Balancing low cell on 30kWh leaf after it was abandoned for a year

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LMF5000

Active member
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Oct 20, 2017
Messages
41
We just got a good deal on a used leaf with very low mileage. Previous owner left it abandoned in a garage for a year and one of the cells seems to have self-discharged more than the rest and is now sitting at 50mV lower than the rest of the pack.

I've tested at full charge and at 25% indicated (actually 33%SOC on leafspy) and it always remains 50mV below the rest. To me this implies that the capacity is still there, it just needs to be synchronised with the rest of the pack.

We did a range test on the pack, from 100% to 25% on the car's display took 116km of driving so the capacity seems to be there - I just wish to correct the imbalance for peace of mind.

Is there any way to do that? The cell didn't balance much on its first full charge.

Leafspy screenshots below, at discharged and full states.


 
You can try taking the pack down to "turtle" and then charge to full, but I suspect you won't see much difference. My guess is that cell is "weak" and may only get worse. Don't shoot the messenger!
 
Considering that the state of charge was only 33% when the reading was taken, 49mv isn't a particularly bad reading. I'd just drive the car down to ~20% when it's convenient, drive and charge it normally otherwise, and see how much the cell re-balances. Even if it stays exactly the same, you got a good deal.
 
LMF5000 said:
Is there any way to do that?
Click to expand...
Yes, there is, but it's a lot of work. You need to drop the pack, open it up, find the appropriate module, and with all the appropriate safety gear, apply a charge from a bench current limited power supply. When it's at about the pack average, stop the charge. Then reassemble the pack, possibly re-sealing it and/or repressurizing it, and reinstall.

As indicated above, 50 mV of imbalance is way too small a value to go to all that much trouble. The BMS will likely re-balance it in time. It probably doesn't attempt to balance below a certain state of charge, as (by my understanding) the balancing is purely dissipative, so that might explain why the imbalance wasn't corrected for at least the last part of that long storage.

The cell didn't balance much on its first full charge.
Click to expand...
Leafs have a slightly unusual balance regime, or at least the earlier models did. I imagine that a 30 kWh model qualifies as early. The balancing current is full time, even then the car is turned off, but the balancing current is very small. A figure of 10 mA comes to mind. So unlike some other cars that do a quicker balancing only when charged to 100%, Leafs do a very slow balancing all the time (possibly except when the state of charge falls below a threshold). So it will take time, and fully charging doesn't actually quicken that process, other than keep the battery above the balancing threshold SoC.
 
coulomb said:
LMF5000 said:
Is there any way to do that?
Click to expand...
Yes, there is, but it's a lot of work. You need to drop the pack, open it up, find the appropriate module, and with all the appropriate safety gear, apply a charge from a bench current limited power supply. When it's at about the pack average, stop the charge. Then reassemble the pack, possibly re-sealing it and/or repressurizing it, and reinstall.

As indicated above, 50 mV of imbalance is way too small a value to go to all that much trouble. The BMS will likely re-balance it in time. It probably doesn't attempt to balance below a certain state of charge, as (by my understanding) the balancing is purely dissipative, so that might explain why the imbalance wasn't corrected for at least the last part of that long storage.

The cell didn't balance much on its first full charge.
Click to expand...
Leafs have a slightly unusual balance regime, or at least the earlier models did. I imagine that a 30 kWh model qualifies as early. The balancing current is full time, even then the car is turned off, but the balancing current is very small. A figure of 10 mA comes to mind. So unlike some other cars that do a quicker balancing only when charged to 100%, Leafs do a very slow balancing all the time (possibly except when the state of charge falls below a threshold). So it will take time, and fully charging doesn't actually quicken that process, other than keep the battery above the balancing threshold SoC.
Click to expand...

That's interesting, thanks for the info. If I had access to the cell it would be very easy for me to charge it with my computerised hobby charger (I've been into RC toys and power inverters for over a decade) - problem is getting to the cell as you rightly pointed out! :D Wish there was a way to reach the individual BMS leads from the outside of the battery to give a charge to a particular cell without breaking any seals.

When we found the car it was at a 70% SoC - so presumably the threshhold for activating the balancers is higher than that. Is there any information on what the threshhold is (in terms of SoC or absolute cell voltage)? I'm not keen on keeping the battery at an SoC higher than 80% because of the extra wear. I have two nissan leafs myself and use charge timers to make sure they live at between 30% and 80% on the GOM at all times.
 
LMF5000 said:
When we found the car it was at a 70% SoC - so presumably the threshhold for activating the balancers is higher than that. Is there any information on what the threshhold is (in terms of SoC or absolute cell voltage)? I'm not keen on keeping the battery at an SoC higher than 80% because of the extra wear. I have two nissan leafs myself and use charge timers to make sure they live at between 30% and 80% on the GOM at all times.
Click to expand...
To be clear:
  • even if you "opened up" the pack and (manually) charged the weak cell, my guess is it would return to its previous "weak" level (and get worse over time).
  • the (internal) balancing activity takes place very slowly over many cycles (as previously mentioned) as long as you aren't at a very low SOC (like LBW).
Again, the reason your car didn't maintain balance probably has nothing to do with what SOC you were at or how long it was sitting--you simply have a weak/bad cell. I know this because I just got done doing this dance, and the only (long-term) solution was replacement of the problem cell/module.
 
50 mV is not a really large unbalance. If the car gives adequate driving range between charges for your needs, just drive it and charge it normally for a while--it will probably balance itself over time. Try to discharge close to low battery warning (or lower if you are comfortable) and charge fully each charge cycle. This will allow the LBC (lithium battery controller) to learn the characteristics of the cells and do its best to balance them over time. It is a slow process, but there is no detriment to the unbalance unless range becomes an issue.
 
It makes sense to do the above for a while, if only because at this point you don't actually know if that cell is weak enough to be an issue or not. If it improves, or even just remains the same, it isn't.
 
Stanton said:
LMF5000 said:
When we found the car it was at a 70% SoC - so presumably the threshhold for activating the balancers is higher than that. Is there any information on what the threshhold is (in terms of SoC or absolute cell voltage)? I'm not keen on keeping the battery at an SoC higher than 80% because of the extra wear. I have two nissan leafs myself and use charge timers to make sure they live at between 30% and 80% on the GOM at all times.
Click to expand...
To be clear:
  • even if you "opened up" the pack and (manually) charged the weak cell, my guess is it would return to its previous "weak" level (and get worse over time).
  • the (internal) balancing activity takes place very slowly over many cycles (as previously mentioned) as long as you aren't at a very low SOC (like LBW).
Again, the reason your car didn't maintain balance probably has nothing to do with what SOC you were at or how long it was sitting--you simply have a weak/bad cell. I know this because I just got done doing this dance, and the only (long-term) solution was replacement of the problem cell/module.
Click to expand...

In my experience, a weak cell is one that has low capacity, high internal resistance (IR), or both. That would manifest as a higher voltage during charging and a lower voltage during discharging, since the weak cell is getting filled up or emptied prematurely compared to the rest of the pack.

In my case this cell is staying 50mV below the rest of the pack consistently, which seems to imply that the capacity and IR are similar to the rest of the pack. It's still an outlier due to the low voltage, but I believe this is due to elevated self-discharge, which ordinarily wouldn't be a problem if it's this mild (since the car would get driven almost every day) but became serious since the car spent 12 months in disuse.

During charging (this would be the second charge since we bought the car), I observed a 30mV difference, so things might be improving. I will monitor it again after a few more cycles. Hopefully the dodgy cell doesn't develop increasingly high self-discharge levels. It doesn't seem to have much of an impact on range, so far this car does over 115km between 100% and 25% on the GOM
 
LMF5000 said:
When we found the car it was at a 70% SoC - so presumably the threshold for activating the balancers is higher than that.
Click to expand...
Oh, I assumed that you would have found it at a much lower SoC, say 10-20%. That's pretty remarkably low discharge over a year.

I would expect the balancing to be active down to 30% or lower. But that's just a guess.
 
Hi,
that "weak" cell is about 1/3rd the way into the large big bank of cells. In our case it was cell 49, hat was the top cell on the RH side bank.
I had the same issue with my brothers 24kwh nissan leaf.
We decided to buy a replacement cell, charge it to max, charged the leaf to max, dropped the pack and opened it, and replaced that 1 cell.

All ok since.

I didnt bother trying to rebalance it, as the grief of resealing and reinstalling the pack and then the possibility of dropping it and doing it all over again for the sake of maybe £80 for a replacement cell wasnt worth the stress.
We just swapped it out.

Not a had job to do if you have access to the right equipment. Took us about 8 hours, but thats not rushing, a chat, few cups of coffee and dinner inbetween.

Id swap it out if i were in your position to be honest.

Anthony.
 

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