at the time (rather than describe a specific electrochemical mechanism responsible for the symptom), it seems perfectly appropriate to me--and plenty of other people before me--to describe electrochemical losses generically as "Peukert loss". If some guys want to call it something else, fine, but this isn't really a key point in the present discussion. 

The real disconnect comes when we start to consider how these losses play out in a pack situation. In a Li system, these are far more important than in other chemistry types, so this isn't "using lead thought for Li", but rather a sharpening of our awareness that is necessitated by the appearance of new behavioral issues that are only germane to Li secondary cells. Few people really understand that Peukert losses are not a pack level phenomenon, but a cell level phenomenon. That is, as I have already explained in a gross example, series cell groups do not experience these losses equally. In most battery types this doesn't really matter because they are regularly charged to over 100% SOC, allowing the cells in those packs balance themselves via safe electrochemical shunting mechanisms that kick in at that charge level. This does not happen in Li chemistries, which is why we need balancers to perform that function. So, what I'm describing exists in all types of cells, but the problem corrects itself with other chemistries so we don't notice or care it's happening. With Li, we can no longer afford to ignore what is really going on here. 

In addition to electrochemical losses, we also need to consider the impedance of connections between cells. That is, if a series connection between cells has greater impedance than the other connections in the pack, the cells adjacent to that connection alone will suffer elevated losses across that connection--THE PACK DOES NOT SUPPORT THESE LOSSES EQUALLY ACROSS ALL CELLS. One cell will be affected during charge, the other adjacent cell will be affected during discharge. 

Now, I can appreciate that this is a difficult thing to wrap your brain around. I have an advantage unique to most guys here in that I've stared at A LOT of data from packs at all stages of life and conditions. These anomalous behaviors that arise again and again and again have a way of getting your attention over time. There is loads of data at A123 from both used and brand new batteries to support all of what I'm saying here. I have a "gross demonstration" experiment planned in my head that should make this undeniably obvious to anyone, but finding the time to run it...probably going to be a while. If anyone with a reasonably accurate battery cycler wants to take a crack at running it, I'd be happy to draw it up for them.


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## PStechPaul (May 1, 2012)

I think I tried to explain the difference between the internal impedance effect and the Peukert effect, and the fact that Peukert's formula does not match the result of resistive losses alone. Here is how internal resistance affects the current and energy that can be stored into and retrieved from a lithium cell. 

Assume a 100 A-h cell with a nominal voltage of 3.3V and 1 mOhms internal resistance. A 100 ampere load has a resistance of 32 mOhms plus the resistance of the cell to obtain 100 amps. It should provide this current for a full hour during which time there will be 10 watts of losses and 320 watts to the load and an efficiency of about 97%. Although the cell could provide 330 watt-hours into a higher resistance load, it can provide only 97% at 1C.

Now, for 10C, or 1000 amps, the total load is 3.3 mOhms, or an external load of 2.3 mOhms. Now the losses are 1000 watts and the load gets 2300 watts, for an efficiency of 70%. It should be able to do this for 1/10 of an hour or 6 minutes. The same A-h out as was put in, but a lot less energy delivered to the load.

The same calculations do not work for a lead-acid battery, which is the reason for the more complex exponential formula and not something simpler as may define a lithium cell. I think the actual SOC as defined by A-h actually drops in a lead cell, so instead of the electrons always flowing out of the battery into the load, some of them recombine within the cell, and capacity is actually lost because of the high current. There is some residual charge that can be extracted into a lower current load, but the total A-h out will not equal the A-h in.


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## EVfun (Mar 14, 2010)

Please don't think my comments are intended as any type of personal attack against you. I think we agree that striving for 100% SOC or visiting 0% SOC are bad ideas. This is one of the biggest differences between Lead and Li ion. Lead acid batteries need to visit 100% SOC regularly to avoid capacity loss from a build-up of crystalized Lead sulphate (and Lead acid is fairly tolerant of overcharge provided it is less than 10% per charge.) There is no compelling reason to visit 100% SOC with Li ion (except as done during forming by the manufacturer to build an interface layer.) You don't want to leave a lot on the table with any battery chemistry, but into the upper 90's will do for Lithium chemistries.

You covered "*The consequences of overdischarge*" and "*The consequences of overcharge*" very well. I just happen to disagree with the section "*Cell "drift" is inevitable*." I think we need more structured tests and certainly more real world tests before we know exactly what to expect. I do believe that some form of cell drift in old age will surface, but we can't test that by abusing some test cells (I can show you self discharge that way.) I see no reason to believe Peukert's losses have anything to do with it, as he was not measuring an actual capacity loss. I am wondering if old age will cause capacity loss off the bottom, off the top (this is more about where it is found than the cause), or a reduction is charge efficiency (which is very close to 100% for LiFePO4 cells in good shape and not being overcharged.)


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## wb9k (Apr 9, 2015)

PStechPaul said:


> I think I tried to explain the difference between the internal impedance effect and the Peukert effect, and the fact that Peukert's formula does not match the result of resistive losses alone. Here is how internal resistance affects the current and energy that can be stored into and retrieved from a lithium cell.
> 
> Assume a 100 A-h cell with a nominal voltage of 3.3V and 1 mOhms internal resistance. A 100 ampere load has a resistance of 32 mOhms plus the resistance of the cell to obtain 100 amps. It should provide this current for a full hour during which time there will be 10 watts of losses and 320 watts to the load and an efficiency of about 97%. Although the cell could provide 330 watt-hours into a higher resistance load, it can provide only 97% at 1C.
> 
> ...


Thanks for putting some real numbers in place where I didn't do such a hot job. I agree with everything you've said that I actually understand . I apparently haven't found your comments yet if what you're referring to is in this thread--what page are they on? I'm curious to understand some of the details you're talking about here. You would be the first person I've seen put a different name on internal losses in Li vs. lead or anything else. I'm sure you know that cell impedance is actually dynamic, changing not only from cell to cell, but within single cells based on SOC, instantaneous C-rate, temperature, and perhaps other inputs. 

I would go further and state that the loss across cells in series cannot be perfectly equal except in circumstances so fortuitous as to be virtually impossible. Even the best cells are not perfectly equal in their internal losses. This means drift in balance is a given. I don't think you're disagreeing with me on that point....are you?


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## dcb (Dec 5, 2009)

http://batteryuniversity.com/learn/article/calculating_the_battery_runtime










for a given battery, very little change in ah delivered for different discharge rates (within reason) though delivered at increasingly reduced voltage:
http://na.industrial.panasonic.com/sites/default/pidsa/files/ur18650f.pdf

You really need to know cell temperature in any tests to make any sense out of them really, huge factor. If your tests are warming the cells/etc.


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## wb9k (Apr 9, 2015)

EVfun said:


> Please don't think my comments are intended as any type of personal attack against you. I think we agree that striving for 100% SOC or visiting 0% SOC are bad ideas. This is one of the biggest differences between Lead and Li ion. Lead acid batteries need to visit 100% SOC regularly to avoid capacity loss from a build-up of crystalized Lead sulphate (and Lead acid is fairly tolerant of overcharge provided it is less than 10% per charge.) There is no compelling reason to visit 100% SOC with Li ion (except as done during forming by the manufacturer to build an interface layer.) You don't want to leave a lot on the table with any battery chemistry, but into the upper 90's will do for Lithium chemistries.
> 
> You covered "*The consequences of overdischarge*" and "*The consequences of overcharge*" very well. I just happen to disagree with the section "*Cell "drift" is inevitable*." I think we need more structured tests and certainly more real world tests before we know exactly what to expect. I do believe that some form of cell drift in old age will surface, but we can't test that by abusing some test cells (I can show you self discharge that way.) I see no reason to believe Peukert's losses have anything to do with it, as he was not measuring an actual capacity loss. I am wondering if old age will cause capacity loss off the bottom, off the top (this is more about where it is found than the cause), or a reduction is charge efficiency (which is very close to 100% for LiFePO4 cells in good shape and not being overcharged.)


No offense taken. Thanks for mentioning self-discharge, another trait that will vary from cell to cell and continuously contribute to imbalance.

I'm not adamantly anti-100% SOC...going to that SOC can be very useful for cell diagnostics and balance evaluation. You can balance LFP close enough to prevent gross problems at 3.45 V or so, but they will be more closely balanced if you go to 3.6. This is where we do all our self-discharge testing at A123, because it affords us the best resolution of SOC vs. voltage. 

The data you wish for already exists, it's just not accessible to you. I'm just trying to share the knowledge.....

Please understand that when I say "drift is inevitable", I don't mean that every pack anyone might build will drift wildly, although that could happen if a particularly bad design was put into regular use. But to expect that you can build pack after pack after pack and they will all behave like perfect angels for years if you just bottom balance and then do nothing but cut off charging at 3.6 Volts is a fantasy. That is a recipe for high failure rates, mostly manifesting in lots of mid-to-late life frustration. How long it takes to get there will depend on how overbuilt the pack is versus the demands put upon it.


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## Duncan (Dec 8, 2008)

Re Peukert law

What I am hearing is
Lithium cells are "charge devices" - you put x coulombs in you get the same out
As far as I can see there is no mechanism for "leakage"

So charging/discharging a series string you will always move the same number of coulombs

As a cell deteriorates its "capacity" may change

So If I have started out with a bottom balanced string and one deteriorates I may be able to reach the top or bottom of that cell before it's brothers
If it's the bottom I may kill that cell

The change is the deterioration of the cell - it is just that - not an amount of deterioration for each cycle 
So an extra 1000 cycles if the cell has not further deteriorated does not make it "more unbalanced"

This means that
Re-Balancing the cells may be a useful idea but it needs to be done very infrequently

If a cell loses 20% of capacity in 2000cycles 
And its brother (the bad cell) is 50% worse
Then the imbalance is 1% in 200 cycles - or about a year of use
The good cells will have lost 2% and the "bad" cell 3%

Assuming that you are not using the top and bottom 10% of capacity

So you would need to check/re-balance after about 3 years when you would have lost about 9 % of capacity on your "bad" cell

But you still won't be able to get that capacity back - no matter what type of fancy balancing system you use
That cell is still down 9%


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## pm_dawn (Sep 14, 2009)

wb9k said:


> Just to clarify my own positions.....
> 
> In addition to electrochemical losses, we also need to consider the impedance of connections between cells. That is, if a series connection between cells has greater impedance than the other connections in the pack, the cells adjacent to that connection alone will suffer elevated losses across that connection--THE PACK DOES NOT SUPPORT THESE LOSSES EQUALLY ACROSS ALL CELLS. One cell will be affected during charge, the other adjacent cell will be affected during discharge.


Do you suggest that this also include the connections to and from the pack ?

If so, which cell is affected in what way by the connection with the higher impedance?

Or does this only affect cells if they have a high impedance between any of the cells ?


Best Regards
/Per Eklund


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## dcb (Dec 5, 2009)

also, by losses do you mean some batteries get warmer (i.e. the ones north of the bad connection when charging)? They are all going to get the same amp hours in any event, even if the voltage divsion is a little off.


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## skooler (Mar 26, 2011)

wb9k's argument uses the cells internal resistance as a key point.
It is not as simple as calculating the cells Internal Resistance (IR) at the terminals. 

What we actually need to know is where the internal resistance is within various parts of the cell which cannot be measured at the terminals. A cell is made of many parallel connections - if the resistance is between the parallel connections within the cell you have a problem as only this cells energy will pass through it. 

If the resistance is located elsewhere within the cell (say between all the parallel connections and the terminal) then ALL the current from all the cells in the pack will have to pass this point meaning that the energy lost is equal across all cells. No different to having a bad connection somewhere else in the circuit.

I agree with your points however in practice, with real world experience I have not witnessed any meaningful cell drift over time on any LiFePO4 battery pack. The phenomenon you describe do exist, but nowhere near the level you have described.

To put things into perspective, the difference on the highest and lowest of 74 cells in my RX8 15,000 miles after the initial bottom balance was a few thousandths of a volt when discharged to 2.700 volts (the voltage they were balanced at).


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## dougingraham (Jul 26, 2011)

In 2004 I had the opportunity to work with the then fairly new to the RC market Kokam LiIon cells. I wrote the software for a charger and developed the first balancer for these types of batteries. There was essentially no information available on these cells so I did a lot of testing. One of the characteristics of those pouch cells was that they didn't seem to have any self discharge. This meant that they would stay balanced once balanced. I individually charged four cells and placed them in series. I measured the overnight resting voltage and found that the difference between the highest and lowest cell voltage was about 0.003 volts. I then did 30 2C discharge 1C charge cycles and measured the resting voltage about an hour after each charge or after an overnight rest. An interesting thing happened during this time. The differences between the cell resting voltage lessened rather than increased. The difference had reduced to about 0.0015 volts. About four years ago I repeated this experiment with some A123 M26650 cells and found that LiFe types also do this. And this effect was also noted by John Hardy during his cell testing. I have not specifically tried this on the 18650 laptop Lithium Cobalt type cells but since they behave in all other ways similarly I see no reason to expect them to behave differently.

What does this mean? It means that after an initial top balance you don't need to worry about the cells drifting apart because they don't behave that way. They drift together (at least at the top). My best guess as to why this happens is because when you get close to full the charge acceptance efficiency goes down and the fullest cell accepts less charge and so it ends up over the long term becoming more balanced. You do need to do an initial balance because the self balancing effect is very subtle and you will wear out a pack long before it becomes self balanced if you try to depend on this as your initial balance.

I do know that RC packs can go out of balance when you abuse them. And abuse seems to be fairly normal for them. Excessive discharge and discharge rates, and over heating are just a couple of issues they face. But we don't treat our EV batteries this way (at least not often).

As long as the cells are in good working order and not abused they don't go out of balance. And this has been my experience in my EV. About the only thing you might do would be to monitor the voltage of the cells in a manner that does not place a parasitic load on the cells. This will tell you when a cell is going bad and which one it is. This is about the most that even the OEM's really need to do. They just haven't realized it yet.

This is an easy experiment to reproduce on a bench. Though any of us without a BMS are doing it every day we drive our EVs. I have approximately 550 charges on my pack (1/4 charge) with an initial bottom balance and then a second one a year and a half later when I added in 19 more cells to the pack. Other than adding in the 19 cells there was no reason to rebalance.


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## IamIan (Mar 29, 2009)

dcb said:


> for a given battery, very little change in ah delivered for different discharge rates (within reason) though delivered at *increasingly reduced voltage*:


The difference is that the Peukert type PoV , assumes the same ending voltage point for all the different discharge rates .. If you do that (hold the same ending voltage) .. you will see significantly different amounts of usable capacity .. under different discharge loads .. by the time you reach that same bottom end voltage.



dougingraham said:


> They drift together (at least at the top). My best guess as to why this happens is because when you get close to full the charge acceptance efficiency goes down and the fullest cell accepts less charge and so it ends up over the long term becoming more balanced.


I suspect some of the different mechanisms involved tend toward different results .. each pulling this way or that ... and the net effect will vary considerably depending on the specific context of a specific battery pack.

I saw a similar (trend toward self top-balancing) effect when I did testing on my batch of 56 A123 20Ah pouch cells .. I don't know how representative they are for all other cells.

But ... I can also see how some of the other mechanisms might have a different trend ... Lowering Ohms as SoC increases ... Lowering Ohms as Temperature increases .. Increasing Self Discharge and SEI growth as temperature increases .. Different Self Discharge rates from cell to cell .. Different Aging effects from cell to cell .. etc .. etc.



Duncan said:


> What I am hearing is
> Lithium cells are "charge devices" - you put x coulombs in you get the same out
> As far as I can see there is no mechanism for "leakage"


They are not 100% Ah cycle efficient .. very high .. but not 100%... you will not (Consistently & Repeatably) be able get 100% exactly the same Ah flow on discharge as you applied on charge ... you can get high, but not 100%... and the exact % varies slightly from cell to cell.

Every 1 Ah you see 'go in' on one terminal has (effectively) the same magnitude 1 Ah go out on the opposite terminal at the same time .. +1 -1 = 0 ... there is no Ah storage.

The battery is a series electrical connection (effectively):
|+Amps| Terminal A = |-Amps| Terminal B

Although the self discharge rate is very very low ... it is not zero... it changes with different temperatures .. and it is not the same for all cells.


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## EVfun (Mar 14, 2010)

IamIan said:


> Every 1 Ah you see 'go in' on one terminal has (effectively) the same magnitude 1 Ah go out on the opposite terminal at the same time .. +1 -1 = 0 ... there is no Ah storage.
> 
> The battery is a series electrical connection (effectively):
> |+Amps| Terminal A = |-Amps| Terminal B
> ...


This seems to be a silly argument. Gas tanks hold gas, because it goes in and out in a way we see, but batteries don't hold anything because electrons just travel through them on charge and discharge? Charging moves Lithium atoms over to the graphite negative plate. Discharging moves Lithium atoms over to the positive plate. Every one of those Lithium atoms that makes it from one plate to the other does so because an electron makes the path between the terminals. In a functional sense they "store" (relocate) coulombs (that is what we measure going in and out.) 

The self discharge rate of good LiFePO4 and LiFeYPO4 cells must be *very* low. I've now run top balanced for 2 years without a BMS or intervention. I've got cells that have been stored for 4 years at about 50% SOC without the terminal voltage changing by even 0.01 volt. I have some concerns with bottom balance, but good cells rapidly going out of balance isn't one of them.


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## wb9k (Apr 9, 2015)

EVfun said:


> This seems to be a silly argument. Gas tanks hold gas, because it goes in and out in a way we see, but batteries don't hold anything because electrons just travel through them on charge and discharge? Charging moves Lithium atoms over to the graphite negative plate. Discharging moves Lithium atoms over to the positive plate. Every one of those Lithium atoms that makes it from one plate to the other does so because an electron makes the path between the terminals. In a functional sense they "store" (relocate) coulombs (that is what we measure going in and out.)
> 
> The self discharge rate of good LiFePO4 and LiFeYPO4 cells must be *very* low. I've now run top balanced for 2 years without a BMS or intervention. I've got cells that have been stored for 4 years at about 50% SOC without the terminal voltage changing by even 0.01 volt. I have some concerns with bottom balance, but good cells rapidly going out of balance isn't one of them.


First off, Ian understands how cells work very well. Nice to see!

All batteries store electrical potential in a chemical medium. This is what batteries "do". They don't store "Ah" because Ah is a compound unit that contains time, and you can't hold time in a battery. It is reasonably correct to say that batteries store electrical power, which is expressed in Watts. Coulombs is probably more proper, but not terribly useful in our particular context most of the time. In an LFP cell, it is Li ions that are physically moved back and forth from anode to cathode. The Li is contained entirely in the electrolyte when the cell is originally made, and comprises less than 2% of the cell by weight (at least that's the case for an A123 Amp 20 cell). 

At 50% SOC, you can lose some 20 to 30% SOC without moving 1 mV in OCV. The truth is, you don't really know what the SOC of those cells are anymore. *You cannot use voltage to discern SOC in LFP except at the extreme ends of the charge/discharge curve. *If you want to see how self-discharge rates really vary, take the cells to 3.6 volts and watch them fall over a few days. THEN you can see how similar they are (or aren't). A123's spec allow for 2-3% SOC per month of self-discharge when in storage, but typical SD is a small fraction of that amount in these cells. It is presumed by cell makers that pack builders are competent individuals who include some sort of BMS in their packs that can take up this kind of slack.


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## wb9k (Apr 9, 2015)

dougingraham said:


> In 2004 I had the opportunity to work with the then fairly new to the RC market Kokam LiIon cells. I wrote the software for a charger and developed the first balancer for these types of batteries. There was essentially no information available on these cells so I did a lot of testing. One of the characteristics of those pouch cells was that they didn't seem to have any self discharge. This meant that they would stay balanced once balanced. I individually charged four cells and placed them in series. I measured the overnight resting voltage and found that the difference between the highest and lowest cell voltage was about 0.003 volts. I then did 30 2C discharge 1C charge cycles and measured the resting voltage about an hour after each charge or after an overnight rest. An interesting thing happened during this time. The differences between the cell resting voltage lessened rather than increased. The difference had reduced to about 0.0015 volts. About four years ago I repeated this experiment with some A123 M26650 cells and found that LiFe types also do this. And this effect was also noted by John Hardy during his cell testing. I have not specifically tried this on the 18650 laptop Lithium Cobalt type cells but since they behave in all other ways similarly I see no reason to expect them to behave differently.
> 
> What does this mean? It means that after an initial top balance you don't need to worry about the cells drifting apart because they don't behave that way.


I work for A123 in warranty. What your experiences, cool as they are, say to me is that you have worked with a relatively small number of brand new cells in rarified circumstances during your career. Your experience does not warrant a belief that all batteries are created exactly equal, because they're not. And as they age, they become less and less so. You're dead wrong if you think OEM's will ever adopt a system like you describe. You are placing WAY too much power and responsibility in the driver's hands by not at least having cell-level monitoring for LVC. What happens if one cell loses, say 4% of SOC compared to the pack mean over a few months, and the driver takes the pack to "2%" one day? A cell gets driven negative, and god help them if they don't notice. The cells don't always rupture right away...they're not really dangerous until you charge them again. So they plug in and go to bed thinking all is well.... a proper failure analysis screams that this is a death trap. Do you really think the OEM's would even be ALLOWED to put something like that out there? I'll go out on a limb and presume that you've never worked in automotive, have you? This isn't consumer electronics. Automotive reliability specs are second only to aerospace. Even military specs fall far short in the reliability department by comparison. 

I know I get wound up on this here detail, but this is the kind of stuff that drives me crazy when people start talking about "how easy it is to build electric cars", and then yammer on about how the OEM's are incompetent or in some kind of oil conspiracy. The fact that you would even consider such a design for anyone but yourself and other tech hobbyists makes it blatantly obvious that you have no idea what goes into a mass-market automobile in terms of safety and reliability engineering. And your battery experience is clearly limited as well. Sorry if that seems harsh, but you're peddling rubbish here with an unjustified air of authority. I'm calling BS.


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## wb9k (Apr 9, 2015)

pm_dawn said:


> Do you suggest that this also include the connections to and from the pack ?
> 
> If so, which cell is affected in what way by the connection with the higher impedance?
> 
> ...


I suspect the end connections do impose a similar phenomenon on end cells in a pack (how many here have noticed they seem to drift the most in a typical pack or module?), but I don't have a good handle on why that happens yet. I can explain it within the confines of a pack, but I can't yet explain it where the load is directly involved. That is much harder to understand, for me anyway.


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## PStechPaul (May 1, 2012)

wb9k said:


> All batteries store electrical potential in a chemical medium. This is what batteries "do". They don't store "Ah" because Ah is a compound unit that contains time, and you can't hold time in a battery. It is reasonably correct to say that batteries store electrical power, which is expressed in Watts. Coulombs is probably more proper, but not terribly useful in our particular context most of the time.


This is wrong. Batteries, capacitors, flywheels, and even biomass, store energy, which is properly measured in watt-hours or joules. Batteries are also characterized by power density and energy density, and other properties that may include voltage, current, power, and efficiency. I think we need to get the terms straight before discussing the finer aspects of batteries of various type, manufacture, and application.


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## EVfun (Mar 14, 2010)

wb9k said:


> All batteries store electrical potential in a chemical medium. This is what batteries "do". They don't store "Ah" because Ah is a compound unit that contains time, and you can't hold time in a battery. It is reasonably correct to say that batteries store electrical power, which is expressed in Watts. Coulombs is probably more proper, but not terribly useful in our particular context most of the time. In an LFP cell, it is Li ions that are physically moved back and forth from anode to cathode. The Li is contained entirely in the electrolyte when the cell is originally made, and comprises less than 2% of the cell by weight (at least that's the case for an A123 Amp 20 cell).


Almost every battery out there is rated in amp hours because it works. Perhaps you would prefer it if I said my 60 amp hour cells are able to shuttle about 1.348 x 10^24 Lithium ions across the electrolyte while the electrons make the trip between the terminals. But that can also be expressed as 21,600 Coulombs or 60 amp hours. Cells clearly don't store watts -- what does a 20 watt cell look like? How long does it put out 20 watts, clearly not indefinitely.

After carrying out tests it became clear that watt hours are something to consider for the load, but not a primary concern when charging. If you take 10 amp hours you will have to put very near 10 amp hours back in. If you take 30 amp hours out it will be very near 30 amp hours you have to put back in. The watt hours for every charge will be higher than the watt hours removed and much less useful when charging the cells. The watts hours the cells deliver vary based on the discharge rate. The watts needed to recharge vary depending on the charge rate. The amp hours needed to recharge don't vary because of differences in either the discharge or charge rate.


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## wb9k (Apr 9, 2015)

PStechPaul said:


> This is wrong. Batteries, capacitors, flywheels, and even biomass, store energy, which is properly measured in watt-hours or joules. Batteries are also characterized by power density and energy density, and other properties that may include voltage, current, power, and efficiency. I think we need to get the terms straight before discussing the finer aspects of batteries of various type, manufacture, and application.


Fair enough...sort of. Batteries still don't "hold Ah"--that's not a description of electrochemical processes, which is really the level we're talking at here. You can express battery capacity in Ah, but that's not what moves through the battery, and that concept (capacity, per se) is far removed from the present discussion. The statement is really a quasi-technical way of trying to model battery behaviors, and not a particularly valuable one at that.


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## wb9k (Apr 9, 2015)

EVfun said:


> The amp hours needed to recharge don't vary because of differences in either the discharge or charge rate.


Yes it does, just not enough for you to have noticed it. It is the compounding of this over time that will bite you. 

I'm not trying to be obtuse with the language...there are plenty of right ways to express things, but this terminology, to me, isn't really appropriate....it's too far removed from the aspects we're discussing. The situation is far more complex and dynamic than you are giving it credit for.


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## IamIan (Mar 29, 2009)

Apologizes for length .. sorry , relapse ... still working on personal conciseness.



PStechPaul said:


> I think we need to get the terms straight before discussing the finer aspects of batteries of various type, manufacture, and application.


100% agree.



EVfun said:


> This seems to be a silly argument.


I agree .. except from my PoV it's insisting on something that is not real that is 'silly' ... batteries do not 'store' Ah.

It should just be a .. sure technically they don't .. agreement and than move on.



EVfun said:


> Gas tanks hold gas, because it goes in and out in a way we see, but *batteries don't hold anything* because electrons just travel through them on charge and discharge?


*Bold* = No one claimed that
The correction is that batteries do not store Ah ... because of what specific thing Ah are .. and how a series electrical circuit (like a battery) works.



EVfun said:


> In a functional sense they "store"


I'll agree many many people incorrectly use that Ah storage PoV and phrasing all the time .. and because the Ah cycle efficiency is higher , I understand why that can make it easier to be a functional / useful thing for people to measure.

But none of that changes that it is still technically incorrect... no matter how many people do it .. or how many times they do it.



EVfun said:


> The amp hours needed to recharge don't vary because of differences in either the discharge or charge rate.


Careful and accurate testing has shown that it does vary... it also varies with SoC .. Temperature ... etc.



EVfun said:


> I have some concerns with bottom balance, but good cells *rapidly* going out of balance isn't one of them.


(*bold* added) That I will agree with.
From what I've seen and read thus far .. in 'good' cells it should be as Duncan wrote above.



Duncan said:


> Re-Balancing the cells may be a useful idea but it needs to be done very infrequently


I suspect 1-2 times a year at most .. and depending on the specific details of the context maybe far less might be fine as well.

Of course doing it 50 times a year or 300 times a year is not necessarily bad in itself ... but has as they say "diminishing returns".

All the large OEMs AFAIK seem to have decided to use cell level BMS .. of course a cell level BMS is not the only way .. but it does allow the minimum consumer knowledge level to be set the lowest .. which is good for OEMs trying to sell to the mass market consumer , consumers who knows next to nothing about batteries.

- - - - - - 

To me .. a 'good' pack should (somehow):
1> Detect a 'bad' cell.
2> Prevent overcharging
3> Prevent over discharging
4> Prevent abusive temperature extremes
5> Safety disconnects
6> Be reasonably well balanced

That 'somehow' can be a high (several million) hour MTBF smart BMS on each cell .. or it can be done other ways too.


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## dcb (Dec 5, 2009)

It isn't entirely clear to me how, even with top balancing and a bms, it can prevent over discharging a cell under continuous load (i.e. cruise control set).

Though AH is basically an electron count. And for a given temperature (heck even gas tanks change energy density based on fuel volume and temperature), that seems to be the best indicator of a lithium battery capacity, ions move in relation to electrons.

oh, and
7> must not kill cells or the pack. Some notable failures were key in driving the bottom balance nobms approach.


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## wb9k (Apr 9, 2015)

Ian, you're a much calmer guy than me, my hat's off to you for your diplomacy.  

DBC, Ah, isn't really an electron count....and electron counting doesn't generally work very well anyway for the very reasons I'm stating here. Some power is lost on charge and discharge...you can't just assume all power put to the cell is actually stored. 

When everything is working just as expected, these approaches can be made to work to some level...and that's fine...for *us*. But I start to lose it when people proclaim this is the way all EV's should be made. Car designers MUST plan for part failure and complete neglect by the driver. Not having cell-level monitoring is a total non-starter for this reason alone. 

Top balancing every charge cycle by itself doesn't do anything to prevent overdischarge, except guarantee that you aren't drifting into a situation where your lowest cell is lower than you expect. There's no way to control this when you bottom balance in a "set it and forget it" kind of way. 
Top balancing doesn't absolve you of the need for proper LVC controls. All of these things are part of a good quality BMS...it isn't just about balancing cells, as Ian has so calmly pointed out .

Electronics problems (or, more accurately, electronics and pack design problems) have indeed been the cause of failures, but this is not a valid reason to not control a battery pack--at least not in commercial designs. It *is* a reason to improve designs for better robustness.


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## skooler (Mar 26, 2011)

wb9k said:


> I start to lose it when people proclaim this is the way all EV's should be made.


 Who exactly is claiming that this is how all EV's should be built?

Going back to the reason for this thread, it is to discuss the process for bottom balancing rather than a BMS/no BMS debate.


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## Moltenmetal (Mar 20, 2014)

Too many angels attempting to dance on the head of a very small pin here!

Batteries store electrical potential energy, in the same way that a gas tank stores chemical potential energy. The amount of energy they store or the electrochemical form it is stored in isn't important to the users- of primary importance to a battery user is how much of that potential energy the cell is able to release under a certain set of drain conditions. That is measured by the product of the voltage-current curve during discharge, integrated over the discharge time- which is charge/time x voltage x time which hence has units of energy (J, kWh etc.). If the current is constant, you integrate the voltage with respect to time- and vice versa. If both are more or less constant, the simple product is a close enough approximation. 

The efficiency of stored energy release depends on the discharge rate (relative to the cell capacity) to an extent determined by the cell chemistry etc. The voltage drop which increases as the current is increased at any particular state of charge is modeled as an "internal resistance", and in fact results to some extent from ohmic resistive loss within the cell components, but in most chemistries it arises primarily due to mass transfer effects- it takes some of the source energy to move the electrons (or counterions) faster than they would "like" to do so by diffusion. 

Current is charge per unit time (coulombs per second), so Ah IS a perfectly fine unit of charge- a perfectly functional unit actually, even though it might be a bit weird to talk about distances in miles/hr*hrs.... If you're looking at the chemistry, you're probably going to be working in Faradays anyway (1 mole of electrons).

It has been mentioned that some charge goes, during recharge, to reactions in the cell which are not reversible during discharge. These processes consume components of the cell and result in a reduction of capacity over time. As an example, extraordinarily accurate measurements of the difference in charge in versus charge out under highly controlled conditions can be used to test improvements in Li-ion cell chemistry and to predict cell cycle life. There are other reactions which don't need charge transfer per se, which can also result in degradation of cell components. The nature of these reactions differ with the cell chemistry- some chemistries allow you to renew the electrolyte or to carry out other processes which will reverse some of these reactions and regain lost capacity. And with some chemistries, there are also bulk physical changes of the electrode materials which occur with each cycle which ultimately result in cells failing due to short circuits etc.


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## EVfun (Mar 14, 2010)

IamIan said:


> > Originally Posted by *EVfun*
> > _Gas tanks hold gas, because it goes in and out in a way we see, but *batteries don't hold anything* because electrons just travel through them on charge and discharge?_
> 
> 
> ...


So, what do LiFePO4 cells store? We know they shuttle Lithium between the plates. Further, why not use the "useful" term, the battery manufacturers even use it when referring to the size of batteries they are selling?

I covered the fact that SoC effects amp hours needed to recharge. I agree that temperature also has in impact on the amp hours needed to recharge but was trying not to add more variables to the discussion.


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## dcb (Dec 5, 2009)

Moltenmetal said:


> Too many angels attempting to dance on the head of a very small pin here!


That's sort of the feeling I'm getting too. AH absolutely is amps over time, which is the stuff of coulombs. But instead the response was phrased in terms of "power".

Nobody thinks it is a great idea to sell consumer vehicles without a bms in the current state of affairs. But DIY'ers dont typically have the battery specific data or inclination to come up with a highly precise SOC. 


gobs of info:
http://www.mpoweruk.com/soc.htm
http://www.mpoweruk.com/lithium_failures.htm#soc


I do think, for us, a half pack monitor and a coulomb counter and temperature probes can provide a large return though on reducing the fudge factor, if only to drive an idiot light. But temperature compensation still requires some cell specific testing. So a large safety factor is prudent. And temperature variations between cells is still a PITA.

So I'll add
8> should discharge cells if in danger of being over charged due to temperature drop. (into a battery heater would be best).

But back to Bottom Balance, how much capacity difference are we talking about? I.e. if top balance has a higher average pack voltage, how much more energy does it store compared to bottom balance? Lets assume 100ah and 100 cells. And are battery manufactureres getting more consistent?


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## wb9k (Apr 9, 2015)

dcb said:


> So I'll add
> 8> should discharge cells if in danger of being over charged due to temperature drop. (into a battery heater would be best).
> 
> But back to Bottom Balance, how much capacity difference are we talking about? I.e. if top balance has a higher average pack voltage, how much more energy does it store compared to bottom balance? Lets assume 100ah and 100 cells. And are battery manufactureres getting more consistent?


You don't want to discharge cells that are cold to protect from over charge, you want to not charge cold cells beyond a very strict current limit to prevent permanent capacity loss. Rated charge current falls off with temperature. At very low cell temps, allowable charge current eventually falls to zero. The Li doesn't like to move through the cell the way we want when it's very cold. If you try to push it too fast, the Li will plate permanently onto the cathodes rather than intercalate (temporarily, as desired) into the cathode powder, resulting in rapid, permanent loss of capacity. 

As far as how much capacity difference we're talking about with top vs bottom balancing, if the pack is truly in balance, there should be no difference in the usable capacity of the pack either way. The usable capacity of any pack is always equal to the lowest (usable) capacity cell/cell group in the the series string. The problem with bottom balancing is that you aren't resetting balance on a regular basis, and if you don't have cell level monitoring, you don't really know how much power is in the lowest cell, or who the lowest cell even is after a while. That system relies on the assumptions that what starts out as the "weakest" cell will always remain so and that balance will never drift--both of which are unreliable assumptions at best. When you top balance (and use balancers every, or nearly every, charge cycle), you insure that things are not moving around. Of course, if you have balancers, you must have cell level voltage measurements so the balancers know when to kick in and the charger knows when to stop. Use the same sense circuitry to trigger LVC when any cell--regardless of whether it's one you expect or not--hits the lower safe limit. 

Battery makers are getting more consistent, but nobody has ever produced cells that are exactly alike and, as others have pointed out, temperature is a big factor in moment to moment performance, even if the cells ARE exactly alike. I don't know of a single system built by anyone that can guarantee temperature is perfectly even across an entire pack at all times. And no matter how good cells might get, carmakers cannot assume perfection on the part of anybody...ever. This is especially true of parts that have the potential for creating "thermal events", the automotive euphemism for "fire".


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## dcb (Dec 5, 2009)

wb9k said:


> As far as how much capacity difference we're talking about with top vs bottom balancing, if the pack is truly in balance, there should be no difference in the usable capacity of the pack either way. The usable capacity of any pack is always equal to the lowest (usable) capacity cell/cell group in the the series string.


Ok, I stand corrected on low temp, it helps reduce aging not to store them fully charged, period.

re: capacity difference, my point is how much energy is left untapped, for the scenario given, using actual testing data. I.e. what is the standard deviation in capacity, and is it improving?

re: bottom probs, well, you *can* regularly bottom out the pack, automatically, on whatever schedule you like, it would be handy if your pack was grid-tied though.

And yah, no body is discussing what car manufactureres should do, this is a diy forum...


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## Sunking (Aug 10, 2009)

skooler said:


> Nothing wrong with top balancing, it protects the pack from overcharge but not overdischarge, perhaps getting slightly more capacity from the cells but at a slightly higher risk of something going wrong.


I know this is an old post and do not know if you are still monitoring the post or not. First let me say I Bottom Balance. I do use a Battery Monitor to control my charger, in fact some would call it a BMS because it is an Orion Jr. I just do not use the Top Balance feature. I just use it to monitor cell voltages. 

On the Charge Side I charge at C/2 and the Orion shuts the charger off when it sees any cell at 3.55 volts for more than 15 seconds. I found that 3.55 is required to get a resting voltage of 3.4 volts or roughly 90% SOC after rested. Charging at C/2 makes the voltage set point a little higher.

Second Fail Safe on the charge side is in the charger itself will trip off if the pack voltage reaches 56.8 volts. Has never happened in 6 months of operation. 

On the Discharge side I have two Fail Safes. One is if the Orion sees any cell drop to 2.5 volts or less form more than 15 seconds. Second Fail Safe is if the Motor Controller battery monitor sees the pack voltage reaches 46.4 volts. Has never happened except when I tested it running the battery down with Orion deactivated. 

Lastly I use Coulomb Counting gas gauge. When charged up I should see 96 to 99 AH. I recharge between 10 to 20 AH left on the gauge. Works great as I have three monitors which helps me sleep at night. 

But back to one point I disagree with you on and it is a minor point. It is possible to over charge using a BMS that uses Vampire aka Bleeder Boards. Example if you use say a C/3 constant Current Charger say 33 amps on a 100 AH battery, the Vampire Boards can only shunt up around 1 amp leaving 32 amps still flowing in a fully charged battery. So on down the line until the charger current tapers off when the last few cells finally approach full charge. 

Having said that once fully Top Balanced that problem tends to be minimized until something goes wrong. 

That is my two-cents and how I Bottom Balance. In my minds eye the BMS should be integrated with the charger in a Top Balanced or Bottom Balanced system. In a Top Balanced approach, when that first cell reaches desired capacity, the BMS signals the charger to cut current back to whatever value the Vampire Boards can shunt until all cells reach the set point capacity of the users choice.


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## dougingraham (Jul 26, 2011)

wb9k said:


> You're dead wrong if you think OEM's will ever adopt a system like you describe. You are placing WAY too much power and responsibility in the driver's hands by not at least having cell-level monitoring for LVC.


So you didn't read my posting? I specifically mentioned cell level monitoring. Anything beyond that doesn't make the vehicle or battery pack more reliable and all cell level monitoring can do is tell you which cell is going bad and perhaps stop you from continuing to drive the vehicle.



wb9k said:


> What happens if one cell loses, say 4% of SOC compared to the pack mean over a few months, and the driver takes the pack to "2%" one day? A cell gets driven negative, and god help them if they don't notice. The cells don't always rupture right away...they're not really dangerous until you charge them again.


And this is the big problem. Something you did yesterday can cause you to burn down your house. And is again why a cell level monitor is not a bad thing. I have seen occurrences of this for over 10 years in the RC hobby industry. Only the delay between doing something bad to the pack and charging it is at least 5 days.



wb9k said:


> I know I get wound up on this here detail, but this is the kind of stuff that drives me crazy when people start talking about "how easy it is to build electric cars", and then yammer on about how the OEM's are incompetent or in some kind of oil conspiracy. The fact that you would even consider such a design for anyone but yourself and other tech hobbyists makes it blatantly obvious that you have no idea what goes into a mass-market automobile in terms of safety and reliability engineering. And your battery experience is clearly limited as well. Sorry if that seems harsh, but you're peddling rubbish here with an unjustified air of authority. I'm calling BS.


Well how about that! You certainly did get wound up. This rant can't have come from anything I have written. For the most part I have a lot of respect for the OEM's. In the area of the OEM EV's they have done a good job of producing cars that they can warranty out long enough that they can sell them and get good customer satisfaction ratings.

I wish you well on your conversion.


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## IamIan (Mar 29, 2009)

EVfun said:


> So, what do LiFePO4 cells store?


I'll agree with:


PStechPaul said:


> Batteries, capacitors, flywheels, and even biomass, store energy, which is properly measured in watt-hours or joules.


Energy by definition is the term that means "capacity to do work" .. if you want to talk about the ability (capacity) of something (a battery) to do work .. move an object of mass , apply a force over a distance , change temperature of an object , release some amount of electrical energy , etc .. the correct term is an energy term (Wh , Joules, etc)



EVfun said:


> Further, why not use the "useful" term, the battery manufacturers even use it when referring to the size of batteries they are selling?


I see two sides to this 'penny'.

Side A> Part of Herd.
Do you want to use the term because it's popular .. for whatever reason .. (even if it is known to be incorrect) ... this has real value .. especially to a company that wants to sell a product... If customers were even 1% more likely to buy batteries described in 'pigeon/years' than I would expect OEMs to follow suit and describe them in 'pigeon/years'... or anything else that would help sell the product.

I suspect Ah started off being used correctly .. in relation to batteries the Ah cycle efficiency is higher than the Wh cycle efficiency ... but unfortunately this has lead to the spread of mis-information .. such as the idea and claim that batteries store Ah ... or Ah tell you the capacity to do work .. etc.

Side B> Do you want what you claim and 'teach' others to be correct?
I see no problem with correctly using the terminology ... And avoiding the spread of mis-information (intentional or not) .. I think there is value in correctly informed consumers .. I think the consumer would ultimately be better off applying correct unpopular information .. instead of applying incorrect popular information.



wb9k said:


> Car designers MUST plan for part failure and complete neglect by the driver. Not having cell-level monitoring is a total non-starter for this reason alone.


I agree the designer has to do this ... and if the proper analysis leads them to the cell level BMS as the over all best net MTBF and/or best cost vs benefits... great by all means... And I suspect this type of analysis is why the large OEM are all currently using cell level BMS.

However .. part failure (individual battery cell) can be detected without a BMS on each individual cell... that is one way to do it .. but not the only way.


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## EVfun (Mar 14, 2010)

IamIan said:


> > Originally Posted by EVfun:
> > Further, why not use the "useful" term, the battery manufacturers even use it when referring to the size of batteries they are selling?
> 
> 
> ...


How about _Side C_? I know I have the watt hours needed to do the work. My job is to make sure the batteries live between charges that are sometimes partial charges, and had better all be partial discharges. Amp hours works for this much better than watt hours. I can count them in and out for 10x the battery capacity, without ever being quite full or empty, and be not very far off on the state of charge as determined by the next full charge. I typically concern myself with amp hours for the same reason most battery "gas" gauges use amp hours for calculating SOC. 

Is this fair? Batteries store energy, but sometimes the state of charge of a battery is what is most important to know. Batteries store Joules by holding Coulombs at a voltage potential based on the chemistry.


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## dcb (Dec 5, 2009)

but, in terms of ionic reactions, it is based on electrons.
https://youtu.be/pxP0Cu00sZs?t=815

amps * time is exactly how you measure charge and discharge capacity. It doesn't matter if it is done at 2.5 volts or 4.1 volts or a string of batteries of any arbitrary voltage.

Qc=I*Tc Qd=I*Td
CE (Coulombic Efficiency) = Qd/Qc (Charge out/Charge in)

I see no problem viewing charge in terms of AH. It isn't the same as power or energy, those need to integrate voltage and time, but in terms of monitoring the ionic reactions, AH is fundamental.

You might find kwh more useful, but it isn't the operating principle. Monitoring volts is almost useless anywhere in the middle of the pack for determining state of charge.

Maybe I'm wrong, but I don't see what the big deal about using AH is. CE accounts for the parasitic charge losses without unnecessary terms.

(note, slower charging means worse CE, explaind in vid)

Filling a glass doesn't require knowledge of the psi, just the resulting flow over time.


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## IamIan (Mar 29, 2009)

EVfun said:


> Is this fair? Batteries store energy, but sometimes the state of charge of a battery is what is most important to know.


In the end .. whatever works for you. 

- - - - - 

But .. If you are asking my own personal PoV.

If you ever want to consider the batteries ability to do any work .. the correct term would be an energy term .. Using other (non-energy) terms to describe or think about the batteries ability/capacity to do work .. is incorrect and is what perpetuates this mis-information ... as innocent as the intent might be.

I would argue that knowing the SoC is never more important than knowing the batteries capacity to do work ... The whole point of having a battery at all is for it's capacity to do work.

I would still say as I wrote before though:


IamIan said:


> I suspect Ah started off being used correctly .. in relation to batteries the Ah cycle efficiency is higher than the Wh cycle efficiency


I see nothing at all wrong (in and of itself) with acknowledging and using this known aspect of Ah battery cycle efficiency.

However the benefit it gives with 'easiness' and such for making a type of 'fuel gauge' ... it comes along with a 'dark-side' ...







it leads many people down that same path toward that mis-information and mis-understanding.

Soo .. from my PoV .. even if Energy terms (like Wh) might be 'harder' to use / implement as a type of 'fuel gauge' ... it comes with the added benefit of correctly reinforcing the correct terminology for what one is describing and doing .. and if done to track SoE as accurately as the Ah track SoC ... the SoE fuel gauge is a more accurate representation of the remaining capacity to do work (ie move the car).


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## dcb (Dec 5, 2009)

IamIan said:


> I would argue that knowing the SoC is never more important than knowing the batteries capacity to do work ... The whole point of having a battery at all is for it's capacity to do work.


When it comes to the reasoning behind balancing (i.e. in bottom balancing thread), and optimizing battery life, or even BMS's/monitoring in general, AH is key. Managing the state of charge within certain limits is the sole concern.

Which again, is a nice thing about bottom balancing, they all die at the same time without killing one another


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## Sunking (Aug 10, 2009)

Well from working with batteries professionally for 35 years i can tell you SOC voltages are virtually meaningless because it tells you nothing about the energy capacity contained inside, or the health of the battery in question.

I can show and name countless examples of batteries with a X rated AH. For example 100 AH at some given voltage, whatever voltage floats your boat. A 100% SOC, say 12.6 volts on a FLA battery indicated 100% SOC. Do a load test you and suddenly find it only has 25 AH. Next specimen may yield 95 AH. 

The only accurate way to determine the health of a battery is to do a capacity test under controlled conditions. Otherwise it is a WAG. 

Coulomb Counting can be useful if it is calibrated routinely. It wil at least allow you to track capacity loss., and a fair idea of the real SOE which is what you really want to know in an EV application. In practice it is a Ball Park meter.


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## joamanya89 (Feb 13, 2012)

Hi, I would like to know, you say about dischargeing cells until 2,7v and chargeing them to 3,5v.

Is it just for buttom balancing moment, or never again I should discharge my battery while I´m driving my ev under 2,7v and never again charge them to 3,7v?


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## Sunking (Aug 10, 2009)

joamanya89 said:


> Hi, I would like to know, you say about dischargeing cells until 2,7v and chargeing them to 3,5v.
> 
> Is it just for buttom balancing moment, or never again I should discharge my battery while I´m driving my ev under 2,7v and never again charge them to 3,7v?


Your confusion is understandable, manyhere don't fully understand. 

With respect to LiFeP04 Chi-Com batteries rested 100% SOC voltage = roughly 3.4725 volts or something close to that. It is a hard number to nail down. On the other side rested 0% SOC is generally accepted to be 2.5 volts. Note I said RESTED Voltages, not working voltages. Big Difference. 

In a Bottom Balanced System you will have two know Reference Points. Where 0% SOC is and the known capacity of ALL CELLS being 0 AH in them. You establish that reference from the start by connecting all cells in parallel and discharge them to anywhere from 2.3 to 2.7 vpc with 2.5 being your finale finished target voltage. It does not have to be exactly 2.5 volts. At 2.3 to 2.7 volts there is no meaningful capacity left in them. What you are left with is a Known Starting Point reference of voltage and capacity. Think of it as they are now calibrated..

Now connect them in series and charge them while monitoring all cell voltages. Charge at a Constant Current until the first cell reaches roughly 3.6 volts, and not total charger voltage when that happens. That cell is the weakest of the bunch and dictates pack capacity. Allow it to rest for a few hours and recheck voltage. It should settle around 3.4 volts is where you are aiming so that cell is up around 90 to 95% SOC. All other cell voltages will be slightly lower because they have a little more capacity not being used. 

If the first try gets you to 90 to 95% SOC on the weakest cell, you now know where to set the charger because you noted the voltage when the first cell reached 3.6 volts under charge. If it settles a little higher or lower make slight adjustments until you find the sweet spot. 

So when done you know exactly where the bottom is and what the pack voltage is at the bottom. Say a 45 cells x 2.5 volts = 112.5 volts. Set your Motor Controller LVD to 130 volts and you will never over discharge your batteries.

You have also established a known top voltage and AH capacity. A full tank of gas. If you have 100 AH cells, and the weakest is 105 AH and yyou charge to 95% SOC on that cell you know every cell has 99.75 AH and what voltage you should see when fully charged and rested. If will be something lower than 153 volts. 

Once done it is almost impossible to touch 100% over charge, and over discharge. 

With A Top Balanced system you really do not know much other than 100% SOC voltage on all the cells. It tells you nothing about capacity or SOC once you start discharging. All you know is 100% SOC voltage.


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## IamIan (Mar 29, 2009)

dcb said:


> IamIan said:
> 
> 
> > I would argue that knowing the SoC is never more important than knowing the batteries capacity to do work ... The whole point of having a battery at all is for it's capacity to do work.
> ...


*Bold* Added.
Personally I'd disagree.

The only reason to have a battery in the 1st place is because you want it's capacity to do work ... otherwise if as you suggested .. than there is no room left to have any concern for the batteries capacity to ever do any work .. ie ever move the car .. run lights , or any other type of work of any kind .. it's just a large paper weight.

From my PoV that is clearly not the 'sole' concern ... instead it is one is actually more concerned as a higher priority about the batteries capacity to do work .. it is that higher concern that leads someone to the lesser concern to care at all about battery balance and it's effects on that battery capacity to do work.

Capacity to do work in one trip .. and total capacity to do work over the entire lifetime of the battery itself.



dcb said:


> Which again, is a nice thing about bottom balancing, they all die at the same time without killing one another


Yup.

- - - - - - - 

Just a note ... SoC balancing is not the exclusive type of balancing either ... SoE balancing is also possible ... top / bottom / other... I suspect a Bottom SoE balanced pack should hold all the same benefits a bottom SoC balanced pack does... and describing that balance as such would be correctly in line with one's actual higher priority concern about the batteries capacity to do work.


But maybe that's just me.


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## Sunking (Aug 10, 2009)

IamIan said:


> Just a note ... SoC balancing is not the exclusive type of balancing either ... SoE balancing is also possible ... top / bottom / other... I suspect a Bottom SoE balanced pack should hold all the same benefits a bottom SoC balanced pack does... and describing that balance as such would be correctly in line with one's actual higher priority concern about the batteries capacity to do work.


Not sure I follow your logic. Bu there is my take on Bottom vs Top Balance. 

Top, Middle or Bottom works for a commercial EV manufacture. They can do things the DIy cannot even remotely do except in their dreams. A large car company uses the best batteries money can buy for cheap. They don't need to use cheap Chi-Com crap. But that is just the start. They buy the batteries by 100's of thousands at a time. They can test and segregate the cells to be matched up within 1% tolerance. It does not matter if they Bottom, Middle, or Top Balance as all the cells are balanced and grouped by design. All they have to do is make sure pack voltage never goes above 90 or below 10%. User cannot intervene. That is how they offer 8 and 10 year warranties. User has no control.

DIY not a snowball chance in he!! you can do that, but you can mimic that concept with Bottom Balance, you cannot mimic that with Top Balance because cell capacities are not equal. Only way to know capacity is with Bottom Balance. Once you have a Zero Reference Point the Top is easy to find. It is the weakest cell/link in the chain. If you have 45 100 AH cells and the weakest is 99 AH you have a 99 AH pack regardless is some cells are 120 AH. You cannot use that extra 20 AH unless you use Active Balancing which is not worth the expense, complexity, and nightmare it is. 

With Bottom Balance it is simple, and you know what is in the tank at any time without worrying about over charging and more importantly over discharging.


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## EVfun (Mar 14, 2010)

Sunking said:


> Now connect them in series and charge them while monitoring all cell voltages. Charge at a Constant Current until the first cell reaches roughly 3.6 volts, and not total charger voltage when that happens. That cell is the weakest of the bunch and dictates pack capacity. Allow it to rest for a few hours and recheck voltage. It should settle around 3.4 volts is where you are aiming so that cell is up around 90 to 95% SOC. All other cell voltages will be slightly lower because they have a little more capacity not being used.
> 
> If the first try gets you to 90 to 95% SOC on the weakest cell, you now know where to set the charger because you noted the voltage when the first cell reached 3.6 volts under charge. If it settles a little higher or lower make slight adjustments until you find the sweet spot.


This requires a charger where you can change the voltage settings in small increments and one where the output voltage is quite stable despite changes in the charging current and temperature. I could do that with my PFC-20. If just one cells does the upward launch of being fully charged its 0.1 volt change from 3.5 to 3.6 might only correspond to a 0.2 volt change in the total pack voltage (the others will be coming up but much more slowly.) I don't like to rely on that kind of accuracy, especially when I may not be around to watch the situation. 

I'm also uncomfortably with the weakest cell in the pack reaching the highest charging voltage every day. 



Sunking said:


> So when done you know exactly where the bottom is and what the pack voltage is at the bottom. Say a 45 cells x 2.5 volts = 112.5 volts. Set your Motor Controller LVD to 130 volts and you will never over discharge your batteries.
> 
> You have also established a known top voltage and AH capacity. A full tank of gas. If you have 100 AH cells, and the weakest is 105 AH and yyou charge to 95% SOC on that cell you know every cell has 99.75 AH and what voltage you should see when fully charged and rested. If will be something lower than 153 volts.
> 
> Once done it is almost impossible to touch 100% over charge, and over discharge.


So set your LVD to 130 volts? Many here have talked about needing to be able to pull the pack under 2.5 vpc to have useable available amps on colder days. I can certainly get under 2.88 vpc (130/45) on cold mornings in the early spring. 



Sunking said:


> With A Top Balanced system you really do not know much other than 100% SOC voltage on all the cells. It tells you nothing about capacity or SOC once you start discharging. All you know is 100% SOC voltage.


This is a common misunderstanding. After I top balanced next thing I did was fully discharge the pack while monitoring all the cells. (Like you advise doing on the first charge of a bottom balanced pack.) I know my available capacity and weakest cell(s) just like you do with a bottom balanced pack. There is even a little red dot on the smallest cell.

I'm simply more comfortable with knowing the charger will have a very strong signal to terminate charge. All the cells pop up together when its time to stop charging. Even if the shut-off timer fails no cell will go over a safe voltage. If one cell where to internally short no other cell would go to an excessive voltage. I haven't ruined a cell yet by over discharge, but if I ever do it is strictly my fault, its my right foot. There are plenty of low voltage alarms from the controller before I get very close to killing a cell. My range test only got the lowest cells down to 3.0x volts resting.

Bottom balancing seems to work pretty well in the field. Top balancing without a BMS also seems to work pretty well in the field. In both cases I'm only taking about owner/builder cars where they maintain the pack. I'm actually a bit concerned about both methods as we have more of these conversions where the pack is in the late stage of life. We need to see more packs built like this fail from old age before we know if they fail safely. I couldn't get away with this at work, but my EV in the parking lot is running top balanced no BMS.


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## joamanya89 (Feb 13, 2012)

Sunking said:


> With respect to LiFeP04 Chi-Com batteries rested 100% SOC voltage = roughly 3.4725 volts or something close to that. It is a hard number to nail down. On the other side rested 0% SOC is generally accepted to be 2.5 volts. Note I said RESTED Voltages, not working voltages. Big Difference.


Ok, but shouldn´t be talking about percentage? I know not all the batteries are the same but is is hard if every one just talk about it own battery pack.

If I have understud all of you, I should NEVER charge my battery pack above the 90% and never discharge it under 10% right? and what about the 80%DOD or 60%DOD that a lot of web pages and also so datasheets talk about?



Sunking said:


> In a Bottom Balanced System you will have two know Reference Points. Where 0% SOC is and the known capacity of ALL CELLS being 0 AH in them. You establish that reference from the start by connecting all cells in parallel and discharge them to anywhere from 2.3 to 2.7 vpc with 2.5 being your finale finished target voltage. It does not have to be exactly 2.5 volts. At 2.3 to 2.7 volts there is no meaningful capacity left in them. What you are left with is a Known Starting Point reference of voltage and capacity. Think of it as they are now calibrated..


Lets si if I follow you, let say a li ion cell which minimum voltage is at 2,4v if I´m not wrong, and it maximum voltage is 4,2v, right?

so 10% is 0,18v, then I should never discharge it under 2,58v and never chould charge it above 4,02v. is it correct?



Sunking said:


> Now connect them in series and charge them while monitoring all cell voltages. Charge at a Constant Current until the first cell reaches roughly 3.6 volts, and not total charger voltage when that happens. That cell is the weakest of the bunch and dictates pack capacity. Allow it to rest for a few hours and recheck voltage. It should settle around 3.4 volts is where you are aiming so that cell is up around 90 to 95% SOC. All other cell voltages will be slightly lower because they have a little more capacity not being used.


As you can see I´m a newer here, so is there a charger that alows you to check each cell or I have to do it while is chargeing with my multimeter?



Sunking said:


> So when done you know exactly where the bottom is and what the pack voltage is at the bottom. Say a 45 cells x 2.5 volts = 112.5 volts. Set your Motor Controller LVD to 130 volts and you will never over discharge your batteries.


Why if my 10% is at 112,5v I should set the controller LVD to 130v?

And before finishing my questions, thank you very much for your time.


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## Sunking (Aug 10, 2009)

joamanya89 said:


> Ok, but shouldn´t be talking about percentage? I know not all the batteries are the same but is is hard if every one just talk about it own battery pack.


We are talking about percentages. Only thing that changes is scale 10% of $100 is $10. 10% of $1,000,0000 is $100,000



joamanya89 said:


> If I have understud all of you, I should NEVER charge my battery pack above the 90% and never discharge it under 10% right? and what about the 80%DOD or 60%DOD that a lot of web pages and also so datasheets talk about?


You just have not made the connection or understand the terms. SOC and DOD are interchangeable terms and mean the same thing except opposite reference points.

0% SOC = 100% DOD. your tank is empty
10% SOC = 90% DOD
90% SOC = 10% DOD
100% SOC = 0% DOD your tank is full

So for example a manufacture my state a 100 AH battery and 80% is usable for maximum cycle life (80 AH). You want to operate the battery between 10 and 90% SOS or 90% to 10% DOD. Which way floats your boat? 

Very common to see a Chi-Com state cycle life like:

1000 cycles 100%
1500 cycles 80%

What they don't say because they assume you know is at 1500 cycles you operate between 10 and 90% to obtain 80% usable capacity.



joamanya89 said:


> See if I follow you, let say a li ion cell which minimum voltage is at 2,4v if I´m not wrong, and it maximum voltage is *4,2v, right*?


Forget you ever heard that number 4.2 vpc. Otherwise you will face serious consequences. 

There are a few ways to get to 100% SOC. There is fast and furious for risk takers, and slow and safe with a method in between. I use fast and furious, but only because I bottom balance. To go to 100% the safe way is CC/CV which in reality it is Constant Voltage with Current Limit.

Set supply voltage to 3.6 to 3.65 vpc. You can start a war about which voltage to use. Just pick one and stay quite. Limit current to manufactures recommended C-Rate usually C/2 to C/5 range. Terminate when charge current stops or reduces to .01 to .03C. This assumes your cells are Top Balanced and using Balance Boards. 

LVD is a personal choice and there is no right answer other than no lower than 2.5 vpc. There is very little if any charge left below 2.9 vpc. That right there gives you a range using a 45 S pack of 112.5 to 130.5 volts. So anywhere from 112.5 to 130 works. You decide what works for you. 

But there is a technical issue with LVD. 2.5 to 2.9 are at rest voltages with no current. Draw 1C or 100 amps from a 100 AH battery and you have .19 volt drop per cell you need to account for to avoid premature ejac#&(%$, er I mean low voltage disconnect. With that info you can safely go a bit lower than 2.5 vpc. I prefer to error on the side of caution and recommend no lower than 2.5 vpc. You decide what pain level you like. Start at LVD = 126 volts. If you find yourself tripping off line and see the battery voltages at rest still indicate more than 10%, lower the LVD setting incrementally until you get where you want and comfortable with. If the voltages are too low for comfort bump the voltage up a bit. You get to decide, not us. 

Personally I use a battery monitor on my Bottom Balanced pack. an Orion Jr. and have 3 fail safes to prevent over discharge.

1 Bottom Balance is a Fail Safe Mode
2. Orion battery monitor initiates LVD if any cell falls to or below 2.5 vpc for more than 15 seconds.
3. I use a 16S pack and set my Motor Controller to LVD at 46 volts.

That makes odds astronautical of over discharging. I also wear suspenders and a belt. My pants are not likely ever to fall off my skinny butt.


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## IamIan (Mar 29, 2009)

Sunking said:


> IamIan said:
> 
> 
> > Just a note ... SoC balancing is not the exclusive type of balancing either ... SoE balancing is also possible ... top / bottom / other... I suspect a Bottom SoE balanced pack should hold all the same benefits a bottom SoC balanced pack does... and describing that balance as such would be correctly in line with one's actual higher priority concern about the batteries capacity to do work.
> ...


SoC refers to charge ... ie Ah , coulombs, etc.
SoE refers to energy ... ie Wh , joules , etc.

Cell balance itself is relative / comparative .. as such it can be done with either PoV .. SoC or SoE.



Sunking said:


> But there is my take on Bottom vs Top Balance.


Fair enough.

Here's my PoV:
Use the right tool for the job.

Meaning .. Each method has both pros and cons .. it is best to be honest about both pros and cons (make an honest effort to avoid one's own personal bias) ... When designing for a specific context .. it is best to pick the method that works best (minimizes cons and maximize pros) for that specific context .. I do not think that any one type (top, bottom, SoC, SoE, etc.) is always superior to all other types under all possible contexts / conditions .. not for Large OEMs .. and also not for DIY hobbyist.

But ... to each their own.


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