Help me understand my cell voltages at the end of a charge

[cajocars]

I am using a Seplos BMS with a 280Ah 48V pack made of LF280K cells (the one with double stud terminal)

I have an active balancer but I’m not using it at the moment

I’m charging at night with 50A, then when one of the alarms is triggered it switches to 9A and then lower the current to stay at 56V. I think the charge ends when the coulomb counter counts 280Ah.


Why are some cells increasing voltage and some decreasing voltage at the end of the charge? They are effectively swapping places, so an active balancer would be counter productive!

 

[400bird]

Looks like your current balancer is doing it's job. There's almost no energy stored between 3.5 and 3.55 volts, it doesn't take much balance current to pull a cell down from 3.55 to 3.5, in the meantime time your still putting energy into the cells, so the others (cells that are not being balanced) are raising

 

[cajocars]

Looks like your current balancer is doing it's job. There's almost no energy stored between 3.5 and 3.55 volts, it doesn't take much balance current to pull a cell down from 3.55 to 3.5, in the meantime time your still putting energy into the cells, so the others (cells that are not being balanced) are raising

The active balancer was off

 

[400bird]

Then nothing about those spikes makes any sense to me.

 

[cajocars]

Then nothing about those spikes makes any sense to me.

Could it be the passive balancer?

 

[400bird]

?
Yes, now it makes sense again.

 

[Craig]

As long as your cells are all within a 100 milivolts or less at full charge I think you are fine. There are many things going on with cells that can affect their voltage at the top end. If they get further out than that then I would turn on the active balance just to stop any one cell from running. I kind of picture cells like big buckets of water that hold 280 gallons however the buckets are a a slight bit different in configuration they all hold the same amount however one bucket may be an inch shorter but an inch wider , same volume different configuration, since this is the case at the extreme ends things get a little wonky.

 

[cajocars]

This is with both active balancer and passive balancer off

 

[cajocars]

I'm still trying to understand this behavior. I have both the passive and the active balancers off here
Single cell overvoltage protection 3.65V (triggers 100% and stops the charge)
Total overvoltage protection 56V (when the pack reached this voltage, the current starts to taper)







Which are the most charged cells? I think it's the ones that have higher voltage when the current is almost 0A, after 5AM.
The passive and active balancers (before I disabled them) were set to start at 3.45V, so IF it started balancing at 4AM, it would bring down the cells that seem to have the highest voltage, while in reality those cells are the least full.
A typical cell internal resistance is 0.15mOhm and a bad busbar connection is 0.5mOhm; 0.65mOhm at 50A would only cause 32.5mV voltage drop, not 350mV (highest cell (1)@0A- lowest cell(2)@50A+ (2)@50A- (1)@50A)

EDIT 18/11 (active balancer ON when current drops <2A)