Testing for bad cells in parallel configuration?

[theyerb]

I'm sure it's here somewhere, but I'm really tied of looking :? I have a batteryspace LiMnNi 36v 12ah pack that stopped working last week. It will actually put out just a little juice, but any significant load, and it cuts out completely. I believe the onboard BMS is tripping because one of the cell groups is bad. I've taken the dang thing apart, but how am I supposed to test for a bad cell when there are 3 cells in parallel for each of the 10 series?? I can only measure the voltage for all three cells, not each individually (to my knowledge).

tldr: how do I test for the bad cell in parallel configurations?

 

[Ypedal]

no other way but to split them from parallel into single cells.'

If the bad cell is bad enough, it may have destroyed the other 2 along with it.. all depends on resting voltage. Below 3.0 ~ 2.5v the group is likely shot.

 

[dnmun]

first, why do you believe what you believe? did you test with a different BMS? what are voltages during charging, then how fast does your 'bad' group drop?

if one is bad, and it has been that way for a while there is the real risk that the bad one has killed its neighbors even though they still have some life left. discharging to really low voltages is hard, but they may yet be ok.

post up some pictures please so we can recommend where to cut if you wanna do a binary search.

 

[dogman dan]

Find the bad group, replace the entire group. Chances are 99% that the other cells have now been hammered anyway.

 

[amberwolf]

Also, to find the low group(s), you can test the voltage of each group while under load. The one(s) with problem cell(s) will likely drop in voltage more than the others during the load test, if that is what is causing the BMS to cut out.

 

[John in CR]

Find the bad group, replace the entire group. Chances are 99% that the other cells have now been hammered anyway.

+1 . It's not one bad cell. It's 3 and easy to find.

 

[theyerb]

The problem is either with the BMS or the battery cells; based on the BMS cutting power after about a half second, I diagnosed it as a faulty cell or cell group, though I might be wrong. Gotta start somewhere. Plus I called Batteryspace and the engineer said it was likely a faulty cell after I described the problem. I will test each cell group under load to see what's going on. Thanks for the input

 

[docnjoj]

Hey Yerb
It sounds like you are right on. Put a load on and measure cells by groups (3s?) and find the one that drops low early or immediately. How old is the battery, since Batteryspace sells quite expensive cells and batteries? I will be interested to see how they handle warantee issues like yours.
otherDoc