BMS cutting off charging early

I have a 12s battery that is cutting off early and not charging all the way :frowning:

I opened it up and measured each P group and found they are not balanced:

The BMS wasn’t balancing and it cuts off charging ~70% when the best cell reaches 4.2v (group 7, 11, 12).
And it cuts off discharge early at ~40% when the lowest cell reaches below 3v (group 2,4,5)
Ok that makes logical sense.

I replaced the BMS with this one (discharge only):

I charged it but it is still only charging to ~70%
This BMS seems to cut off charging when the best cell reaches 4.2v as well…

Is that normal behavior? I thought it was suppose to keep charging the other cells and drain the “done” groups with the resistor.
Or are these groups way too unbalanced for the BMS to handle and I must manually balance them first?


Yes the balancing is very little with a BMS, you must first manually charge each cell to the equal Voltage.
I do it before the assembly with a icharger but you can also leave the BMS connected.

Also like always check the cells from time to time.

my daddy told me about those wise words

1 Like

The vast majority of BMS’s only balance while charging and at nearly full charge, 4.15V or so per cell. Theyre really not designed to balance a severely out of balance pack only to maintain the balance of a pack. You will continue to have this issue no matter the BMS you switch to.

You need to balance those groups yourself. My recommendation would be a lipo charger. You could do it one cell group at a time. Charger them all to 3.8 or whatever, 4.2 would work as well. And then you should have. A happy pack and BMS.


thanks for your help guys!
I’ll balance them and then hook it up