3.3, 3.3, 3.3, 2.3 ?!

[BroomJM]

I recently had a 2nd-life battery do something weird. The BMS shut down the discharge because one of the cells (it's a 2P-4S, 240AH, FLP battery, so technically it was 2 of the 8 cells) went to 2.3 volts. The other cells were all closer to 3.3 volts. I thought for sure something had gotten damaged beyond repair, but I slowly charged up the 2 cells until the deviation was less than 50mv. I let those two cells "rest" for about 4 hours and they showed very little drop in voltage, no more than you would expect right after charging. I was then able to start charging the entire pack and it seems to be charging normally.

Any thoughts on what could cause one of the cells to be fully discharged, while the other three were normal? I need to check all of the balanced leads and the harness, itself, to ensure that is behaving normally. (The cells are staying in balance, as they charge...within 20mv.)

 

[sunshine_eggo]

Resting cells in the operating range will have pretty even cell voltages, and voltage isn't a good indicator of balance between the legs. 50mV isn't great either.

Battery was top balanced?
Regularly charged to 3.45V/cell and higher with all 4 cells at small dV?

If so, that cell is simply notably lower capacity than its pack mates.

 

[BroomJM]

Resting cells in the operating range will have pretty even cell voltages, and voltage isn't a good indicator of balance between the legs. 50mV isn't great either.

Battery was top balanced?
Regularly charged to 3.45V/cell and higher with all 4 cells at small dV?

If so, that cell is simply notably lower capacity than its pack mates.

I had cycled this battery dozens of times over the last 6 months, in the 30% - 100% range, with deviation between (pairs of) cells no more than 10-20mv. It just suddenly had one pair of cells drop to 2.5 volts, while the others were at 3.25 to 3.3 volts. It was like only that one pair of cells discharged down to 2.5 and then the BMS cut off. I'll have to watch and see what the cell pairs do during the next discharge. I might run a test on them to see if the capacity is greatly reduced...it just sucks because that test would take a full 24 hours, if it's still at 240ah.

 

[sunshine_eggo]

I had cycled this battery dozens of times over the last 6 months, in the 30% - 100% range, with deviation between (pairs of) cells no more than 10-20mv. It just suddenly had one pair of cells drop to 2.5 volts, while the others were at 3.25 to 3.3 volts. It was like only that one pair of cells discharged down to 2.5 and then the BMS cut off. I'll have to watch and see what the cell pairs do during the next discharge. I might run a test on them to see if the capacity is greatly reduced...it just sucks because that test would take a full 24 hours, if it's still at 240ah.

Stated another way, in the operating range voltage deviations between even significantly imbalanced cells tend to be negligible. Deviations are only meaningful above 3.4V and below 3.1V.

I assume the BMS is the % SoC to which you're referring. Those can get out of whack. If you can confidently say that when the BMS indicated 100% that all cells were > 3.45V and within 10-20mV, then yes, that's a fully charged reasonably well top balanced battery.

You may be discovering the downside of 2P4S instead of 4S2P. Parallel cells are masked by their parallel mates. One of the two cells may have failed and can't reliably hold charge with an excessive self-discharge rate.