Top balancing issue

[dddem]

Hello,

I have a 100Ah 2p8s pack made of EVE 50Ah cells I recovered from a commercial rack pack. The bus bars appear to be welded or glued to the cells' terminals so no way to reconfigure the cells. I performed a top balance by charging each pair of cells to 3.65V with a bench PS (while they are still connected together obviously since the busbars cannot be removed). Went around the pairs several times to be sure they were all equal at 3.65V.

Problem: after discharging the pack to 24V (measured capacity 90Ah when discharged stopped), all the cells pairs were between 2.87V and 3.02V. After recharging the pack as a whole, the charge is stopped by the BMS with one pair at 3.63V and the lowest at 3.4V.

Is this supposed to happen? I thought that my top balancing would ensure that all the cells would reach their higher voltage simultaneously?

If not, what else could be wrong?

 

[DThames]

If you have used cells, the cells will have some variation in the capacity. If you take them low on discharge then charge backup, yes this is normal. I suggest you.
1. Discharge only to the point that a cell starts going lower than the others. The first one that nears full discharge will go down quickly after some amount of discharge. Stop your discharge before that starts happening.
2. Charge to 3.55v per cell or even 3.50v per cell. Set your charge voltage lower and your boost time longer, so it will hold 3.50v long enough to fully charge to that voltage. This will normally give you some head room for that first cell that is fully charged and races ahead to 3.65v BMS charge cut off.

I have some older packs and this works well for me. The total usable capacity is less but the BMS never turns off the battery while charging. An active balancer might help.

 

[MattiFin]

Old cells have unequal self-dischage rates and charge efficiencies leading to situations like this. As mentioned slightly lower charge voltage and active balancer might help.

You could run couple of cycles from 10% to 90% and see if the voltage delta between weakest and strongest cell keeps growing.