@lemur Oh it's different on that one. I thought you meant it was just balancing slow.
That is very weird. I wonder if there is a corroded link on that BMS or a failing resistor that's creating a larger voltage drop by the time it gets to the bit that reads the voltages.

When I made my battery pack from spring loaded contacts the first time (and then gave up and welded them) the voltage drop was insane. Might be something similar be it contact resistance on the port or some biffed trace/ component.