12s 4p Battery uneven discharge at first and last in the series

[Mikebike]

Bought new Samsung cells and DIY kit from reputable supplier.

Used a single cell charger to get the cells close. Didn't check internal resistance of each cell. They were close in voltage from the shipper.
Then assembled parallel sections. (4p) no issues.
Then made the series connections. All seemed good.
Have A BMS and see the parallel group voltages with sense wires.

The damn battery won't balance properly and the first and last parallel groups in the series seem to change the most?
Is there something inherent in the charge/discharge rates of the first and last groups in a series of connected paralled batteries (ie closest groups to the main battery minus/plus leads) that causes them to drain/charge faster than the groups in the center of the battery??

 

[fechter]

I've seen this happen when the main battery wires coming from the BMS don't go straight to the end cells. The voltage drop in the sensing wires is enough to cause an imbalance. You should also check to see if the BMS is draining those cells slowly. You can do this by charging and let it sit for a while and take voltage measurements periodically. The voltages should all remain steady.

If you have a picture or diagram of how the BMS is attached to the cells, we can see if it looks right.

 

[Barncat]

Mikebike- search for "end cell effect". i'm no tech expert, but have noticed that one or both of the end cell groups on my packs tends to charge faster than the rest (on basic Yarlan 3A-4A chargers). one pack is 20s5p used MJ1, the other 15s5p new p42a- neither with BMS. this excess p-group charge settles very quickly after unplugging the charger, though can still remain some .03-.04V above the rest for a bit. this accelerated charge seems to kick in more above approx 4.1V. all groups tend to rebalance pretty quickly once you start riding and load them up, and in my case end a 30 mile ride within .00-.02V of each other which doesn't bother me... experts may disagree...

to some extent this effect can be minimized by letting your pack rest a bit during charging when the high groups hit 4.1 or so but that's not always practical. you'd think your BMS would be handling this for you. i suppose the most important thing is not to let a couple groups overshoot by too much on charge.