Bringing one cell in a pack back to life

[riktit]

Hi there, I have two 10s packs that I bought used. They are lithium ion. You may check out more details from Molicel here: http://www.wamtechnik.com/files/specs/642.pdf

The packs are 10s4p. There are two 5s balance connectors coming off of each pack. When I measured the voltage across the cells, I found that one parallel group on each pack is reading at .1 V The overall voltage of each pack is 37 volts, and the rest of the cells are within .5V of eachother.

I have an iCharger 1010B+ that I plan to attempt a revival with. I would like to know if anyone can offer advice on what to do/not to do during my attempt. I would also like to know what I can do if the cells are not recoverable, and how it will affect the rest of the pack if I don't rebuild the pack without the (possibly) dead cells.

Thank you.

 

[fechter]

I'm not sure what options the iCharger has, but for a near zero starting voltage, you want to limit the current to somewhere around 100ma or less until the voltage comes up to about 3.0v, then you can charge at a normal rate. If only one cell is this low, I would try to charge it separately. Once it's balanced to the rest of the cells, a normal charger should be OK.

If you have any power supply, you can use a resistor or light bulb to limit the current, but you have to keep watch on the voltage if there is nothing to keep it from getting too high. If you have a bench supply, you can just set the current low and set the voltage to 4.2v.

When cells are near zero, the internal resistance will be very high so a high charge current can cause overheating, fire, bad stuff. I've recovered quite a few cells from this state and had them work OK. Just be extra careful during charging as damaged cells are more likely to have thermal issues.

 

[d8veh]

Normally, there's a reason why one cell goes down like that, so your chance of recovering your battery isn't high. It's worth a try if you have the wherewithal to charge it, especially if it's a cell at the end of the string that's used to power the BMS. Personally, if it's a cell group in the middle of the string (not no.1 or 10), I wouldn't waste my time.

 

[dogman dan]

Well, keep those ready to blow cells outside is my advice. I'm going to become a harpy on this now, but that's what happens when your battery burns your house.

A "good" battery burned my house. I sure would keep a bad one outside.

 

[amberwolf]

I've done pack repair a few times, and while it is possible to revive cells, sometimes, I wouldn't.

With all the possible problems that dead cells can bring to the table, I would instead remove the dead group from one pack, and transplant a good group from the other pack to it. That would be a more reliable repair.

Even if it's just that the BMS drained the cells down slowly over time, it's still possible the cells are damaged and won't perform the same as the others.

That could mean something as benign as a little capacity loss, or something as potentially catastrophic as really high internal resistance, and/or internal shorts, meaning they'll heat up a lot as current flows thru them, and it's possible the heat could induce a fire.


If you do decide to revive them anyway, Fechter's plan is good--but I'd also never trust them, and keep them outside / in a fireproof place.

 

[riktit]

Thanks for the advice guys, I'll let you know how it goes once I try to revive it.

 

[riktit]

Update: It took a while, and required some tricking the charger into charging the cells to the point it could begin a balance. The cells are now less than .02V out of balance, and that is what they are holding about one week after charging them. Since they aren't self-discharging, I have high hopes for when I actually begin to use them on the bike. The only way I could monitor voltage drop across the single cell out of balance in each pack, was to discharge using my iCharger, which can only discharge at about 50 watts. Given that each battery is 36V, that turned out to be just over 1 amp. I didn't notice any voltage drop. Good.