backchannel: balancing LFP cells

from this reddit thread. OP is reporting undercharge likely caused by cell imbalance. Quotes below are from a “confidently incorrect” commenter.

Cells don’t “equalize” on discharge… It’s not a guessing game.

Keep both these points in mind for the next claims. I’ve reordered the text a bit for clarity.

Yea, discharge to a low of 20%… The 20% discharge will allow for “bottom” balancing.

• So they don’t balance on discharge but discharging to 20% SoC will balance them. Got it.
• I’ve never seen or read of a passive balancing circuit in a drop-in LFP that bottom balances. Typically they top-balance starting at a given cell voltage like 3.5vpc. Active balancers can be set up to balance at any SoC/voltage but that introduces other problems.¹

and charge until full which is around 30 minutes AFTER the charge light turns green. That 30 minutes is when the top balancing takes place.

Does it? Or are we guessing at the behavior of both the charger and the BMS?

Top balancing occurs when the cells’ voltages equalize at their highest point

Top balancing occurs when the balancing circuit’s setpoint is reached. Whatever that is. It does not have to be at the cells’ highest voltage point.

1. My SFK bank has both passive top-balancer and and active balancer board. The latter has options to be OFF, ON, or ON ABOVE A GIVEN VPC. I haven’t looked at the docs in a while but IIRC mine is set to turn on at 3.325vpc. The delta stays 2-5mV. Right now it’s charging at net 35A and the delta is 3mV. ↩