# What to do about one imbalanced cell?



## halestorm (Apr 28, 2009)

I have a Chevy Volt pack, 108 total cells (two parallel strings of 54 series cells). I'm using the Elithion Pro BMS. I have Vcell-max set = 4.01v and have driven a few thousand miles over the past couple of months. All has gone reasonably well until last week.

I'm not getting a full charge on the pack because a single cell is reaching max before all of the others. Specifically, I have one cell which is at 4.01v, while the other 107 cells are at 3.86v. (And that seems to be getting worse -- a few days ago I had one cell at 4.01 and 107 cells at 3.92v)

1) How does this happen? and 
2) How do I correct for it?

Because of the nature of construction of the Chevy Volt batteries, I cannot remove a single cell. Can I connect a resistive load to that one cell (while all of the others remain in series) and try to drain it down? I would guess I should disconnect the Elithion during that process, but am not 100% certain of that.


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## halestorm (Apr 28, 2009)

add

3) why isn't the Elithion balancing the cell? It's been plugged in since Friday but has made no headway in reducing the voltage of this errant cell.


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## Duncan (Dec 8, 2008)

Hi halestrom

I am a cynical old fart - so I would be looking at your Elithium BMS with a jaundiced eye 

Assuming that your problem is actually a dying cell - and not a BMS problem the long term solution would be to lose that cell
You don't need to remove it - just bleed it down to zero and make up a connection across it to effectively keep it shorted out

You lose 3.7v but everything else should be OK

Like I said that is a last resort - plan Z - only after you have exonerated the BMS


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## halestorm (Apr 28, 2009)

I'll guess I'll have to email Elithion tomorrow. I was trying to avoid that since they charge to answer emails.


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## piotrsko (Dec 9, 2007)

fist of all it is a battery not a cell, unless you disassembled it and are running them singly.

Do the high charge cell groups move around or is it always the same bunch? Has to be the same group to indicate a capacity issue. Otherwise it is a LIPO idiosyncrasy which I find to be initiated by high current discharges like > 5C.

I have found that they need to start at the same point to end at the same point. You are putting the same amount of joules into each group unless you have a separate power source for each grouping. 

So run the pack down to low min cutoff and look for high values. bleed off the high valued ones to the same voltages as the rest of the pack and then re-charge. rinse, repeat.

they will take forever to bleed when they are full unless you have something that will tolerate large currents at 3 volts.

i believe this is also referred to as bottom balancing.


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## halestorm (Apr 28, 2009)

piotrsko said:


> fist of all it is a battery not a cell, unless you disassembled it and are running them singly.


I'm referring to a single "slice" within one of the modules. It's technically 3 cells in parallel.



piotrsko said:


> Do the high charge cell groups move around or is it always the same bunch? Has to be the same group to indicate a capacity issue. Otherwise it is a LIPO idiosyncrasy which I find to be initiated by high current discharges like > 5C.


There's no bunch, it's only this one "slice" which is different from the others. My system is only capable of pulling 150A or so, about 1.6C

For some reason the Elithion is not even balancing this cell, but it is balancing all the rest, keeping them at 3.86v -- it is even reporting that it is doing so, even though I have the min balance voltage set to 3.95v, so it should not be balancing anything BUT the high cell.


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## GoElectric (Nov 15, 2015)

Interesting - sounds like the BMS then?

Jim


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## twright (Aug 20, 2013)

I am not familiar with the Volt cells, so please excuse the following if it doesn't make any sense:

How much capacity is there in a Volt cell between 3.86V and 4.01V?

If its only an amp-hour or two, don't worry about it. Your pack will be just fine.


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## WolfTronix (Feb 8, 2016)

"I'm referring to a single "slice" within one of the modules. It's technically 3 cells in parallel."

Verify the voltage of that cell with another volt meter... 
The BMS is probably correct, you probably lost one of the 3 cells in parallel.

So you have a 2/3 capacity "cell" in your pack. And your BMS just prevented a fire.


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## halestorm (Apr 28, 2009)

WolfTronix said:


> Verify the voltage of that cell with another volt meter... The BMS is probably correct, you probably lost one of the 3 cells in parallel.


yes, I did, and that cell is high, the BMS is correct about that.



WolfTronix said:


> So you have a 2/3 capacity "cell" in your pack. And your BMS just prevented a fire.


 That would be unfortunate if that happened, as I'd have to replace the entire 12 cell module.

I was lazy in that, once I got the batteries connected I wanted to drive the car and didn't hook up the cooling water yet. Do you think that would have made a difference? It's never been terribly hot here lately and I hear mixed answers on the importance of the cooling water. I'll hook it up now. I hope my eagerness to drive didn't hurt 1/3 of one of the cells.


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## GoElectric (Nov 15, 2015)

If what you say about Elithion is correct, no failure there and you are lucky, except you still need to get it fixed. Perhaps you could search elsewhere and see if others have reported a similar error/failure? 

Regardless of the cause, the fact the situation is still changing makes me wonder if something worse could happen. Clearly you are concerned and monitoring the situation carefully. Without verifying everything I could, I wouldn't drive the car. 

Interesting to hear what the result is.


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## GoElectric (Nov 15, 2015)

Hi. Twelve volts??

I doubt your cells got too hot unless you were abusing those batteries currentwise - don't know where you have been reading that. 

(You don't have any temperature probes hooked-up?)

Jim


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## WolfTronix (Feb 8, 2016)

halestorm said:


> yes, I did, and that cell is high, the BMS is correct about that.
> 
> That would be unfortunate if that happened, as I'd have to replace the entire 12v module.
> 
> I was lazy in that, once I got the batteries connected I wanted to drive the car and didn't hook up the cooling water yet. Do you think that would have made a difference? It's never been terribly hot here lately and I hear mixed answers on the importance of the cooling water. I'll hook it up now. I hope my eagerness to drive didn't hurt 1/3 of one of the cells.


Probably not... if it was just a quick test run...

More likely it was from the wreck... 
Most used Leaf, Volt, Tesla, etc... cells came from wrecked cars... 
All the cells where exposed to a large G force and it is possible that a cell tab wield cracked.


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## halestorm (Apr 28, 2009)

GoElectric said:


> If what you say about Elithion is correct, no failure there and you are lucky, except you still need to get it fixed.


The fact that the Elithion BMS isn't balancing that one cell is suggestive that there is a problem with that cell board, or perhaps just that one balancing resistor, and needs more diagnostics.



GoElectric said:


> Regardless of the cause, the fact the situation is still changing makes me wonder if something worse could happen. Clearly you are concerned and monitoring the situation carefully. Without verifying everything I could, I wouldn't drive the car.


Indeed, I am not driving the car since I caught this.


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## halestorm (Apr 28, 2009)

GoElectric said:


> Hi. Twelve volts??
> 
> I doubt your cells got too hot unless you were abusing those batteries currentwise - don't know where you have been reading that.
> 
> ...


I said "12v module" but I meant to say "12 _cell_ module"; sorry, I've corrected the typo in the earlier post. The Chevy Volt batteries are made up of "modules" of two different sizes, 12 and 6 cells (where a "cell" is really 3p, 4v 15Ah cells, so you get 24v and 48v, 45Ah modules). Since the 3p cells are in parallel they are treated as a single cell. So on every 12-cell module, I have 12 BMS boards. The composition of the physical modules isn't important, electrically. I have a number of 6- and 12-cell modules arranged such that I have two 54-cell 216v 45Ah "batteries" in parallel to net 90Ah. In the end, I have 108 individual cells that are monitored by the BMS.

I've driven the car quite a bit; it's become my daily driver for about the last two months, so I've gone through many discharge/charge cycles. My system full open on the freeway only draws about 150A or so (maybe it's 160) so that's only 1.6C or so, less than 2C anyway.

I don't have the temperature probes connected, no. A few times on warm days (it's not been hot here the last two months) after hard drives I put my hand on the side of the batteries -- not a great test but figured if they got hot then I'd feel it -- and nothing was warm. The Elithion BMS cell boards have temperature sensors on them, and they have never been too warm.


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## dougingraham (Jul 26, 2011)

halestorm said:


> add
> 
> 3) why isn't the Elithion balancing the cell? It's been plugged in since Friday but has made no headway in reducing the voltage of this errant cell.


Sounds like the shunt balancer for that one cell is shot. If there is a board per cell swap the board for that cell with another board and see if the problem moves.

Alternatively you can manually lower that one cell and disable all the shunt balancing until you can replace that board.


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## halestorm (Apr 28, 2009)

In short, the problem seems to have been a bad balancing resistor on the cell board.

Now for the long version.

First, recall that the system was balancing for V>= 3.85 Friday through Sunday, even though I had it programmed to balance only for V>=3.95. I finally turned it off Sunday night. After powering-on again on Monday, it came up balancing for V>=3.95 as it should. Something must have reset. What? Why? Anyway, in this state now, only one cell load was requested, the errant 4.01v cell, of course. The LED on that cell board was lit, indicating it was balancing, but after several minutes there was no change in temperature. I swapped in a different cell board and within several seconds the temperature started to rise. After a few hours the voltage dropped slightly to 3.98v.

So it's clear that the balancing resistor (and/or something else on that cell board) is bad, and I'm glad I had spare boards. Could a bad balancing resistor alone caused this cell to run amok? Or is there a chance that there is something else wrong with this cell? I suppose I don't know if there isn't something else wrong with this board, aside from the balancing resistor. I'll take a closer look at it some time, to see if I can determine if the resistor is bad or not.

Anyway, it's slow to balance but I'm back on the path to a balanced pack again... I just hope it stays that way.

As an aside, ... I added an extra bleed resistor to that cell to help it along, but didn't want to leave it that way unattended overnight. Since this was the only cell it needed to balance, I put a fan on that cell board last night so it would stay cool and keep balancing all night. When I went to bed the min balance voltage was 3.95v, as I had it set. By this morning however, the Elithion BMS had once again adjusted the min balance voltage (to 3.92v this time) so many cells were being balanced. I emailed Elithion about that and they just said that their algorithm for determining min balance voltage is proprietary. Not sure why there is even an algorithm, since it's supposed to be a value I can set. I suppose it has to do with the fact that the BMS can also turn the charger on. As it bleeds down the one high cell, it gets to a point that it can enable charging. When it does that, then the other cells start to come up, and maybe at this point the BMS takes over and starts tweaking my min balance voltage, to keep all those 107 other guys balanced as they come up. That's conjecture. The one thing I do know is that replacing the cell board has allowed it to start balancing again.


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## dougingraham (Jul 26, 2011)

halestorm said:


> The one thing I do know is that replacing the cell board has allowed it to start balancing again.


If properly sized resistors are pretty reliable components. There is most likely a FET or transistor that has failed or the driver for the FET or transistor has failed.

The problem with all BMS devices is that they are far more likely to fail than the battery they are purportedly trying to protect. The failure you experienced in this case was benign. The BMS would stop the charge because that one cell was at your voltage limit. Because of this failure of the one cell board the balancing action on all the other boards would lower their voltage a little bit each day leaving this one cell gaining in voltage every day. In effect the BMS was unbalancing this one cell a little bit more every day and then stopping the charge when that cell reached what the BMS considered fully charged. Other kinds of failures are less benign. I doubt that your cell has suffered any kind of issue due to this. You should be back to full range when the cell is back in balance.

Best Wishes


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## halestorm (Apr 28, 2009)

dougingraham said:


> If properly sized resistors are pretty reliable components.


It may or may not be properly sized; it's a tiny surface mount resistor as indicated in the attached photo. It's certainly not capable of dissipating much power. I don't know what it's rated at, but at 18 ohms it is being asked to dissipate almost 1 Watt!


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