# Initial Lithium pack balance?



## EVfun (Mar 14, 2010)

I'm about to parallel the buggy pack up to balance it. The pack is 32 Thunder Sky cells. I'm planning a top balance as it will be easier for me to limit the discharge and charging should be a little easier to adjust when they all come up together. 


I've been pouring over the threads for recommendations about the setup and didn't find as much as I'd hoped for. Here is what I have in mind:

```
+ ------------------------    
    |  |  |  |  |  |  |  | 
    _  _  _  _  _  _  _  _    
    -  -  -  -  -  -  -  -    
    |  |  |  |  |  |  |  |    
    ------------------------ -
```
Well, mine will be 32 cells long but the idea is to put a ring terminal with 2 wires in it at each positive and each negative terminal and bring the positive and negative out on opposite ends so wiring resistance and any voltage drop from the charger should be equal (as much as possible) for every cell. Right now I have the cells in series so I will charge in series while watching to get them as close as possible before paralleling them. My plan is to balance them at 3.7 volts per cell as that is a little higher than my target charging voltage of about 3.6 volts per cell.

What size wire is recommended? I know the wire should be large enough to handle the charger current, but that only requires 16 gauge wire. Is larger needed to allow the cells to self balance? Each wire (from + to + or from - to -) will be about 6 inches long and the wires coming off each end for the charger will be about 3 feet long. 

Will the cells move into balance slowly if they are not being charged or discharged? I'm in that big flat middle range with all 32 cells reading 3.31 volts each. 

Is there anything I'm missing here?

Thanks for your input!


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## nimblemotors (Oct 1, 2010)

I would suggest to actually test the capacity of each cell before building a pack with them. Given the ability to do that, you can top each cell, not sure why you want to parallel all of them.



EVfun said:


> I'm about to parallel the buggy pack up to balance it. The pack is 32 Thunder Sky cells. I'm planning a top balance as it will be easier for me to limit the discharge and charging should be a little easier to adjust when they all come up together.
> 
> 
> I've been pouring over the threads for recommendations about the setup and didn't find as much as I'd hoped for. Here is what I have in mind:
> ...


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## EVfun (Mar 14, 2010)

I actually don't have the equipment to capacity test cells. The usual goal is to simply balance them, either at the top or bottom depending on how charge termination and discharge cut-off are being handled. 

This is the most information initial balance or equalizing I found in the forum.


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## nimblemotors (Oct 1, 2010)

that is a good point. But look at the published cycle life, 80% 2000 cycles, 70% 3000 cycles, so you really don't want to take ANY cell down to full discharge, not even the weakest ones, since it will in effect increase the cost of your batteries by 50% (if we believe their cycle life claims, which apparantly nobody has confirmed!! )

So the best is to count the amp/hours and consider it empty once 70% rated capacity has been drained, do not depend on voltage.
And check each cell to see the smallest capacity cell, and use it as your 70% value. So if you have 100ah cells, and one is 90ah, remove only 63ah from a full charged pack. yep it will reduce your range. 



gottdi said:


> I'd do the bottom balance for sure. The problem lies when you are out on a drive and you are getting to the point where the pack will begin to drop but is still showing good voltage you may in fact have a cell that has gone below the safe point and you now have a full pack with plenty of poop left driving it's current through a single or maybe a couple cells which will drive it to the ground and kill it. The problem is you will not know it's happening until it's too late as your voltage meter will still show you should have plenty of power left. Now bottom balanced cells will have a ragged top but if you charge your batteries and have a decent cut off point you may have a cell that during the charge is in the 4.1 volt range but maybe not quite that high. Since it is not a static voltage reading of 4.1 volts your still going to do just fine. If a cell actually went into the danger zone with the charger your cell is only being charged by the charger and you don't have a full pack of cells trying to drive current through that cell. The cell will more than likely be just fine. You will kill a cell faster if you top balance and have a cell or two reach that danger zone too fast and you'd not be aware. Things happen real fast. I did a video about bloated cells from over discharge. You can only use the capacity of the lowest common denominator so whether you top or bottom balance your usable capacity will always be limited to those cells. The higher capacity cells will just be along for the ride. Since you WILL from time to time take your car to the limits of discharge your best bet is to have the whole pack reach that deep discharge at the same time so you don't fry a cell or two. I guarantee you will at some point take your car to the limits. It is far safer for cells and your vehicle to have your cells bottom balanced. Just because its easy to top balance does not mean it is a good thing to do. I put my pack in a middle of the road balance and lost two cells at the bottom while out on a drive. One cell bloated up like a fat pig. No venting but is sure got hot and bloated up real bad. Got a video of dissecting a cell and finding out a couple things. Don't top balance your cells. When things happen at the bottom they happen fast. It was the last couple miles.
> 
> Now that the pack is bottom balanced sort of I have not had my pack get hot nor any single cell get hot. I am going to balance them again better at about 2.6 volts static then charge them up. I will have my end voltage cut back set for 2.8 and my cutoff at 2.5 volts. My charge will be at 3.6 and terminate at 2 amps. NO BMS.
> 
> ...


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## EVfun (Mar 14, 2010)

gottdi said:


> No, your not correct. You take your cells to the bottom. Balance them and then only take them to the low portion before the knee.


I was really hoping this wouldn't devolve into another top balance vs. bottom balance debate. Top balancing is more suited to my application. I have even tighter control over discharge cut-off than charge termination due to a little feature called "low lift lockout." (and yes I will go there, gas cars do)

How 'bout the rest of the process, ignoring the chosen voltage for the moment? Will the cells self equalize if paralleled while resting at 3.31 volts each? My understanding has been that voltage is a good indicator of SOC at all levels, except that you would need to accurately measure to within a few ten-thousandths of a volt when the cell voltage was in the flat middle range. What wire gauge is recommended?


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## nimblemotors (Oct 1, 2010)

cells are paralleled to increase capacity, it forces them all to the same voltage. I'm not convinced this is a good thing, unless they are matched cells, but could be wrong. I really think you need to confirm how matched they are before putting them in a pack. It isn't difficult to manually do it, I'd start with bottom balance, just drain a serial pack very slowly and use a DVM to check each cell as it drains until one starts to reach empty, then quit the serial discharge, and do each cell. This can take a while, it took me three days to balance all 38 cells in a prius pack.
Once they are all discharged, then do the same charging, in serial until you are near full (in terms of amp/hours) then slow it down and check each cell as they reach full. Now you can see which cells reach full first, then charge each cell individually to see how much they differ.




EVfun said:


> I was really hoping this wouldn't devolve into another top balance vs. bottom balance debate. Top balancing is more suited to my application. I have even tighter control over discharge cut-off than charge termination due to a little feature called "low lift lockout." (and yes I will go there, gas cars do)
> 
> How 'bout the rest of the process, ignoring the chosen voltage for the moment? Will the cells self equalize if paralleled while resting at 3.31 volts each? My understanding has been that voltage is a good indicator of SOC at all levels, except that you would need to accurately measure to within a few ten-thousandths of a volt when the cell voltage was in the flat middle range. What wire gauge is recommended?


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## DIYguy (Sep 18, 2008)

EVfun said:


> How 'bout the rest of the process, ignoring the chosen voltage for the moment? Will the cells self equalize if paralleled while resting at 3.31 volts each?


Yes...............


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## EVfun (Mar 14, 2010)

gottdi said:


> If you top then you will need to know what cells drop out first and set your cut off above that. Bet you find it's harder to control than you think. I won't debate but will state facts.


I think you misunderstand how I intend to terminate discharge. I will be counting amp hours out and stop after discharging my smallest cell to about 20% SOC. The amp hour counter can drive the battery level gauge and its low lift lockout function can short the pot box. I figure if gas cars can abruptly run out of fuel sometime around when the gauge says empty an EV can shut down the pack when the amp hour counter says exactly empty. Anyway, if safety was a primary concern in the buggy I might start by installing seat belts.


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## Roy Von Rogers (Mar 21, 2009)

In theory this is what you need to do to set up a pack for the first time.

If the pack is in parallel put a load on the pack til the pack is empty with no load, depending on which cells you have, 2.9v to 3v is a generally good figure.

Now connect the pack in series and charge it CC/CV to 3.5v x cells, if you have 32 cells thats 112v.

The problem is that finding something for a discharge device at 3 volts is difficult. Another option is to assemble them 4 at a time in series, since many times they come already packed that way. Connect a 12v load to the 4 pack and discharge to 12v no load. Do that to all, and then connect all in series and let them sit for a day or so. Check the voltage after previous mentioned time span, and if the pack is somewhere at 96v, consider the pack empty and charge pack CC/CV to112v, the pack is now full.

Get yourself a good amp hour dash counter and dont drive your pack past 75 to 80% of pack capacity. Monitor your pack via either a 3 wire half pack unbalance indicator, or if you want to check all, get an inexpensive Cellog device available at hobby shop dealers, be aware you need to mod the cellog device so it powers up on all wires.

Just make sure that any monitoring device is properly wired and fused. DONT voltage balance these cells to the top via a shunt balancer, its a waste of money and does exactly the OPPOSITE of what you want to do, and could be dangerous to boot.

If you have a 100ah pack and you fill it with 100 ah, the battery is full, any cell thats above the lowest single cell capacity you need to ignore, for its NOT USABLE energy.

Roy


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## DIYguy (Sep 18, 2008)

Paralleling the pack will equalize it at what ever the avg SOC is. Light gauge wire is fine since current will be pretty low.

If you want to top balance, you need three things. A DVM that measures to 3 decimal places, a method to charge and a method to discharge. Ideally, for top balance, you want to be up around the 3.5 volt range or so, to get a decent balance. Charge the pack till you get close. Charge/discharge individual cells until you get them the same (resting), to three decimal places.

I'm sure there are lots of variations to this...but that's basically it.


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## EVfun (Mar 14, 2010)

DIYguy said:


> Paralleling the pack will equalize it at what ever the avg SOC is. Light gauge wire is fine since current will be pretty low.
> 
> If you want to top balance, you need three things. A DVM that measures to 3 decimal places, a method to charge and a method to discharge. Ideally, for top balance, you want to be up around the 3.5 volt range or so, to get a decent balance. Charge the pack till you get close. Charge/discharge individual cells until you get them the same (resting), to three decimal places.


 Why charge/dischareg individual cells to get them the same? Couldn't I just run the cells in parallel up to 3.65 volts and hold until the current tapers to a fraction of an amp? I was hoping if I did that, then disconnected the charger and then left them in parallel for an extra 24 hours, the pack would be well balanced at the top.


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## EVfun (Mar 14, 2010)

gottdi said:


> I understand. You going to use a zeva? I will be using one for amp counting.


Yes, that is what I've got. The other available amp hour counters should work equally well provided they are equally accurate. 

Other ones that come to mind are the new unit from Dimitri, the TBS meter, the offering from Metric Mind, and the old E-meter.

I wanted to add that I don't KNOW this will work, but if I toast a cell or two on a top balanced pack it was the smallest cell anyway, right? I have a few spares so it would mostly be the time to rebalance the pack. I'm not present all the time when charging (but am while discharging -- driving) and I really want the cells to behave when I'm away. I want to be able to charge to 114 to 115 volts (32 cells) and let the current taper off without concern that any are going high in my garage at night. I want to do that without using a BMS. If my first attempt at counting amps out doesn't work out well perhaps I'll try another one.


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## DC Braveheart (Oct 12, 2008)

I don't suppose some bright spark around these parts could come up with a simple (inexpensive ) circuit that could be attached to a cell to discharge it to a set voltage (say 2.8V) and then stop to allow easy bottom balancing (I'd have thought a zener diode in conjunction with a resistive load of some sort could be use to control that in some fashion ... but it's a bit beyond me).

Also, if I understand all the above, it's probably not a good idea to bottom balance and then use a shunt-type top-balancing BMS during charging? My logic is that if some cells have a lower capacity their voltage will go higher quicker during charging, the current shunt will kick in and they will end up with less stored charge. Then when you discharge them they'll go lower then all the other cells and possible get damaged? Let me know if I've understood this correctly.


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## DIYguy (Sep 18, 2008)

EVfun said:


> Why charge/dischareg individual cells to get them the same? Couldn't I just run the cells in parallel up to 3.65 volts and hold until the current tapers to a fraction of an amp? I was hoping if I did that, then disconnected the charger and then left them in parallel for an extra 24 hours, the pack would be well balanced at the top.


Sure... but do you have a charger that will charge at 3.65 volts? If so, do you have a few weeks/months to wait? lol

Just charge them in series...then balance them off... or tie them in parallel then check to see how close they are. Just depends how fussy u want to be I guess..


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## EVfun (Mar 14, 2010)

If you are going to run a BMS with shunting regulators you shouldn't bottom balance. The effect of the shunt regulators is to top balance the pack. I would recommend top balancing to help the regs along (they generally can't shunt very fast.)

For the buggy pack I am planing to top balance without running a BMS. Discharge will be limited by amp hour counting.


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## DIYguy (Sep 18, 2008)

EVfun said:


> If you are going to run a BMS with shunting regulators you shouldn't bottom balance. The effect of the shunt regulators is to top balance the pack. I would recommend top balancing to help the regs along (they generally can't shunt very fast.)


I would agree with that.


EVfun said:


> For the buggy pack I am planing to top balance without running a BMS. Discharge will be limited by amp hour counting.


If you are going BMS-less, I would have to side with Pete. Bottom balance and cut charge short. The risk is much lower.


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## EVfun (Mar 14, 2010)

DIYguy said:


> Sure... but do you have a charger that will charge at 3.65 volts? If so, do you have a few weeks/months to wait? lol
> 
> Just charge them in series...then balance them off... or tie them in parallel then check to see how close they are. Just depends how fussy u want to be I guess..


I will initially charge in series because it is easy to do. I will terminate the charge so the first cell to rise doesn't exceed 3.6 volts. Then I will then parallel the cells and hopefully it won't take to long to bring them all up. 

I figured I could just take my cheap 10 amp 6 volt charger and adjust the output by running the input power through a variac. I can put a motor run capacitor on the output (to read the output with a DMM) and adjust the variac to whatever I want. I would run that in the evening when I was home and hopefully they would come up within a few days. I figure it will be pretty easy to put in 32 amp hours per evening (about 1 amp hour per cell) and the cells would be starting out nearly full.


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## brainzel (Jun 15, 2009)

I get aditional 7 cells to my existing 38 cell pack (SE/CALB 130Ah) within the next days and so I have to deal with the same problems.

The new cells will be expected at ~50% SOC.
So I only have to options:
Charge them to upper knee 3.5V or discharge them to 2.8V lower knee and do the same with my "old" pack.

*But what does this meen practical?*
top-balance:
- charge the old pack in series to 3,45V / cell
- single charge each old cell up to 3,5V / cell and let them rest
- single charge each new cell up to 3,5V / cell and let them rest
- wire them together and drive the car down to max. ~20% SOC

bottom-balance:
- dirve the car down to ~20% SOC
- discharge each old cell down to 2,8V / cell and let them rest
- discharge each new cell down to 2,8V / cell and let them rest
- charge the old pack in series to 3,5V / cell
- wire them together and drive the car down to max. ~20% SOC

optional in both procedures, but perhaps unnecessary if you did a good job above?:
- wire all cells in parallel and let them rest/equalize before serial charge

"top" seems to be much quicker and easyer to handle theoretical, but my mind wants me to do it at the bottom ;-)

Are these the ways to go? Feel free to comment it please.


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## ruckus (Apr 15, 2009)

EVfun said:


> I'm about to parallel the buggy pack up to balance it. The pack is 32 Thunder Sky cells. I'm planning a top balance as it will be easier for me to limit the discharge and charging should be a little easier to adjust when they all come up together.
> 
> 
> I've been pouring over the threads for recommendations about the setup and didn't find as much as I'd hoped for. Here is what I have in mind:
> ...



I just paralleled 30 Thundersky 260ah batts. I wouldn't worry about putting the + and - out different sides of the pack as they will automatically "balance" themselves if left in parallel for a bit. Even though I had done quite a bit of 12v motor testing and 12v car circuit testing (lights, etc), there was no discernible difference in the cell voltages and no spark when connecting them in parallel. 

I hooked up a 3.65v .7A charger. I know it will take a year to charge the pack full, but if I had done this when the batts first arrived they would now be full.  ..and balanced.

Please be VERY suspect of those who claim top or bottom balance is better. They are both equally problematic. If you read through the forums you will find multiple stories from those with LOTS OF EXPERIENCE that bottom balance and charge to knee with no bms DOES NOT WORK -sooner or later the weakest cell will be overcharged and go "pop" which causes the charger to charge even harder and the next cell goes pop on down the line until your pack is gone. It is JUST as dangerous as top balance and driving down to the knee only to have the same weak cell turn to dust.

This is why I started the bms thread to see if anybody had a good way to balance across the ENTIRE spectrum to avoid the Lilliputian arguments.
http://www.diyelectriccar.com/forums/showthread.php/future-bmsi-55352.html?p=230156#post230156

How many amps can you charge at 3.65v? my .7A is beyond weak...  I guess if I got 2 or 3 and put them in parallel it would only take 3-6 months.


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## Roy Von Rogers (Mar 21, 2009)

ruckus said:


> Please be VERY suspect of those who claim top or bottom balance is better. They are both equally problematic. If you read through the forums you will find multiple stories from those with LOTS OF EXPERIENCE that bottom balance and charge to knee with no bms DOES NOT WORK -sooner or later the weakest cell will be overcharged and go "pop" which causes the charger to charge even harder and the next cell goes pop on down the line until your pack is gone. It is JUST as dangerous as top balance and driving down to the knee only to have the same weak cell turn to dust.


If you start out with an empty 100ah battery pack and you fill it with 100ah, the battery pack if full, after all thats what you purchased. If any cells are above the lowest capacity cell due to variance in production, its unusable capacity. And if you voltage charge each cell to its max capacitance, you have an unbalanced pack, exactly what you DONT want.

Yes you can fill each cell to its full capacity and then use a downcounter to not exeed the max capacity of your pack, the qestion is why would you want to do so, since it puts the cell in to destruct mode if you ever get too close to the bottom, and there is no realiable indication while driving that a cell is going in to destruct mode. Remember these vehicles may be driven by your wife/daughter/girlfriend, so KISS needs to be applied.

Anyone that has watched JR at EV-TV charge a set of redwood sticks, should by now understand why these cells need to be charged that way.

No one that I know recommends that these batteries do not need some monitoring, thats what needs to be discussed. What is the best and simplest way to monitor a pack so anyone will have an indication that there is a problem, just like the check engine light on an ICE vehicle, and a fail save charging system in case there is a failure while the vehicle is being charged.


Roy


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## ruckus (Apr 15, 2009)

Please explain how battery failure from bottom balancing and then accidental overcharge is better than battery failure from top balancing and then accidental over-discharge??? 

Battery failure is battery failure.

If you drive the lowest cell into the ground you have lost one cell.

If one cell goes pop while charging the charger won't shut off and you will lose the whole pack. This is better?

I am trying to avoid the bottom vs top argument by saying they BOTH create vulnerabilities which must be carefully monitored. 

This cannot be denied.

Cheers


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## DIYguy (Sep 18, 2008)

ruckus said:


> Please explain how battery failure from bottom balancing and then accidental overcharge is better than battery failure from top balancing and then accidental over-discharge???


Gawd I wish I didn't bite.... again... lol 

If you top balance and over discharge, you will pass much higher current through the cell reversing it and killing it almost certainly. Several hundred amps is likely. JR did this three times in a row to prove his point.
If you overcharge at top, you will pass what your charger is capable of... and most likely all it is passing in CV mode (could be CC mode, but more likely CV mode) The current is drastically lower and likelihood of killing it is remote. Damage yes, perhaps. 
Top or bottom balance each has its place and advantages/disadvantages. I'm not advocating one over the other... just sayin, if u are going BMS-less, there is less risk with bottom balance.

When u run out of gas with an ICE... ur walking. When u start to run out of electrons...ur kinda "not done" all at once. BMS, no BMS, fuel gauge, bells and whistles... or god damn elephant on the hood.... my wife or daughter would drive the damn car till it won't go anymore. (even u might just push it a bit) Bottom balance will just eventually stop. Tow it home, charge it up... and other than perhaps shortening the # of cycles a bit... it will most likely be just fine. THAT'S THE REAL DIFFERENCE... So... I would suggest a bottom limit that could be fed to a controller that will stop at some point, and I think that is what most do or will do, or plan on doing.


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## Roy Von Rogers (Mar 21, 2009)

ruckus said:


> Please explain how battery failure from bottom balancing and then accidental overcharge is better than battery failure from top balancing and then accidental over-discharge???
> 
> Battery failure is battery failure.
> 
> ...


The problem is that there are two scenerios involved here and they need seperate attention.

1) how to fill all cells with the proper capacity (charging)so it cant be driven in to destruction via unequal discharge.

2) how to prevent any cell to be overcharged in to destruction.

To fill cells with the correct amount of power means to start with an empty battery pack, and charge pack till the first cell hits just below the knee voltage, and count the amp hours that went in to the pack, thats your base line of pack capacity. 

At that point you can consider the pack fully charged, no matter if the rest of the possible higher capacity cells are at a lower voltages, your pack capacity is determined by the lowest capacity cell. And while I'm on that subject...stop calling it the weakest cell, its not a weak cell, its just the baseline cell, unless of course its less then stated capacity, then you turn it back to the supplier as a bad cell.

Within the charging system you need to have a realiable way of terminating the charging process. All chargers need to have a build in charge termination (charge curve). Also one needs to have some backup system in case of charger failure. Such back up can consist of one or more monitoring components. No matter what terminates the charger, no charger should ever restart after any charge termination, no matter if the charge was proper, or halted by a safety component. If halted by safety component a warning system needs to alert owner of such failure, and such warning must be reset manually. Normal charge termination has to have a latching relay so charger can only restart by unplugging charge plug first.

So, if all cells have an equal amount of capacity and you have a proper amp hour down counter, you can confidently drive by looking at your capacity gauge and no cell should hit destruct mode, for all capacities go down equally.

On a personal note, I'm of the opinion that most are not purchasing the right size battery packs. If one expects to drive an average of 50 miles, the vehicle should have a 100 mile pack, if the expectation is higher, one should size the pack high enough so one should never reach the bottom of a pack exept in an unusual circumstance. After all the most important thing about an EV is electricity (Batteries), if you cant afford the most important item in an EV, maybe you shouldnt be building one, till you have the proper funds.

Roy


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## MN Driver (Sep 29, 2009)

Both situations work, you can 'top' balance or 'bottom' balance and still not lose cells.

If you top balance and use a BMS and you don't want to lose cells, be sure your BMS has a low voltage cutoff function. It's simple, you won't lose cells that way through overdischarge and you have all of your cells in line when charging. This is where Rickard stopped paying attention because he killed cells on the Gem and completely ignored that most BMS's have a low voltage cutoff built in to protect them. Setting it at a low voltage below where a discharge would be at low temperature and high discharge, such as 2 volts and you save your cells and keep driving.

On the other side of the coin, you have everything lined up at the bottom, hopefully. Seems its moved a bunch in Rickard's case in the Speedster with some cells down under a volt and others just below 3v, but either way we'll say its in line when discharged, you set the voltage of your charger so that it is at its max voltage at the end of charge when the highest voltage cell hits the highest voltage you want it to be.


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## EVfun (Mar 14, 2010)

DIYguy said:


> When u run out of gas with an ICE... ur walking. When u start to run out of electrons...ur kinda "not done" all at once. BMS, no BMS, fuel gauge, bells and whistles... or god damn elephant on the hood.... my wife or daughter would drive the damn car till it won't go anymore.


Or just have the low lift lockout function of the amp hour counter short out the pot box. That will safely stop you from discharging your pack more than whatever state of charge you wish to define as empty. When you run the pack low on my EV... "ur walking."

I drove that buggy around for 8 years with a gas engine and a reserve handle instead of a gas gauge (gotta love old Beetle "technology.") To run out of "fuel" and be able to read that is about to happen is an upgrade for that car.


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## Roy Von Rogers (Mar 21, 2009)

MN Driver said:


> Both situations work, you can 'top' balance or 'bottom' balance and still not lose cells.
> 
> If you top balance and use a BMS and you don't want to lose cells, be sure your BMS has a low voltage cutoff function. It's simple, you won't lose cells that way through overdischarge and you have all of your cells in line when charging. This is where Rickard stopped paying attention because he killed cells on the Gem and completely ignored that most BMS's have a low voltage cutoff built in to protect them. Setting it at a low voltage below where a discharge would be at low temperature and high discharge, such as 2 volts and you save your cells and keep driving.
> 
> On the other side of the coin, you have everything lined up at the bottom, hopefully. Seems its moved a bunch in Rickard's case in the Speedster with some cells down under a volt and others just below 3v, but either way we'll say its in line when discharged, you set the voltage of your charger so that it is at its max voltage at the end of charge when the highest voltage cell hits the highest voltage you want it to be.


You can do whatever you please, afterall its your EV and your hard earned money. I've been in the technical field for over fifty years, what do I know, maybe nothing.

If you notice, I dont post a lot, for some of the stuff that gets posted in here is ridicules, and plain stupid at times, I'm not even going to waste my time and energy arguing over it. I dont have a dog in the hunt, I dont sell magic bullets or blue elephant guns, my business is not related to EV's, and when I give my services to my customers, they pay me $85 an hour plus parts and are happy cause I'm reasonably priced.

And the comment you made about Jack, shows you werent paying attention or it was beyond your pay grade.

Roy


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## brainzel (Jun 15, 2009)

I would like to read more about the practice, see some pics or discuss the way to got to top or bottom (in my case bottom), not about BMS or "religious matters" of top/bottom.


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## Roy Von Rogers (Mar 21, 2009)

brainzel said:


> I would like to read more about the practice, see some pics or discuss the way to got to top or bottom (in my case bottom), not about BMS or "religious matters" of top/bottom.


I explained it the best I could in post#13...

Btw, read your offer an der deutsche ebay....I'm tempted..lol

Roy


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## brainzel (Jun 15, 2009)

Roy Von Rogers said:


> Btw, read your offer an der deutsche ebay....I'm tempted..lol


Take your chance, Roy ;-)



Roy Von Rogers said:


> If the pack is in parallel put a load on the pack til the pack is empty with no load, depending on which cells you have, 2.9v to 3v is a generally good figure.
> Now connect the pack in series and charge it CC/CV to 3.5v x cells, if you have 32 cells thats 112v.


Yes Roy, I would agree, but discharging at 3.2Volts to 2.9V with a 130Ah pack is a tough thing 
Additional the two level problem of new and old cells ...

My current plan:
- drive the 38 cell pack to ~50% by Ah-meter
- wire the complete 45 cell pack
- drive around to another ~30% by watching the new and old battery voltage
- stop at own garage
- start the electric heater (3kW)
- wait until the voltage drops to 2,8V at the first cell
- take all cells out of the car
- get every single cell manually down to 2,8V or 2.7V by the same load
- wire them parallel
- equalize for a night
- voltage measurement
- if they are all at one level, start charging in series to 3.5V / cell

How does this sounds to you?


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## brainzel (Jun 15, 2009)

Roy Von Rogers said:


> Anyone that has watched JR at EV-TV charge a set of redwood sticks, should by now understand why these cells need to be charged that way.


In tribute to Jack, to visualize the "redwood stick model" the and *not* to start any fire (top/bottom balance), I made a hopefully helpful grafic.

It shows the two ways to discharge an about ~130Ah CALB pack at 1C and what could happen, if you don't know your pack.


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## Roy Von Rogers (Mar 21, 2009)

brainzel said:


> Take your chance, Roy ;-)
> 
> 
> Yes Roy, I would agree, but discharging at 3.2Volts to 2.9V with a 130Ah pack is a tough thing
> ...


Well I did mention that doing four at a time would be easier to discharge, since its about a 12v pack, and there are many loads available at 12 volts..lol

Roy


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## MN Driver (Sep 29, 2009)

Roy Von Rogers said:


> You can do whatever you please, afterall its your EV and your hard earned money. I've been in the technical field for over fifty years, what do I know, maybe nothing.
> 
> If you notice, I dont post a lot, for some of the stuff that gets posted in here is ridicules, and plain stupid at times, I'm not even going to waste my time and energy arguing over it. I dont have a dog in the hunt, I dont sell magic bullets or blue elephant guns, my business is not related to EV's, and when I give my services to my customers, they pay me $85 an hour plus parts and are happy cause I'm reasonably priced.
> 
> ...


I'm not arguing over it(top/bottom balancing) either, I was posting to indicate that it can be done both ways successfully. You read into it the wrong way and don't even know how I would do it myself, seems you've made an inaccurate assumption.

Beyond my pay grade? I didn't know you were going to pull a personal attack over your opinion. A civil chat is usually what most people come to this forum for, didn't know you would decide to go abusive. Seems Jack has gotten to you. "I dont sell magic bullets or blue elephant guns" Sounds like Jack. ...if you want to come across as respectful, which clearly you don't seem to be, you might want to step back and take a breath. I was only stating the fact that his initial balance was no longer as close as it was before and that the cells did drift. I'm not debunking or trying to say anything against bottom balancing other than that you really still need to be careful and at least check once and awhile because things clearly moved a little for him.


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## Roy Von Rogers (Mar 21, 2009)

MN Driver said:


> I'm not arguing over it(top/bottom balancing) either, I was posting to indicate that it can be done both ways successfully. You read into it the wrong way and don't even know how I would do it myself, seems you've made an inaccurate assumption.
> 
> Beyond my pay grade? I didn't know you were going to pull a personal attack over your opinion. A civil chat is usually what most people come to this forum for, didn't know you would decide to go abusive. Seems Jack has gotten to you. "I dont sell magic bullets or blue elephant guns" Sounds like Jack. ...if you want to come across as respectful, which clearly you don't seem to be, you might want to step back and take a breath. I was only stating the fact that his initial balance was no longer as close as it was before and that the cells did drift. I'm not debunking or trying to say anything against bottom balancing other than that you really still need to be careful and at least check once and awhile because things clearly moved a little for him.


 
This is where Rickard stopped paying attention because he killed cells on the Gem and completely ignored that most BMS's have a low voltage cutoff built in to protect them.


I was refering to your above statement, he wasnt ignoring anything, he did it on purpose to see what happens when one top balances. And in the "charging redwood sticks" showed why. And brainzel put up a marvelous presentation of it earlier.

I may have to send brainzel a "DANKE FÜR DIE BLUMEN"..

Roy


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## EVfun (Mar 14, 2010)

I'm not buying your brand of Lithium religion Roy. He did something no one should do and it ruined cells. That's not news worthy. They can be destroyed just as easily with a charger. One end or the other will be a bit jagged and you have to put some additional protection on it.


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## MN Driver (Sep 29, 2009)

Roy Von Rogers said:


> This is where Rickard stopped paying attention because he killed cells on the Gem and completely ignored that most BMS's have a low voltage cutoff built in to protect them.
> 
> 
> I was refering to your above statement, he wasnt ignoring anything, he did it on purpose to see what happens when one top balances. And in the "charging redwood sticks" showed why. And brainzel put up a marvelous presentation of it earlier.
> ...


You seem to be ignoring what I said too, even though you quote *exactly what he ignored*. If you top balance and you have a *low voltage cutoff* set to a low enough level to protect your cells but not disrupt normal driving. You won't kill cells!! You are saying that when one top balances that they will. With a low voltage cutoff, you won't because that is what is protecting your pack.

You are telling me that he isn't ignoring a necessary part of driving when driving low SOC with a top balanced pack. You should have a low voltage cutoff. He ignored this. Don't tell me he didn't because if he had this in his top balanced pack, his Gem wouldn't have lost cells.

Do you see what I'm saying?? The redwood sticks show that the capacity is different so the voltage profile is going to be uneven at the end of discharge opposite of where you balance. This is obvious, dealing with it in the appropriate manner for how you balance is required. Shunts are not required either so if you fear them, they aren't needed as a top balance is easy to measure at the end of charge and doesn't need to be brought down to a low voltage to check the cells.

You can do it either way but just be sure you know what you are doing and why you are doing it.


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## dtbaker (Jan 5, 2008)

MN Driver said:


> You can do it either way but just be sure you know what you are doing and why you are doing it.


...exactly. My personal decision to top balance is based on a couple things:

- its easy to do an initial balance with cells in parallel and a variable power supply set to some value in the upper knee.

- the TOP voltage I want to be in balance so the charger has the best chance of catching all cells close to target voltage without needing added shunting balancers. I do a full charge EVERY DAY, and would prefer to have my charger handle it automatically as designed per charge curve. As long as the top is close to balanced, total pack voltage will trigger charger without any single cell going ballistic.

- the ragged bottom low cell death can be avoided in my case mostly because my typical daily errand miles will only use about 50% DOD. I don't plan on attempting a 'run to the last drop' test. Worst case is I could add a low-voltage alarm on the pack voltage if it hits 2.9*38. Sure this requires some driver training, but if my wife drives the car 55 miles when I tell her to go a max of 40, then I guess I deserve to lose a few cells.


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## EVfun (Mar 14, 2010)

dtbaker said:


> [snip]I do a full charge EVERY DAY [snip]
> 
> - the ragged bottom low cell death can be avoided in my case mostly because my typical daily errand miles will only use about 50% DOD. I don't plan on attempting a 'run to the last drop' test. Worst case is I could add a low-voltage alarm on the pack voltage if it hits 2.9*38. Sure this requires some driver training, but if my wife drives the car 55 miles when I tell her to go a max of 40, then I guess I deserve to lose a few cells.


That first point I clipped out of your post is one of the main reasons I started looking toward level at the top. Most charges will take the pack to the top, but few discharges will take the cells near the bottom. 

Your idea on how to alarm for the bottom won't work with a top balanced pack. The point of top balancing is that they all come up through 3.6 volts together, but then don't go below 3 volts together. You might have 12 at 3.1 volts, 12 at 3.0 volts, 12 at 2.9 volts, 1 going down fast at 2.6 volts, and the last 1 dying at -0.3 volts. In this case the pack low voltage alarm is happily letting you drive without caution. 

Something more is needed for low level cut-off if going with a top balanced pack. Cell monitors (BMS) that detect low cell voltage is one way. They don't need shunts or even high voltage detection for this. I couldn't find anything that seemed right for the unique application of my EV Buggy. Perhaps a battery bridge would work, and I'm pretty sure cell block monitoring could work (for example, watch 4 cell blocks and don't allow any block below 11.6 volts.) 

I'm going to use an amp hour counter to cut off the discharge when the pack should be considered empty. Most of these units offer a "low lift lockout" function. On forklifts this is used to keep the forks from being raised when the pack is too low but it could be used to short out the pot-box.


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## dtbaker (Jan 5, 2008)

I don't really ever want to KNOW which cell is the short capacity, so I hate to think of installing a low-volt monitor/trigger alarm on all..... perhaps an audible alarm just on raw 'trip miles' ?! 40 miles and the yellow alert siren starts?


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## EVfun (Mar 14, 2010)

dtbaker said:


> I don't really ever want to KNOW which cell is the short capacity, so I hate to think of installing a low-volt monitor/trigger alarm on all..... perhaps an audible alarm just on raw 'trip miles' ?! 40 miles and the yellow alert siren starts?


Something like that may work so long as you don't live in San Francisco or Seattle (hills.) (or let me drive)

I think counting amp hours should work so I intend to try it. I think this can run the fuel gauge and use the low SOC alarm function to shut the car down by shorting out the pot box (or make lights and buzzers go crazy inside the car, or any combination of annoying things designed to convince the driver it would be better to just get a tow.) Since it is counting amp hours how I drive or where I go should automatically be taken into account. There are other amp hour counters with these functions, Dimitri is coming out with version 2 of his amp hour counter and it includes a low alarm, the TBS E-expert Pro comes with a low amp hours relay, and it was an option on the old E-meter too. Pretty much any forklift amp hour counter has this functionality, called a low lift lockout.


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## dtbaker (Jan 5, 2008)

EVfun said:


> I think counting amp hours should work so I intend to try it.


me too... only issue I am seeing so far is gaining confidence in the calibration of the amp counter. The 
'cycle analyst' meter is inexpensive, but also seems not to agree with hand-held meultimeter despite repeat efforts to callibrate and zero amps, etc.


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## ruckus (Apr 15, 2009)

dtbaker said:


> I don't really ever want to KNOW which cell is the short capacity


Why not? Find the weakest cell, slap on a guage/meter, Done.




dtbaker said:


> 40 miles and the yellow alert siren starts?


I don't want to sound snotty, but this is really dumb. How about you smash the gas gauge in your ICE car and use the odometer to figure out when to put in gas? Sooner or later you will forget to hit the trip meter, do a partial fill, etc. and have NO CLUE how much energy you have in the vehicle. I can see you hopping out on the side of the interstate and trying to measure cells to figure out if you can make it home. Ridiculous. 

How about a power outage? You wake up in the morning and are not sure how many hours it got charged. A simple gauge on the lowest capacity cell would tell all. But your proposed system would have no clue. You even admit the cell counters don't agree with one another, so how could you trust it?

Applying Murphy's law, why would you trust a more complicated method over a simpler method? There is more to go wrong. A counter has to count. This means it has to measure AND keep a tally of the measurement. A Voltage meter merely has to measure. So simple. One gauge.

I don't think a bms is manditory, but you have to compensate by monitoring.


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## EVfun (Mar 14, 2010)

ruckus said:


> Why not? Find the weakest cell, slap on a guage/meter, Done.


More likely the cell is done. You just placed a small additional load on your weakest cell. If you are going to operate a LiFePO4 pack without a BMS (and want it to work) you have an obligation not to imbalance your pack with small parasitic loads. There are about 8800 hours in a year to slowly bleed off charge. Pretty soon your pack isn't balanced at the top or the bottom.


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## dtbaker (Jan 5, 2008)

ruckus said:


> Why not? Find the weakest cell, slap on a guage/meter, Done.


first, slapping a gauge one one cell would make a weak cell weaker and CREATE imbalance. second, just how do you propose to identify the 'weakest' cell.... discharge all the way? sounds brutal on the cell. My point is that I hope to never even come close to 100% DOD on ANY cell. 





ruckus said:


> I don't want to sound snotty, but this is really dumb.


well you do sound snotty, and thats not a good idea in a Forum where we all try to HELP each other through areas of ignorance. much different than being dumb.



ruckus said:


> How about you smash the gas gauge in your ICE car and use the odometer to figure out when to put in gas?


you've never had a car with a broken gas gauge? This is EXACTLY what you do....



ruckus said:


> How about a power outage? You wake up in the morning and are not sure how many hours it got charged. A simple gauge on the lowest capacity cell would tell all. But your proposed system would have no clue.


?? I do have a Cycle Analyst on the PACK, which does count amp-hr, I also typically have my power in thru a kill-a-watt meter out of curiosity, so I actually DO have areally good idea how much energy is in the pack. I also have a pretty set routine of errands and know my town well enough that I could probably tally daily miles without a trip od.



ruckus said:


> Applying Murphy's law, why would you trust a more complicated method over a simpler method?


ok, lets.... the simplest solution is ONE gauge on pack, and top balancing so that the charger handles the end-of-charge.


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## dtbaker (Jan 5, 2008)

gottdi said:


> ..you will always have cells that have different capacities. It is just the bottom is the safer side of balancing. Safer for the cells.
> I find it interesting you insist it is the top where you want to balance.


absolutely there will be differences in cell capacity. I maintain that in MY case I believe it is safest for MY cells to top balance because I am depending on my charger automatic CA->CV trigger to end my charge automatically , without cell-level shunting, or manual timer to end charge way early. Since I want to depend on my cells all coming up to 3.65v as nearly as together as possible, that is by definition top-balancing.

I acknowledge the potential for catastrophic discharge if I get down to 100%DOD where the short cell(s) would bite the dust. I just happen to feel that the chances of me ever going below 80% DOD are slim enough to 'risk' going with top balance to gracefully manage my daily charge cycle as simply and dependably as possible. There are numerous ways to prevent the over discharge, and in my case I am comfortable NOT using cell-level monitoring or alarms to protect the bottom end.

All the chargers out there have charge curve set to trigger on pack voltage approaching some set value at the TOP in the knee. If you balance at the bottom, you have to add SOMETHING to monitor at cell level and stop charge when the first cell hits the upper target.

this is why I still believe that in my case top balance is better. Its not a personal or religious issue, and I am open to new thought, but as far as I can tell top balance enables a charger automatic curve to work the way it is supposed to.


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## Guest (Mar 4, 2011)

> All the chargers out there have charge curve set to trigger on pack voltage approaching some set value at the TOP in the knee. If you balance at the bottom, you have to add SOMETHING to monitor at cell level and stop charge when the first cell hits the upper target.


No actually you don't. If you know the cell(s) starts to jump fast at lets say 3.8 then you can set your CV to 3.65 or 3.7 and hold there until your amperage drops to a predetermined point like 2 amps or 0 amps. That will keep you from ever having a cell go over the top. I do understand your way and if you are sure you won't go to the low end then by all means go with top. For you the risk low. For me the risk is high. Cost of cells are to pricey to take that risk.

I know where my cells start to jump so keep my CV set point to 3.65 and I have one cell that reaches 3.78 volts at the end of charge. All my cells reach my end point at the same level. So you see I am safe at both the top and bottom. Safer at the bottom but each person must make those risk assessments. 

Just promoting the safer side of the risk assessment. 



Pete


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## spdas (Nov 28, 2009)

Pete, would you not then just knock a few AH off the cell that is 3.78v? If you have a 3.78 then you must have some undercharged cells or one cell lagging around 3.30 or so @ maybe 3/4 charge if you are charging series @3.65v?
Francis


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## dtbaker (Jan 5, 2008)

spdas said:


> Pete, would you not then just knock a few AH off the cell that is 3.78v? If you have a 3.78 then you must have some undercharged cells or one cell lagging around 3.30 or so @ maybe 3/4 charge if you are charging series @3.65v?
> Francis


if he took a few AH out of the high cell, that would be top balancing.


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## ruckus (Apr 15, 2009)

Why will the bottom balance crowd not acknowledge that some who tried their method FAILED because their batteries went "pop" under charging. This causes the charger to never hit hvc and destroys the entire pack.

This is no different than a top-balancer running to 90% dod and killing a cell.

Both ends have risks. Both catch people and kill cells. This has been well documented, so I am surprised to hear folks saying it is safe. It is not. You might get away with it for a while, but it won't work on a societal level.

This is why the manufacturers recommend a bms. They should know, they build the batteries in the first place.

What I don't understand is the anti-bms crowd relies on a charger to shut off at the right time, but don't trust a bms to do the same. Is there something I am missing? Probably.

Please be clear, I am not advocating one method or system over another. I am merely saying that both bottom and top balancing have risks and both methods have been PROVEN to fail. Sometimes catastrophically. 

Why are folks so against cell-level shunting? If the cells stay as balanced as you expect, then there would be very little time between the first and last cell getting "full" and charger shutoff. Again, what am I missing? Sure, JR says they are fire hazards, but so are controllers. and batteries. and chargers.

The bottom balance folks say they can avoid damage during charging by setting the charger a bit low. How is this different than a top-balancer who sets their controller cut-back to a conservatively high voltage?


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## dtbaker (Jan 5, 2008)

ruckus said:


> ...I am surprised to hear folks saying it is safe.


neither is 100% safe... just a matter of assessing your personal case. building your own EV is not a 'production quality' venture by any stretch suitable for every driver.



ruckus said:


> Why are folks so against cell-level shunting?


I don't know about other folks.... my my objections to cell-level BMS, especially an active shunting one, are cost and introducing MANY more points of potential failure.


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## EVfun (Mar 14, 2010)

I'm not so sure it is SAFER Pete. While it can happen much quicker at the bottom you will be present counting the seconds because you will be doing the killing. At the top it usually happens while you are away, possibly asleep. Either way, it was just the smallest cell, right? (getting tired of that one anyway  )

I figure that if I don't try anything new I cannot possibly learn anything new. So I decided that the top is worth a try. Amp hour counting can shut down the bottom, it should be reliable but that is what I'm aiming to find out. It seems to me that if you are bottom balanced then almost every cycle you have a cell going up to the 3.7 to 3.9 volt range. If I balance at the top then perhaps once every 10 cycles I take one down to 2.8 to 3.0 (likely not even that often.) It seems to me that top balance will generally keep the smaller cells away from the ends more of the time because more charges are to full than discharges are to empty.


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## ruckus (Apr 15, 2009)

I am thinking EVfun's logic is sounding solid. Let's pretend he top balances and NEVER goes below 50% dod. His batteries will be very happy for a very long time.

However, if he bottom balances and does the same, he will be pushing a few batts into the danger zone EVERY CHARGE. That is obviously a worse scenario which will lead to shorter cell life, if not sudden failure.

Now, if he is planning to pull 80-90% out every time, then I agree that bottom balance would be good, but the battery specs clearly state that this behavior will shorten the life by almost 50%, so why do it?

The engineer who designed the first John Deer tractor (never produced) noted that every design will fail rather quickly when run at 100% capacity. He stated that you must build a design for 150% of the intended task if it is to prove reliable.

The same is true of EV's. 70% dod should be a worse-case scenario, not a regular event. If the batts live 60% longer by reducing the dod by 10%, then reducing it another 10% will increase longevity even more. 

Everyone seems to want to cheat the specs by undersizing their pack, pulling out high C-rates, and pushing the discharge to the limit. This can be done, but it is NOT a recipe for longevity or reliability.

Correct?


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## MN Driver (Sep 29, 2009)

It depends on what people predict in the future.

If we think we will see a better battery at a lower cost 5 years down the line, then someone buying a larger, heavier, more expensive, yet more capable pack might bring about a heavier, more energy consuming, less fun to drive car. There is a balance point to it and I personally think that leaving 30% on the table is good protection against a forced detour, being stuck in a funky stop and go traffic situation, or for when we under predict the total miles to a trip. My highway work commute is relatively short and actually half of the range I want my car to be and I'm actually looking to get around 100 miles to 0% and plan on owning an 80 mile car if I want to hang on to the battery for a long time. ...but to plan out a 50 mile at 80% discharge car is going to be far cheaper. I'd use my gas car more for the very occasional trips to the outer suburbs but I need that car to drive cross country anyway so I figure its not bad to give it some use maybe the one time I month I go over 50 miles round trip in addition to the times I need to leave my metro area.

I'm not entirely convinced that we will see a dramatically shorter pack life pulling, say 1C at constant. A guy from the UK drains year 2008 LiFePO4 batteries rated at 4C for 15 seconds and 1/3C constant discharge/charge and charges them at over 1C for 10-15 seconds at a time with regen and has discharged them in 45 minutes at a constant current a number of times. One cell out of the 50 was lost early on from internal resistance skyrocketing with a bloated cell and was replaced under warranty.

It's hard to tell what cycle life we get but I'm more inclined to take a chance and buy cheap cells and beat on them a little bit and if I lose seem to start getting any issues, I'll buy more cells and raise my voltage to get back capacity and make the ride from them a little gentler. I still think that cycle life is going to be fairly flat as long as we can keep them cool. I've seen 140 degrees F(60C) from tests as being the highest you want them before they degrade quickly. I'm looking to aim for 120 degrees F as the max and measure at the hottest terminal that I find in my initial shakedowns of the car after its ready.


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## ruckus (Apr 15, 2009)

Let me throw out a calc or two:

If a car running 60mph on the highway uses a steady 12KW, then the pack will have to be 24kw at 1c in order for the expected load to be .5c. 24kw / 200ah cells is 120v. This means we need 38 200ah batts. At $250 per cell this is $9500. At 70% dod this pack has 140ah. 1.4 hours @ 60mph = 84 mile range.

Now, if I take the same $9500 and buy gas (currently at $3.15) I get 3015 gallons. At 25 mpg this is 75,400 miles. Divide this by our EV range of 84 miles and you get 900 charge cycles. At 30 mpg it is just over 1,000 cycles.

The $2-3 nightly charge will add another 20%, so you are definitely breaking into the green if you can get 1500+ charge cycles from the system, even if electricity goes up. (gas will definitely be going up).

If the math is correct, even if you only get 3000 cycles (.5c @ 70% instead of .3c @ 70%) this is HALF the cost of driving gas. Given this, isn't it smarter to oversize the pack and get the 3000 cycles rather than push it?

3000 x 84 = 250,000 miles!! A decent life-cycle by any measure.


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## JRP3 (Mar 7, 2008)

ruckus said:


> Why will the bottom balance crowd not acknowledge that some who tried their method FAILED because their batteries went "pop" under charging. This causes the charger to never hit hvc and destroys the entire pack.


Can you provide any links showing where this happened with the large format prismatic cells from CALB or Thundersky? I've never heard of anyone doing what you describe.



> Why are folks so against cell-level shunting? If the cells stay as balanced as you expect, then there would be very little time between the first and last cell getting "full" and charger shutoff. Again, what am I missing? Sure, JR says they are fire hazards, but so are controllers. and batteries. and chargers.


Because people installed cell level BMS's and had them fail, draining and killing a number of their cells. I'd like to see a comparison between the number of cells lost with and without BMS's but we'll probably never have that data. The fact is the closer your cells are grouped the less need you have for a BMS and the less it matters if you top or bottom balance. Keep the cells at the same temperature and use split pack monitoring to tell you if something has gone wrong. Putting devices on each cell which can fail and drain the cell seems risky until we see greater long term reliability from those devices.


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## EVfun (Mar 14, 2010)

Perhaps someone has an "I don't care" pack and wants to do an experiment...

It seems to me that the weakness of top balancing is that you can kill a cell on discharge while the pack voltage as a whole is O.K. In minutes, seconds even, I could kill a cell by reversal if I go to far. I'm going to use amp hour counting to deal with that risk.

It seems to me that the weakness of bottom balancing is that you can kill a cell on charge if that top voltage set point and/or time at that voltage is not strictly adhered too. If your charger drifts a couple of volts between summer running on 120 vac to winter and running on 240 vac that couple of extra volts gets taken out on one or just a few cells, quickly running them over 4 volts. I don't know what step others are taking to mitigate this risk.

What if 1/2 the pack was top balanced, say to 3.70 volts and tapering down to 0.1 amps. The other half of the back was bottom balanced and then charged, manipulating the charger a bit at the end (a Manzanita Micro PFC would be perfect for this) so the charge was terminated as the first cell hit 3.70 volts at 1/2 amp. Now put the pack together in series and turn on the charger, adjust up the voltage so the top balanced cells are again sitting at 3.7 volts with little current flowing. You would now have a pack that should protect itself against over discharge because half the pack is going to stop dropping voltage like a rock as their bottom is reached. It should be sharp enough and big enough to easily program you controller to simply not allow the pack voltage to go low enough to hurt all those cells. It would be much easier for your charger to protect your pack because fully half the cells start climbing fast as they reach full together. Even if the charger drifted a couple of volts, or the timer failed one night, the top balanced cells would rise up just an extra tenth of a volt or less and force the current to essentially zero. After 5 years of driving it would be real interesting to do internal resistance and capacity tests on a number of cells from each half too!


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## EVfun (Mar 14, 2010)

I just paralleled the 10 cells I have that are not going into the buggy. The first 8 had been sitting with their BMS cell modules on them for about 4 months. They where still sitting at 3.32 volts each (the EVworks modules only draw about 2 milliamps.) The other 2 had never been charged since shipped from Thundersky. They dropped about 0.01 volt each over the last 13 months, both where resting at 3.29 volts. 

The BMS was removed and all 10 where put in parallel with 16 gauge wire, good tin plated wire and tin plated copper ring terminals. Each wire is 6 inches long and 2 of them are crimped into each ring terminal. I could measure the current flow between the cells by measuring the drop on the wires. Initially, I had about 1 amp flowing into the never charged 2 cells from the other 8 (2.1 millivolt drop between terminals.) You could see how all 8 batteries behind shared that current by watching the millivolts drop as you checked backwards away from the 2 never charged cells. After 5 hours all the cells now show as 3.31 volts and the 2 cells never previously charged as down to taking about 1/4 amp from the other 8. It is great to see the charge shuttling so clearly visible with the millivolt meter.


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## JRP3 (Mar 7, 2008)

Just remember that cells at 3.31V are not necessarily at the same SOC. Also, why leave BMS modules on cells that aren't being used? A bad module can kill a cell without you knowing until too late, we've seen it happen.


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## tomofreno (Mar 3, 2009)

> If you know the cell(s) starts to jump fast at lets say 3.8 then you can set your CV to 3.65 or 3.7 and hold there until your amperage drops to a predetermined point like 2 amps or 0 amps. That will keep you from ever having a cell go over the top.


 No it won't. Voltage at a given SOC during charge is a function of cell temperature and charge current. If you bottom balance, and set the voltage limit on your charger as you describe above at a cell temperature of 60 F, when you charge at 95 F you may well overcharge your lowest capacity cell unless all the cells are very close in capacity. Because at 90 F the ir of the cells will be lower resulting in less voltage drop per cell while charging and lower pack voltage for a given SOC. The charger will keep on charging until it hits the PACK voltage you have set, possibly driving the low capacity cell to over charge since most of the other cells are still down on the flatter part of the V versus Ah curve so pack voltage doesn't change much with Ah added. Similar may occur when charging at a lower current than the one you did the above set up at. I have seen this many times with my Manzanita. (Edit: Either I with my dvm, or the minibms have been there to shut it down before it overcharged a cell, except for once when the I had killed the minibms LM339 chip and didn't know it, and I got out there a bit late. It drove two cells to 3.95V that time before I caught it.)

Edit: The same will hold for electricar's relay which shuts off current to the charger at a given pack voltage. If the pack is bottom balanced, and the relay trigger voltage is set in winter when the pack is colder, the charger may well overcharge the lowest capacity cell before the relay triggers in warmer summer temperatures.


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## EVfun (Mar 14, 2010)

JRP3 said:


> Just remember that cells at 3.31V are not necessarily at the same SOC. Also, why leave BMS modules on cells that aren't being used? A bad module can kill a cell without you knowing until too late, we've seen it happen.


Have you seen it happen when the cells are not being charged (AC line racket) or discharged (controller ripple)? I have not seen a BMS module fail sitting calmly on a cell that is not in use. I have seen a BMS module drain a cell dead, but I know the amount of power these units draw. I figured they would slowly pull the cells down from 100% SOC, where they where at after the last charge. Lithium cells shouldn't be stored at 100% SOC, it actually accelerates aging some. Also, it is a pain to remove a BMS when there are 8 cell modules with soldered wires (BMS signal loop) running between them. 

It appears that the 8 cells where at the same SOC based on the current flowing between them. The 2 never charged cells are on the left of the row. The highest current is in the jumper to that pair of cells. As I check jumpers farther right the current drops at each cell (so they are sharing the milliamps of load to charge the 2 never charged cells.) The 2 never charged cells are sharing the current from the other 8 based on the millivolt drop in the jumper between them. This morning the current flowing to the 2 never charged cells is down to about 100 milliamps. Yesterday it started at 1 amp and by evening was down to 250 milliamps. The cells are all reading 3.31 volts right now, except the 2 at the far right flick back and forth between 3.31 and 3.32.

I can accurately measure down to 1/10 millivolt, but that sensitive scale only goes up to 300 millivolts. It allows me to watch the current flow in a 16 gauge wire pretty well (about 0.0021 ohms per 6 inches.) I've been told that if I could accurately measure cell voltages to 4th digit right of the decimal I actually could determine relative SOC based on voltage. I have no means the accurately test that.


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## tomofreno (Mar 3, 2009)

> I figure that if I don't try anything new I cannot possibly learn anything new. So I decided that the top is worth a try.


 That's the way I felt about it, so I recently top balanced my pack of CALB cells after running for about 1 1/4 years, 8k miles, bottom balanced. I gave the impetus for it some time back in the bms or no bms thread: If bottom balanced there is wide variability in SOC the lowest capacity cell is charged to due to voltage for a given SOC when charging being a function of cell temperature and charging current. Near end of charge, when bottom balanced, only the lowest capacity cells are near or on the exponential part of the V versus Ah curve, the rest are down on the part of the curve with much lower slope, so pack voltage increase with Ah added is relatively small. As a result, the variability in pack voltage with charge current level and cell temperature for a given SOC causes considerable variability in final SOC the lowest capacity cell is charged to if all cells are not very close in capacity. 

If a pack is top balanced the change in pack voltage with SOC near end of charge is much larger since all cells are near or on the exponential part of the V versus Ah curve. In this case variability of pack voltage with charge current level and cell temperature for a given SOC doesn't result in nearly as much variability in the SOC the lowest capacity cell is charged to, as the voltage increase near end of charge causes the charger to cut back current quickly to a very low level if it is set up so it's limit voltage is hit a bit below this point. 

Running bottom balanced I've seen the SOC of the lowest cell vary by 10Ah between charging in early morning when cells are cool, and in the afternoon when they are warmer. If the charger limit voltage is set for full charge at the cooler temperature the lowest capacity cell will be overcharged at the higher temperature.

I have now run with charging currents between 7 and 27A top balanced. The charger limit voltage is set such that the charger shuts off 2 to 5Ah short of "full" (defined for these tests as when any cell hits 3.5V) when charging at higher currents, and shuts off at about full or within 1 Ah of it for all other charge current levels. At 7A a few shunts turned on at 3.5V briefly before the charger shut off, but charge current was down to less than 1A by then and dropping quickly so when they shut off it was at about 0.2A (I added the shunts to the minibms cell level boards when I decided to try running top balanced). Before, when bottom balanced, if the charger limit voltage were set this way I would have overcharged my lowest capacity cell at currents below around 18-20A. This will of course depend on how closely your cells are matched in capacity. Mine are within around 2-3% as far as I can tell. (Edit: that includes 4 cells that were not part of the original pack)

I have a TBS gauge for charge counting and never discharge my lowest capacity cell below 30% SOC, and usually only 35 to 40% SOC, so no danger of over discharging it. Also, the minibms LVC turns on a buzzer and the "Check Engine" light, and cuts the throttle by 50% if an LVC event occurs. It also shuts off the charger when a cell hits 3.6V should the charger fail to shut off before then (and I'm usually watching). 

I also recently mounted the un-isolated Manzanita on rubber feet with nylon bolts so it is isolated from the chassis (12V ground) like the pack is. So now if I drop a bleed resistor or cell log 8 lead or while hooking it up to a cell while charging, I will no longer short AC to ground possibly damaging the minibms (that's how I killed the LM339 chip). I have a GFCI breaker, but it doesn't act quickly enough to save chips from damage.

So far I like running top balanced better because of the ability to run the charger at widely different currents and have it shut off before over charging a cell. I've now taken to turning on the charger at night and charging at 7 to 12A, so it shuts off about an hour after I rise in the morning. I also charge at 28A at an out of town RV park to extend my range, and in this case the charger shuts off a bit less than full, depending on cell temperature (usually warmer since the car has traveled around 25 miles). I likely will have to adjust the charger limit voltage this spring or summer. The nice thing about winter is the cell heaters keep the cells at the set point temperature unless I've driven and warmed them up. So usually the cell temperature at start of charge is at the set point eliminating one variable, though they do heat up a couple degree at the highest charge currents depending on total Ah of charging.

Edit: btw, I've run with shunts on for over 45 minutes with charge current set to about 3/4A as a test. They feel just slightly warm to the touch.


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## JRP3 (Mar 7, 2008)

Ah you've gone to the dark side   You've laid out a good argument for running top balanced, it does make the finish charge more accurate. If your pack is large enough that you stay far away from the bottom in all situations I'm sure it won't be a problem. I'm planning on playing with reducing my finish voltage to stay in the lower part of the knee and adjust for temperature differences by adjusting the charge current. Having once gone into turtle mode I still prefer the comfort of bottom balancing. I'd say for recent CALB purchasers it's all a moot point since the cells seem to be so closely matched. At some point these top/bottom discussions will seem quaint


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## Guest (Mar 29, 2011)

I already did that. All my cells were balanced and charged at the top at 3.7 volts. I took the car to 25 miles and the pack still showed I was well above the danger zone but had lost two batteries in the pack and they were HOT. They also had a very low voltage when I stopped compared to the rest. They were driven into the ground and it did not take long. I never new i had two cells going south until I stopped. Two swelled like two fat pigs. I guarantee you will bust a battery and I was not the first to show this. The experiment is repeatable all the time. I guarantee it. I now bottom balance my pack. Never have had a cell over 3.8 and I take my cells to 3.6. I have even recharged while the batteries were warm from a drive. So cold or hot the differences are small so if your pack is reasonably matched then you won't have a problem with a bottom balance and busting out a cell on charge. Even if a cell did to into the 4 volt range it would not do what those two cells did when I drove full pack voltage and amperage through them. Charging will only pump in by that time a few amps so there is no danger unless the controller malfunctioned. If that happens then your screwed. 




EVfun said:


> Perhaps someone has an "I don't care" pack and wants to do an experiment...
> 
> It seems to me that the weakness of top balancing is that you can kill a cell on discharge while the pack voltage as a whole is O.K. In minutes, seconds even, I could kill a cell by reversal if I go to far. I'm going to use amp hour counting to deal with that risk.
> 
> ...


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## Guest (Mar 29, 2011)

> and never discharge my lowest capacity cell below 30% SOC, and usually only 35 to 40% SOC, so no danger of over discharging it.


Now if we all stayed in that range of SOC then it would not matter one bit if you top or bottom balance. But we all know that isn't the norm. We know that just about every one if not everyone has taken their ev to the bitter end of the charge. It happens and it won't ever stop. In that light you do need to be sure your bottom is not a ragged bottom or you will drive pack current through the low cells and quickly destroy them. A ragged top with a couple cells in the higher zone but not crazy will do just fine and never see pack amperage driven through them. EVER. That is what kills an out of balance pack at the bottom. A ragged bottom WILL kill cells. It has been shown and proven and has been repeated to further enforce that proof. There is no doubt. No argument. No Shit. If your pack is new from the factory you neither need to top or bottom balance it because it comes balanced in the middle where it should be. Keep off the bitter bottom and bitter top cold or hot and you will be just fine. I noticed no difference in charging a hot pack vs a cold pack in pack end voltages. Must it be done AGAIN? How many times does it need to be done and shown before anyone listens. How many cells must die before anyone listens. The testing is so you don't have to loose cells. The things are expensive. 

Pete


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## JRP3 (Mar 7, 2008)

gottdi said:


> I already did that. All my cells were balanced and charged at the top at 3.7 volts. I took the car to 25 miles and the pack still showed I was well above the danger zone but had lost two batteries in the pack and they were HOT.


What's your normal range with that pack? If it's more than 25 miles then something else was wrong with those cells since even with bottom balancing you can only get as much range as your smallest cells will allow.


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## EVfun (Mar 14, 2010)

gottdi said:


> I already did that. All my cells were balanced and charged at the top at 3.7 volts. I took the car to 25 miles and the pack still showed I was well above the danger zone but had lost two batteries in the pack and they were HOT.


If you top balanced your pack you did not do this:



> What if 1/2 the pack was top balanced, say to 3.70 volts and tapering down to 0.1 amps. The other half of the back was bottom balanced and then charged, manipulating the charger a bit at the end (a Manzanita Micro PFC would be perfect for this) so the charge was terminated as the first cell hit 3.70 volts at 1/2 amp. Now put the pack together in series and turn on the charger, adjust up the voltage so the top balanced cells are again sitting at 3.7 volts with little current flowing. You would now have a pack that should protect itself against over discharge because half the pack is going to stop dropping voltage like a rock as their bottom is reached. It should be sharp enough and big enough to easily program you controller to simply not allow the pack voltage to go low enough to hurt all those cells. It would be much easier for your charger to protect your pack because fully half the cells start climbing fast as they reach full together. Even if the charger drifted a couple of volts, or the timer failed one night, the top balanced cells would rise up just an extra tenth of a volt or less and force the current to essentially zero.


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## tomofreno (Mar 3, 2009)

> Now if we all stayed in that range of SOC then it would not matter one bit if you top or bottom balance. But we all know that isn't the norm. We know that just about every one if not everyone has taken their ev to the bitter end of the charge. It happens and it won't ever stop.


 No, not everyone has taken their pack "to the bitter end".



> A ragged bottom WILL kill cells. It has been shown and proven and has been repeated to further enforce that proof. There is no doubt. No argument. No Shit.


 No it won't. It will only kill cells if you drive cells to zero charge and reverse them. If you use a charge counter to ensure your lowest capacity cell is not discharged below 20% SOC a "ragged bottom" is no problem.


> I noticed no difference in charging a hot pack vs a cold pack in pack end voltages. Must it be done AGAIN? How many times does it need to be done and shown before anyone listens. How many cells must die before anyone listens.


 I have. Many times. Cell voltage during charge is a function of SOC, cell temperature, and charge current.


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## tomofreno (Mar 3, 2009)

> Ah you've gone to the dark side


 Apparently Pete thinks so.



> You've laid out a good argument for running top balanced, it does make the finish charge more accurate. If your pack is large enough that you stay far away from the bottom in all situations I'm sure it won't be a problem.


 I think so. I'll measure cell voltage readings down around 30% SOC to see what the variability is out of curiosity. 


> I'm planning on playing with reducing my finish voltage to stay in the lower part of the knee and adjust for temperature differences by adjusting the charge current.


 I did play with that a bit - using higher charge current at higher temperatures, lower current at lower temperatures to try to cancel out the effects. It did seem to reduce the variability, but I didn't do enough to quantify it.


> Having once gone into turtle mode I still prefer the comfort of bottom balancing.


 That would definitely put the fear in you! My tact was to resolve to never discharge my lowest capacity cell below 30%. I have enough range that I can easily do that without any inconvenience. I think as you've said for a long time, you can go a long way toward avoiding trouble by just buying more, or larger capacity cells, so you have more than enough range for your needs and don't need to charge or discharge to near the limits. I'd say that, and a charge counter may be the best protection you can have.


> I'd say for recent CALB purchasers it's all a moot point since the cells seem to be so closely matched. At some point these top/bottom discussions will seem quaint.


 Yeah, boy those sound like nice packs! They will have it easier, but I don't know if their packs will remain that way for years. I know the CMU prof said there was no mechanism for them to drift apart, but that doesn't seem correct to me. We know they loose capacity as they age. I see no reason to think they will all loose capacity at exactly the same rate. Seems more likely it is a random process with some variance.


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## tomofreno (Mar 3, 2009)

> What if 1/2 the pack was top balanced, say to 3.70 volts and tapering down to 0.1 amps. The other half of the back was bottom balanced and then charged, manipulating the charger a bit at the end (a Manzanita Micro PFC would be perfect for this) so the charge was terminated as the first cell hit 3.70 volts at 1/2 amp. Now put the pack together in series and turn on the charger, adjust up the voltage so the top balanced cells are again sitting at 3.7 volts with little current flowing. You would now have a pack that should protect itself against over discharge because half the pack is going to stop dropping voltage like a rock as their bottom is reached. It should be sharp enough and big enough to easily program you controller to simply not allow the pack voltage to go low enough to hurt all those cells. It would be much easier for your charger to protect your pack because fully half the cells start climbing fast as they reach full together. Even if the charger drifted a couple of volts, or the timer failed one night, the top balanced cells would rise up just an extra tenth of a volt or less and force the current to essentially zero.


 One concern I would have is if your lowest capacity cell is a few percent smaller than the others and is in the top balanced half, you may over discharge it before the bottom balanced half starts down the exponential part of the discharge curve triggering the charger to cut back. Similarly, if it is in the bottom balanced half you may drive it up the exponential part of the charge curve before the top balanced cells get there and kick off the charger. Seems like it would work for well-matched cells though.


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## EVfun (Mar 14, 2010)

I'm certainly not going to go to the trouble of doing the 2 half pack experiment. I believe all the cells would have to be tested for capacity and sorted so the smaller cells where in the bottom balanced half and all the larger ones in the top balanced half. Then just a 1 or 2 amp hour difference should cause fully half the pack to react together at each end of their capacity. 

Right now I have 32 cells bolted down in the Buggy. I'm playing with 10 cells that won't be initially used in the Buggy. They will be back-ups if I manage to destroy 1 or 2 cells (or find 1 or 2 that are noticeably smaller.) Perhaps some will be added in the future to raise the pack voltage. 

It is interesting to watch them balance themselves at 3.31 volts. Each 1/10th millivolt on a jumper between cells represents something between 15 and 40 milliamps of current moving. This is straining the limits of my old Fluke DMM to measure voltages that small. It would be very slow to line them up this way, it is only practical when they are taken to either the top or bottom knee. Still, it is educational to watch them move just milliamps, slowly trying to work themselves together. I'm taking some charge out of them with a headlamp. My goal is to take them down so they are resting at 3.29 volts each. LiFePO4 cells really shouldn't be stored fully charged as it is supposed to accelerate aging some.


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## Guest (Mar 30, 2011)

tomofreno said:


> One concern I would have is if your lowest capacity cell is a few percent smaller than the others and is in the top balanced half, you may over discharge it before the bottom balanced half starts down the exponential part of the discharge curve triggering the charger to cut back. Similarly, if it is in the bottom balanced half you may drive it up the exponential part of the charge curve before the top balanced cells get there and kick off the charger. Seems like it would work for well-matched cells though.



The top balanced batteries will be in greater danger of being driven into the ground than the ones that are bottom balanced. If that cell is pushed into the upper limits like 4.2 volts while the others are at 3.6 the one higher one will only have minimal amps driven through it compared to the other. If your cells are that far out of whack you need to do some better matching. 

Yes you need a well matched pack to be safe. The safest place is lower end balanced if you must balance your pack. If new you should not have to do so and you still no matter what need to stay off the bottom and off the top. 

It is a simple matter. But if you or someone else drives your car and does not know and drives it beyond the limits set then your cells will be safer than if you top balanced and had your pack taken too far into the bottom. The first cell that reaches the low point will be at GREAT risk.


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## EVfun (Mar 14, 2010)

gottdi said:


> The safest place is lower end balanced if you must balance your pack.


What would happen to your bottom balanced pack if the timer on your charger failed so it ran an extra 4 hours until you caught it? I recently had a charger fail that way. Voltage regulation was functioning perfectly but it quit shutting off. My pack agrees on the top so it was a few hundredths of a volt per cell with the current to low to measure when I caught it just before going to bed.


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## dtbaker (Jan 5, 2008)

EVfun said:


> What would happen to your bottom balanced pack if the timer on your charger failed so it ran an extra 4 hours until you caught it? I recently had a charger fail that way. Voltage regulation was functioning perfectly but it quit shutting off. My pack agrees on the top so it was a few hundredths of a volt per cell with the current to low to measure when I caught it just before going to bed.



I had a timer fail too.... it was a cheap home unit rated for 15amps, and must have fried when it tried to click off while pulling 12amps. I had it in place at the end of my lead pack's life because the charge would not ever hit the top voltage any more and would just boil'em dry in the gass stage if I didn't kill it.


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## Guest (Mar 30, 2011)

The same thing that would happen to a top balanced pack that had the charger fail to shut off. Except that you'd loose some bottom balanced cells faster. The danger of top balanced packs being driven into the low reaches of the SOC curve would result in bad things happening faster. 

There is ALWAYS a risk even if the cells were mid balanced like you get from the factory. Components can fail and when they do it is always good to have a redundant system to help combat any disaster that could befall your ride. It can happen with any vehicle at any time. The argument is only that the cells are actually safer if you bottom balance if you need to balance your pack. It is not an argument of balance or not but what end is actually safer. The bottom IS safer. It does not mean to take a brand new pack and bottom balance it because you don't need to do that. I had no choice. If you took a perfectly balanced pack and top or bottom balanced it would be kinda stupid. But if you NEED to balance for what ever reason you go to the lower end for balancing. Not the top. ONLY because it is safer. Not that you can't make it work balancing on the top but on the top you must be more careful and attentive. If your friend go to drive it for what ever reason you may be putting your top balanced cells at risk. 

Just be extra careful if you think balancing on the top is OK. 

Pete


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## Guest (Mar 30, 2011)

dtbaker said:


> I had a timer fail too.... it was a cheap home unit rated for 15amps, and must have fried when it tried to click off while pulling 12amps. I had it in place at the end of my lead pack's life because the charge would not ever hit the top voltage any more and would just boil'em dry in the gass stage if I didn't kill it.


Well if you used a cheap home thingie then you got what you paid for. What charger did you have that stopped hitting the top voltage? Sure it was not the batteries that were the problem? 

Pete 

Don't ever use cheap junk. It is not rated for what you need.


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## EVfun (Mar 14, 2010)

gottdi said:


> The same thing that would happen to a top balanced pack that had the charger fail to shut off. Except that you'd loose some bottom balanced cells faster.
> 
> There is ALWAYS a risk even if the cells were mid balanced like you get from the factory.
> 
> ...


I don't think I'd loose a cell overnight if the timer failed. I don't even think one would get to 3.9 volts. I don't intend to find out, those things cost $75 each.

Are you sure the cells come mid-balanced? Looking at this post from Jack's archive it appears that the cells are at least close to bottom balanced.

I figure if the low lift lockout function of an amp hour counter shorts out the pot box that ought to pretty well eliminate cell reversal issues.


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## Guest (Mar 30, 2011)

Well I guess that would depend on what level of power the cells were charging at. Yes you could loose the whole pack. It happens in the middle of the night. It must be the darkness that creeps in and does the damage. 

New cells come from the factory at mid level. I think TS has there batteries at like 50% and Calbs at 60%. It is part of production and how they initially charge the cells. They have the equipment to do so during the manufacturing process and QC process. All cells get charged and they are charged to a specific level. Each batch and each AH level of cells gets the mid charge right from the get go. 

So the answer is YES they come charged balanced and all you need to do is charge and go. You do need to watch them a few cycles to be sure. Most folks will do this and so far all are fine. But like me if you happen to have a bunch that are badly misbalanced then you need to balance them and the safe end is the bottom. That is all. It is not fast work to balance a pack. If you get your cells at different times and different sources but the same manufacturer and AH size you will have an imbalanced pack and you will need to balance the pack and use the lowest common denominator to use for your top and bottom calculations. Be safe. Enjoy your new cells. How many times does one have to repeat this. Is this being done just to see how many times someone will say something. I am getting tired. Just read. It has been shown and shown more than once from more than one person. Me included. 

Pete


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## JRP3 (Mar 7, 2008)

I doubt the cells are actively mid balanced at the factory. They are probably charged and discharged at least once as part of QC, and then they are probably charged to around 50% SOC. The only way to figure charge in the middle of the curve would be to count amp hours. Unless they adjusted the amount of amp hours for each cell according to actual size, which I doubt, then putting say 55 amp hours into cells varying from 110-114ah as mine do would result in actual different SOC. They end up being pretty close but I doubt they are really balanced, since we know it's almost impossible to do so in the middle of the charge curve. Jack never took a new batch of cells out of the box and measured the amp hours in them compared to actual capacity that I'm aware of.


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## Guest (Mar 30, 2011)

I am quite sure that they do this process in batches and I do believe the cells are close enough in AH that they can do a 50% SOC on each cell with no problem. They are not some back yard shop you know. They do have the equipment to do exactly what you said but I am quite sure that its done in batches. If they did not have a way to do that then all the cells would be all over the place in SOC and every cell of every batch would need to be charged one at a time so they all match or to be discharged then charged one at a time until they were all the same. This would just not do. If you took an order of thousands of cells and had to do that I guarantee you would not buy them. 

The company must have a way to put the cells at 50% SOC before ever being boxed and shipped. That way you just charge and go. 

Well I guess we just need to ask the companies, right? Until they say so I guess it is just speculation, right? Or maybe it's not that hard to figure out and I just don't know the process. Ask Jack. I'd almost bet he has asked the manufacturers about the process. I highly doubt he takes information from the air. I'd almost bet he is the one that does go to the source more than anyone else. I usually go to the manufacturer if I can. Some times they just don't respond. 

Never did send for information from TS or Winston. Maybe I should. Maybe we all should. 

Pete


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## tomofreno (Mar 3, 2009)

> The top balanced batteries will be in greater danger of being driven into the ground than the ones that are bottom balanced.


 Yes, but if the probability is low in both cases, as they are for me, then it doesn't much matter.


> If that cell is pushed into the upper limits like 4.2 volts while the others are at 3.6 the one higher one will only have minimal amps driven through it compared to the other.


 Not necessarily. I've had to shut off my charger when it was still putting out over 20A and the lowest capacity cells were on the exponential part of the curve. The current will always be much lower than during discharge of course, but enough to damage a cell if left to over charge long enough.


> But if you or someone else drives your car and does not know and drives it beyond the limits set then your cells will be safer than if you top balanced and had your pack taken too far into the bottom.


 True, but again if the probability is small it doesn't much matter. The TBS gauge is mounted about 1 1/2 feet from the driver at eye level and flashes "Empty" when the lowest capacity cell reaches 35% SOC. The 5 bars that disappear with falling SOC indicate roughly the percentage of charge left to empty. And again, the minibms cuts the throttle, sounds an buzzer mounted at the steering column, and lights the "Check Engine" light if any cell hits 2.5V for more than a few seconds.


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