# Fast way to bottom-balance a lot of cells?



## valerun (Nov 12, 2010)

Hi All - 

need some creative solutions how to bottom-balance 200 100AH cells in relatively short time (say, 2 days). They are 60% charged, so ~36kWhrs to burn... All in boxes so can't really 'drive' them down in a car...

Ideas? 

Thanks,
Valery


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## dexion (Aug 22, 2009)

I suppose you could get a 1500 watt 12v inverter and power stuff for 2 days. If you get 1000 watts into it for 36 hours...a window ac could do it.
A bit of setup is required putting them into 50 4 pack bundles.


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## EVfun (Mar 14, 2010)

Are these in boxes new? If they have never been loaded or charged they most likely don't need any balancing. They are more or less bottom balanced from the factory because they are discharged for testing (individual capacity and internal resistance) before having a certain amount of amp hours stuffed back into them for storage and shipping.


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## Guest (Aug 17, 2011)

EVfun said:


> Are these in boxes new? If they have never been loaded or charged they most likely don't need any balancing. They are more or less bottom balanced from the factory because they are discharged for testing (individual capacity and internal resistance) before having a certain amount of amp hours stuffed back into them for storage and shipping.


I concur with this. If you need to further bottom balance them after the batteries are installed then its easy to just drive them down very very carefully. Then bring them up to your specified voltage one at a time. 

Pete


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## Nathan219 (May 18, 2010)

Your charger will run on DC wont it; that is 10kw per hour, if you had a grid tie inverter you could put it back on the grid just set the charger to the inverter input voltage, might need to do less than 10kw at the voltage level.


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## valerun (Nov 12, 2010)

EVfun said:


> Are these in boxes new? If they have never been loaded or charged they most likely don't need any balancing. They are more or less bottom balanced from the factory because they are discharged for testing (individual capacity and internal resistance) before having a certain amount of amp hours stuffed back into them for storage and shipping.


Hm, that would be nice. Do you have a reference to the exact procedure they are using? I'll ask my CALB rep, too.


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## valerun (Nov 12, 2010)

ok there goes the theory of CALB bottom balancing the cells at the factory...

Today, took 27 "new from the factory" cells, strung in series and started discharging (~25A, or 0.25C)while checking cell voltages by cycling multi-meter through cells. Eventually, one cell went to 2.6 while the top cell was still at 3.1V! Took a smaller cell out. ~1.5AH later, another cell went to 2.6V, with the top cell at 2.9V now. I suspect there is at least 1-2AH more in some cells. 

So there it goes - CALB does NOT bottom balance cells at the factory. So, if you are a bottom-balance follower, you need to do yourself.


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## valerun (Nov 12, 2010)

Nathan219 said:


> Your charger will run on DC wont it; that is 10kw per hour, if you had a grid tie inverter you could put it back on the grid just set the charger to the inverter input voltage, might need to do less than 10kw at the voltage level.


yeah, well, I don't think it's going to boost from 3.2V, though ;-)

Ended up doing 4s35p config and running through 2/0AWG terminating at the ...10ft conduit pipe. 400A. BTW those 2/0AWG cables heat up to 70deg C at 400A!!! So much for running a 1000A controller output with those. Am definitely going with 4/0 AWG for the bimmer


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## JRP3 (Mar 7, 2008)

I would not think the cells were bottom balanced at the factory anyway. Each cell is probably discharged to 50% SOC, since each cell is slightly different in total capacity 50% SOC is going to be a different number of amp hours for each cell. My 100ah cells ranged from 110-114 actual, which means at 50%SOC they were between 55 and 57ah. Newer orders seem to be closer than that but you still might have an amp hour or so difference at the bottom.


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

Check Jack's video at June 24, 2011 http://www.youtube.com/user/marionrickard?feature=mhum#p/u/6/UUpPRS6IspY he has bottom balance ideas 

My 2c
-Y


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## EVfun (Mar 14, 2010)

JRP3 said:


> Each cell is probably discharged to 50% SOC, since each cell is slightly different in total capacity 50% SOC is going to be a different number of amp hours for each cell. My 100ah cells ranged from 110-114 actual, which means at 50%SOC they were between 55 and 57ah.


How did they determine the actual capacity without fully discharging the cells? That is virtually impossible to do in the vast center of capacity. I'm thinking the cells have at least one full cycle on them as part of the initiation charge and capacity plus internal resistance checking. If they just wanted to put them back to about 1/2 charged for shipping they could do that in series easily. Actual capacity testing cannot be done in series or parallel.


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## Roy Von Rogers (Mar 21, 2009)

EVfun said:


> How did they determine the actual capacity without fully discharging the cells? That is virtually impossible to do in the vast center of capacity. I'm thinking the cells have at least one full cycle on them as part of the initiation charge and capacity plus internal resistance checking. If they just wanted to put them back to about 1/2 charged for shipping they could do that in series easily. Actual capacity testing cannot be done in series or parallel.


 
I seem to remember he discharged the cells x amount to bottom, recorded the amount he removed, and then charged them. He recorded total amount in and deducted what he originaly removed..alas thats how he deduced the total of each cell.

At least thats what I rememeber, but I'm getting old  so someone correct me if I'm wrong.

Roy


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## valerun (Nov 12, 2010)

EVfun said:


> How did they determine the actual capacity without fully discharging the cells? That is virtually impossible to do in the vast center of capacity. I'm thinking the cells have at least one full cycle on them as part of the initiation charge and capacity plus internal resistance checking. If they just wanted to put them back to about 1/2 charged for shipping they could do that in series easily. Actual capacity testing cannot be done in series or parallel.


Yes but you can do it two ways - measuring AH out on full discharge or measuring AH in on full charge. Not sure how the cells are coming out of the assembly line but my guess would be they don't have any charge in them. If that's true, then capacity is measured by counting AH in until fully charged. Then cells would be discharged 40% for storage. This would make cells more top-balanced rather than bottom balanced.

Anyway, CALB tech dudes in China should be replying to my latest batch of tech questions next week. The above is one of the questions I asked - let's see what they say. Another question I asked was about safe range of high-frequency ripple current from controller. *Anyone has any idea on this forum?*


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## JRP3 (Mar 7, 2008)

EVfun said:


> How did they determine the actual capacity without fully discharging the cells? That is virtually impossible to do in the vast center of capacity. I'm thinking the cells have at least one full cycle on them as part of the initiation charge and capacity plus internal resistance checking. If they just wanted to put them back to about 1/2 charged for shipping they could do that in series easily. Actual capacity testing cannot be done in series or parallel.


My guess, after manufacturing the cells have no charge, so they are charged up, then discharged, then probably charged up again and discharged to check capacity. As I understand it cells lose a good amount of capacity after their first charge and then settle in to their actual capacity so they need to be cycled more than once. There is some actual capacity recovery in the first 50 or so cycles before it levels off and then starts decreasing. So after a few initial cycles at the factory the capacity is known and then the cells are discharged to 50% SOC for storage and shipping. Additionally, resting voltage can indicate SOC even in the middle with an accurate enough meter. A cell at 3.300 is going to be at 50% SOC or pretty darn close.


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## JRP3 (Mar 7, 2008)

From what I've seen I think TS/Winston may ship at 50% SOC and CALB may ship at 40% SOC. As I remember when I got my SE, (CALB) cells they were below 3.300V.


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## Beemer (Jun 2, 2011)

Yay, when cells are made, they hard cycle charge and discharge to (believe it or not) equalise the cells characteristics. Actually very slightly plating the anode and cathodes.

Whoever makes the cells, sell them at the same SoC.

*valerun:
Another question I asked was about safe range of high-frequency ripple current from controller. Anyone has any idea on this forum?*

These cells have a high internal capacitance. Why are you bothered? They can take up to ten seconds to even look at accepting a charge. Makes regenerative braking a bit of a joke.


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## valerun (Nov 12, 2010)

Beemer said:


> Yay, when cells are made, they hard cycle charge and discharge to (believe it or not) equalise the cells characteristics. Actually very slightly plating the anode and cathodes.
> 
> Whoever makes the cells, sell them at the same SoC.


Hm, why did I get the voltages all over the place on the first discharge, then?.. Oh, well, probably best safe than sorry and parallel all cells, discharge to same low voltage (2.8V or so) and then wire in series for the pack. 

V


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## Beemer (Jun 2, 2011)

valerun said:


> Hm, why did I get the voltages all over the place on the first discharge, then?.. Oh, well, probably best safe than sorry and parallel all cells, discharge to same low voltage (2.8V or so) and then wire in series for the pack.
> 
> V


I have no idea when others have not had this issue. It's not my place to make assertions or guesses on this matter.


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## Goge (Dec 11, 2008)

My CALB cells were not balanced as-shipped. Within 5 Ah maybe, but not balanced.

I top-balance them individually with a hobby charger. I charge the pack to a low average per-cell V, like 3.4V per cell, and then top off each one with a single 5A charger. The charger gives a total amp-hrs input reading, so I can keep a good record of which cells are needing how much to top off.

I guess that second paragraph was a little off-topic, apologies.

-TH


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## JRP3 (Mar 7, 2008)

valerun said:


> Hm, why did I get the voltages all over the place on the first discharge, then?


Because the cells are not all exactly the same capacity and they are not balanced at the factory as I've explained.


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## Beemer (Jun 2, 2011)

Goge said:


> My CALB cells were not balanced as-shipped. Within 5 Ah maybe, but not balanced.
> 
> I top-balance them individually with a hobby charger. I charge the pack to a low average per-cell V, like 3.4V per cell, and then top off each one with a single 5A charger. The charger gives a total amp-hrs input reading, so I can keep a good record of which cells are needing how much to top off.
> 
> ...


If you restrict your charger max voltage so any cell that reaches the ceiling of series voltage, you will find the current taken off that cell will drop. They will balance themselves near the top eventually. If you want an explanation on cells like these go to
http://goldenmotor.com/ 
Click on the "EVBattery" tab on the left and scroll down to the charge/discharge charactoristics graphs. It says:



> " *3. Self balance*
> 
> Alike lead-acid battery, a number of LiFePO4 cells in a battery pack in series connection would balance each other during charging process, due to large overcharge tolerance. This self balance character can allow 10% difference between cells for both voltage and capacity inconsistency."


Going in reverse at the bottom of charge is a different matter. As we all know cells settle down (or up) when load is taken off. That's why its tedious and a good thing not to keep charging into the upper knee lest it de-bottom balances some of your pack..

If all your cells are within their supposed 5% capacity/Internal resistance. Don't overcharge and don't force a deep discharge then you should never have any issues at all


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## JRP3 (Mar 7, 2008)

> *3. Self balance*
> 
> Alike lead-acid battery, a number of LiFePO4 cells in a battery pack in series connection would balance each other during charging process, due to large overcharge tolerance. This self balance character can allow 10% difference between cells for both voltage and capacity inconsistency."


What? LiFePO4 most certainly does not have a large overcharge tolerance! That's how many cells have been killed, overcharging breaks down the electrolyte and can cause venting while the rest of the pack is still charging. A pack with 10% difference will cause a number of cells to severely over charge, you should be aiming for a pack with 1% differences in capacity or less, which is what we have been seeing from recent CALB deliveries.


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## Beemer (Jun 2, 2011)

JRP3 said:


> What? LiFePO4 most certainly does not have a large overcharge tolerance! That's how many cells have been killed, overcharging breaks down the electrolyte and can cause venting while the rest of the pack is still charging. A pack with 10% difference will cause a number of cells to severely over charge, you should be aiming for a pack with 1% differences in capacity or less, which is what we have been seeing from recent CALB deliveries.


It makes no reference to overcharging.
It refers to the upper charge knee.


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## DIYguy (Sep 18, 2008)

there is no mechanism to self balance series connected LiFePo. They do not exhibit this behavior like pb.


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## Beemer (Jun 2, 2011)

DIYguy said:


> there is no mechanism to self balance series connected LiFePo. They do not exhibit this behavior like pb.


So all those people who charge to say, 3.55V and find their cells slowly coming further into line are all liars then... ookkk.

But remember this. What current into a cell at 3.5V when the cell is 3.5V?
On the other hand what current into another cell at say, 3.2V & 2.5mOhm when the charger is giving an av. shared voltage of 3.5V?

Hmmmm.


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## DIYguy (Sep 18, 2008)

Beemer said:


> So all those people who charge to say, 3.55V and find their cells slowly coming further into line are all liars then... ookkk.
> 
> But remember this. What current into a cell at 3.5V when the cell is 3.5V?
> On the other hand what current into another cell at say, 3.2V & 2.5mOhm when the charger is giving an av. shared voltage of 3.5V?
> ...


The current in a series circuit is the same no matter where in the circuit you sample it. It doesn't matter that the individual cell resistances may be slightly different.


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## Beemer (Jun 2, 2011)

DIYguy said:


> The current in a series circuit is the same no matter where in the circuit you sample it. It doesn't matter that the individual cell resistances may be slightly different.


You are completely correct in the limitations of your definition. But.....

Cell resistances are always constant but the current drops on each cell as they reach charge voltage.

So the current is constantly flowing into a cell equal to the CV charge.. How does that happen?

Most chargers knock off when the load drops. Is this a lie too?

So the current flowing into a cell that is a lower voltage as another is the same as the charged cell in series?
Nah!

Are we going in circles now?


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## valerun (Nov 12, 2010)

Beemer said:


> You are completely correct in the limitations of your definition. But.....
> 
> Cell resistances are always constant but the current drops on each cell as they reach charge voltage.
> 
> ...


Yes, the current is the same for all cells in series. All the different IR does is 'slightly' different additional voltage drop.

V


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## JRP3 (Mar 7, 2008)

Beemer said:


> It makes no reference to overcharging.
> It refers to the upper charge knee.


Actually it specifically says overcharge tolerance, and says nothing about the "upper charging knee". Once you get into the higher part of the curve there is less tolerance for overcharging and you will quickly over volt a cell even at low currents.


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## DIYguy (Sep 18, 2008)

Beemer said:


> You are completely correct in the limitations of your definition. But.....
> 
> Cell resistances are always constant but the current drops on each cell as they reach charge voltage.
> 
> ...


It's not my definition. It's a physics law. 

Charging algorithms for lithium are typically CC until they reach a specific voltage, then they switch to CV. They typically sense voltage between pulses and lower current until it reaches a low set value, then terminate charge.
LiFePo has a pretty flat voltage through most of the charge/discharge curve... so, all the cells will appear very "close". The upper and lower knee of the curves will amplify the difference in SOC.


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## Beemer (Jun 2, 2011)

JRP3 said:


> Actually it specifically says overcharge tolerance, and says nothing about the "upper charging knee". Once you get into the higher part of the curve there is less tolerance for overcharging and you will quickly over volt a cell even at low currents.


That's totally true.

Yet the knee itself automatically balances the cells. In my book the upper knee is getting over 3.5V fulfils no purpose. Thats enough, it will be better next time if the pack is reasonably balanced.

However, the upper knee self balances the cells. That's it. That's what happens. That is what everyone with a correctly set charger with no BMS and no damaged cells finds.

Most good chargers give CC and fall into CV before knocking off.


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## Beemer (Jun 2, 2011)

DIYguy said:


> It's not my definition. It's a physics law.
> 
> Charging algorithms for lithium are typically CC until they reach a specific voltage, then they switch to CV. They typically sense voltage between pulses and lower current until it reaches a low set value, then terminate charge.
> LiFePo has a pretty flat voltage through most of the charge/discharge curve... so, all the cells will appear very "close". The upper and lower knee of the curves will amplify the difference in SOC.


Yes, I was taught that rule in school too. And yes, CV being set will help to level the cells.


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## valerun (Nov 12, 2010)

Beemer said:


> Yes, I was taught that rule in school too. And yes, CV being set will help to level the cells.


CV does not balance cells. There is no way to balance the cells in series unless you have BMS. The current is the same in every cell in series string. 

If your cells are grossly unbalanced (e.g. more than 2-3% SoC difference) and you go for a 3.5V CV cutoff on a string of 50-100 cells, you will find (as we did a number of times through monitoring charge on a per-cell basis) that while the AVERAGE per-cell voltage is 3.5V, you will have cells that go above 4.0V and cells that still sit at 3.4V in your string. Over time, this will kill your high cells as their capacity decreases and they hit their bottom sooner and sooner on discharge...

I would hate to see people damage their packs because of something like the above...

V


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## dladd (Jun 1, 2011)

Beemer said:


> So all those people who charge to say, 3.55V and find their cells slowly coming further into line are all liars then... ookkk.


Who says this? Any links?


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## palmer_md (Jul 22, 2011)

You could parallel all the cells and then run them down with the West Mountain Radio Battery Analyzer. Of course running them all in parallel would give you no useable information, but would computer control the discharge and cutoff. To do a reasonable size pack you'd have to have the 1000 watt amplifier package, and a "normal (?)" sized 24kwh pack could be done in about a day. If you had the time and could charge them individually then you could use the standard unit without the amplifier and get a graph of each cell's capacity which might be useful for assembling the pack. This method, with charging and discharging 40-50 cells could take a month, but would provide a lot of data about your pack.

http://www.westmountainradio.com/cba.php


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## Beemer (Jun 2, 2011)

dladd said:


> Who says this? Any links?


Ironically, everyone who has left or not bought into the BMS world and uses LiFePo4 type cells..
And the Goldenmotor link I sent (They sell packs)
and just about every Professor on the job states what I reiterated
and Jack Rickard who's played this game over many cars and miles
and every manu'fr of multiple celled boxes that only monitor on the pack level
and ..... should I add more?

It's very simple. Don't overvolt and your highest cells sag in consumption of current compared to any others catching up.

Did I at any time state overcharging? All BMS units are nearly always attempting to max up cells with nigh on no exception. They are the culprits to cell murder.

As goes this thread. I'm totally all for bottom balancing. Especially if the inter cell quality is poor. (IR/AH)


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## dladd (Jun 1, 2011)

Beemer said:


> Ironically, everyone who has left or not bought into the BMS world and uses LiFePo4 type cells..
> And the Goldenmotor link I sent (They sell packs)


which, btw, all appear to have built in BMS. Why would they do that if they self balance?


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## dtbaker (Jan 5, 2008)

back to the original question folks... I still haven't seen anyone give 'quick' or automatic way to bottom balance a pack full of cells. This does seem to be an issue for having some reasonable faith that all the cells are starting at the same bottom.

How do you discharge to some set voltage a little bit down the knee, and have faith that you can stop the discharge of each cell at the same point with a good accuracy to call all cells 'balanced' ?

The ONLY way you can 'balance' a whole pack at the top OR the bottom is with the pack wired in parallel and either loaded till voltage drops into the lower knee, or charged until you're into the upper knee.


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## Beemer (Jun 2, 2011)

dladd said:


> which, btw, all appear to have built in BMS. Why would they do that if they self balance?


Read their explanation of the BMS responsibilities. In effect they put them in to create value added... A bit like power stations and "carbon capture" and the extra revenue it will generate. Which is really oxygen capture when you think about it.


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## Beemer (Jun 2, 2011)

dtbaker said:


> back to the original question folks... I still haven't seen anyone give 'quick' or automatic way to bottom balance a pack full of cells. This does seem to be an issue for having some reasonable faith that all the cells are starting at the same bottom.
> 
> How do you discharge to some set voltage a little bit down the knee, and have faith that you can stop the discharge of each cell at the same point with a good accuracy?
> 
> even if you have a one-minute per cell discharge process, that seems like a lot more time that wiring in parallel and TOP balancing the whole pack at once to a set voltage.


Top balancing is easy to do and if your cells are fairly good you can start from there and use your car until you get near the bottom. They say cells should be around 5% from each other. Jack Rickard showed how he done it a couple of weeks back on a cobra with a double tier of boxed cells to 2.7V.


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## dtbaker (Jan 5, 2008)

Beemer said:


> Yes, I was taught that rule in school too. And yes, CV being set will help to level the cells.


? only if they are in parallel.


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## Beemer (Jun 2, 2011)

dtbaker said:


> ? only if they are in parallel.


Irrelevant if the IR is same in all cells. Something the manufacturers set when they are pre-conditioned..


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## DIYguy (Sep 18, 2008)

Beemer said:


> Yet the knee itself automatically balances the cells. In my book the upper knee is getting over 3.5V fulfils no purpose. Thats enough, it will be better next time if the pack is reasonably balanced.
> 
> However, the upper knee self balances the cells. That's it. That's what happens. That is what everyone with a correctly set charger with no BMS and no damaged cells finds.


Ok, sorry for whomever derailed this thread. Perhaps a moderator can split this part off. That said, before going any further and letting this fellow lead anyone down a garden path.. . it needs to be cleared up. These statements above are 100%, totally wrong. Sorry mate, but you need to go do some more homework. I have a large pack of lithium iron phosphate with no BMS. So does JRP. The other folks who chimed in know from experience also. What you are saying is totally incorrect. People who have these cells with no BMS will (and have) corrected you. Jack does not say this, nor do folks who know them well. Regardless if they use BMS's or not.... There was one or two pursuing cell "drift"... but pretty much conceded that argument also. 
What pack of lithium do you have... what is your direct experience? Go do some testing... you will see. This is the whole point of manually balancing....it's because they don't self balance. If jack or anyone else truly believed in self balance, there would be no manual balancing...and no need for BMS's for those who top balance.

There are lots of threads on this...but rather than "typing yourself smart", go do some actual tests.... structured ones....and you will see. 

Lead acid (floodies) boil while others catch up. AGM's need to be balanced, or they will vent and get damaged. Lifepo needs to be limited on upper voltage or damage will also occur. Swelling and electrolyte boiling over 4 volts. . . give or take a little depending on brand.


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## DIYguy (Sep 18, 2008)

valerun said:


> Hi All -
> 
> need some creative solutions how to bottom-balance 200 100AH cells in relatively short time (say, 2 days). They are 60% charged, so ~36kWhrs to burn... All in boxes so can't really 'drive' them down in a car...
> 
> ...


Back to the original quandary. Unless you can come up with a significant load bank... I guess paralleling them will have to do until u get them in the car. Why do you need to do this now while they are in boxes anyways?... not enough to keep you busy? lol (I know that's not the case... lol)


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## Beemer (Jun 2, 2011)

DIYguy said:


> Ok, sorry for whomever derailed this thread. Perhaps a moderator can split this part off. That said, before going any further and letting this fellow lead anyone down a garden path.. . it needs to be cleared up. These statements above are 100%, totally wrong. Sorry mate, but you need to go do some more homework. I have a large pack of lithium iron phosphate with no BMS. So does JRP. The other folks who chimed in know from experience also. What you are saying is totally incorrect. People who have these cells with no BMS will (and have) corrected you. Jack does not say this, nor do folks who know them well. Regardless if they use BMS's or not.... There was one or two pursuing cell "drift"... but pretty much conceded that argument also.
> What pack of lithium do you have... what is your direct experience? Go do some testing... you will see. This is the whole point of manually balancing....it's because they don't self balance. If jack or anyone else truly believed in self balance, there would be no manual balancing...and no need for BMS's for those who top balance.
> 
> There are lots of threads on this...but rather than "typing yourself smart", go do some actual tests.... structured ones....and you will see.
> ...


You're right, there is no cell drift but there is subject drift. Why should I mess up my cells. This matter has already been done to death.
http://www.ri.cmu.edu/video_view.html?video_id=60&menu_id=387

Bottom balancing and his best method:
01:49:05 into the film:
http://media2.ev-tv.me/news080611-1280.mov

p.s. Jack does not actually care about bottom balancing unless the cells are in different states of charge through various reasons. However in this video the Cobra will be sold to a member of the public and many cells will not be easily reached.

Surely, wiring the cells in parallel to match them will be effective on the lower knee. Otherwise you have lots of amps at a silly low voltage to lose somehow and that will be very hard to do.


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## palmer_md (Jul 22, 2011)

palmer_md said:


> You could parallel all the cells and then run them down with the West Mountain Radio Battery Analyzer. Of course running them all in parallel would give you no useable information, but would computer control the discharge and cutoff. To do a reasonable size pack you'd have to have the 1000 watt amplifier package, and a "normal (?)" sized 24kwh pack could be done in about a day. If you had the time and could charge them individually then you could use the standard unit without the amplifier and get a graph of each cell's capacity which might be useful for assembling the pack. This method, with charging and discharging 40-50 cells could take a month, but would provide a lot of data about your pack.
> 
> http://www.westmountainradio.com/cba.php



Seems strange quoting myself, but since I posted this there have been two messages stating that nobody came up with a "quick" solution. My post has two solutions, one requires about 24 hours and the other potentially a month. If Valrun is looking to do it in a couple of days, the first solution would work, but it does require the purchase of $2000 in equipment. Probably not something for someone who is going to do this just once, but if you were going to build more than one car, it would probably be a good investment. Since he has a business of converting vehicles, it might be something of interest to him. Actually if I had a business like that I'd probably invest in a unit that does both the automated charging and discharging of cells and logs the data, so I would have a record of every cell that was installed in every vehicle I sold. Automated units for both are probably over $10k but since I've not seen one for sale, it is pure speculation on my part.


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## dtbaker (Jan 5, 2008)

DIYguy said:


> Ok, sorry for whomever derailed this thread.


well said. 

I am totally with DIYguy on this.... I see no evidence or experienced users, or tests showing that LiFePO4 cells 'self-balance' while in series in use. If fact is is questionable as to whether they 'drift' at all relative to each other over time once balanced because of the minute differences in their internal resistance. The only semi-reasonable concern I have seen is that split packs MY see the split sections drift over time IF the average temp of the cells is significantly different.... but I haven't seen any conclusive test or proof of that either.

there are merits to both top and bottom balancing depending on which condition you consider yourself more likely to encounter, and what technology you place your trust in to protect the 'unbalanced' end.

so.... back to the original post... given that the only way to bottom BALANCE is to apply some load while cells are wired in PARALLEL, I would refine the OP to ask what load source have people found that can burn off energy reasonably quickly to drain the pack, and then how are they catching the pack and stopping the load at some set low-voltage target.


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## JRP3 (Mar 7, 2008)

"Self balancing" means what? The cells somehow grow closer in total capacity, the small ones get bigger, and/or the big ones get smaller? By what mechanism? If "self balancing" means they "self top balance", again, by what mechanism? Which would also mean they "self unbalance" at the bottom. 
I run with no BMS, bottom balanced, and have not seen my cells change in relation to each other at the top by more than a small amount. What may happen is that as we know cells actually gain capacity over the first 50 cycles or so. This effect *may *be more pronounced in cells taken to a higher SOC so the smaller cells *may* gain a bit more capacity. This is pure speculation on my part. However as stated my smallest cells still reach full first as shown by their voltage rise. These day's I'm stopping charging when all cells are between 3.40 and 3.41, but if I continue to charge my smallest cells will head to 3.50 while the rest stay below 3.45. No real self balancing.

As to the initial question, I simply installed all my cells in the car, drove till they were at 3.10V average, then pulled them down from there, first in blocks of 4 with a 12V headlight until one or more started dropping lower than the rest, then with a resistive circuit that cut off at 2.98V. I'd probably aim for 2.85-2.9V now to get them deeper into the knee. Remember you need to pull them slightly below target voltage since they sag under load and then wait for them to recover some to check resting voltage.


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## valerun (Nov 12, 2010)

DIYguy said:


> Back to the original quandary. Unless you can come up with a significant load bank... I guess paralleling them will have to do until u get them in the car. Why do you need to do this now while they are in boxes anyways?... not enough to keep you busy? lol (I know that's not the case... lol)


;-)) Well, as I mentioned above, I tried that and the string of 27 cells from the box, from the same batch got very unbalanced (to the tune of 2-3% SoC) on discharge. Good thing I caught it. Since then I suspect they are not really bottom-balanced at the factory... better safe than sorry.


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## valerun (Nov 12, 2010)

palmer_md said:


> Seems strange quoting myself, but since I posted this there have been two messages stating that nobody came up with a "quick" solution. My post has two solutions, one requires about 24 hours and the other potentially a month. If Valrun is looking to do it in a couple of days, the first solution would work, but it does require the purchase of $2000 in equipment. Probably not something for someone who is going to do this just once, but if you were going to build more than one car, it would probably be a good investment. Since he has a business of converting vehicles, it might be something of interest to him. Actually if I had a business like that I'd probably invest in a unit that does both the automated charging and discharging of cells and logs the data, so I would have a record of every cell that was installed in every vehicle I sold. Automated units for both are probably over $10k but since I've not seen one for sale, it is pure speculation on my part.


Thanks palmer. in the end, here's what we've done:
1. connect cells in parallel into groups of 30-50
2. build the 'loads' - ~10 feet of gauge 12 copper wire wound on an aluminum cylinder.
3. Immerse the loads (one per group of cells) into a large tank of water (has to be able to absorb up to 30kWhrs without boiling out - 0.5 liter per cell seems to be working fine
4. assemble a small Arduino-based 'controller' with 6 ADC channels monitoring up to 6 groups of cells. Wire 6 digital outputs to control transistors that control contactors that switch the loads
5. write a small Arduino program that monitors the voltage and cuts the load when string voltage goes below 2.7V. Waits a bit. If the voltage rises above 2.9V again, turn on the loads again. 

this way, we got all 140 cells to 2.845V within 0.001V of each other in a few hours.

V


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## Yukon_Shane (Jul 15, 2010)

I'm not sure if this is exactly what you're looking for but I did find a post that described a "bottom balancer" that Dimitri used to make/sell.

http://www.diyelectriccar.com/forums/showthread.php?t=39025&highlight=Dimitri+bottom+balance

It doesn't appear to be a super fast way to bottom-balance but it would save you having to do it manually with a multimeter and a resistor...

It might be worth asking him about?


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## JRP3 (Mar 7, 2008)

Yup that's the one I used to finish off my bottom balancing.


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## JRP3 (Mar 7, 2008)

valerun said:


> Since then I suspect they are not really bottom-balanced at the factory...


They are not. Why did you think they were?


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## valerun (Nov 12, 2010)

JRP3 said:


> They are not. Why did you think they were?


someone on this thread was pretty convinced about that...


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## EVfun (Mar 14, 2010)

valerun said:


> ;-)) Well, as I mentioned above, I tried that and the string of 27 cells from the box, from the same batch got very unbalanced (to the tune of 2-3% SoC) on discharge. Good thing I caught it. Since then I suspect they are not really bottom-balanced at the factory... better safe than sorry.


If you hook them together in parallel (as received) for long enough I think you would mid-point balance the pack. That would leave them several percent different at both top and bottom. If you want to bottom balance it is important to pick a low SOC that is down in the rapidly dropping range.

I took 8 cells I had that where between 3.29 and 3.31 volts and hooked them in parallel with 6 inches of 16 gauge wires between each ring terminal. I was able to measure current flow in the wires (by reading voltage to 0.0001 volt) for over 2 weeks (about 2 milliohms per jumper, so down to about 50 milliamps.) About a week after I couldn't measure the current flow I took the jumpers off. They are closer at 3.30 volts.


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## GizmoEV (Nov 28, 2009)

Beemer said:


> These cells have a high internal capacitance. Why are you bothered? They can take up to ten seconds to even look at accepting a charge. Makes regenerative braking a bit of a joke.


Not to derail this thread again but do you have any reference or data to support this statement? If they have high internal capacitance then the energy from regen is capacitively stored at the minimum, isn't it?


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