Balancing cells

[mkpieters]

I am having a difficult time balancing my prismatic cells. i first connect all + and all- for a day so they can equalize i then connect in a 4s 12 volt configuration and put a load on them to drain down to about 3 volts. once they are all about 3 volts i then charge them up. at first all is great all the way up to about 3.35 to 3.40 volts that is when there always is one cell that shoots up to 3.65 volts and of course the BMS stops the charging.
what am i doing wrong? should i discharge down lower when balancing?
thanks

 

[MisterSandals]

Have you tried top balancing?
I just went thru this process, seems to work but not sure how much my cells really needed it.

To top balance, drain your cells down as you did and leave them overnight to equalize in parallel as you did. Then charge them up in parallel to 3.65v. At this point i disconnected them all and let them settle to their resting voltage overnight. See if that helps or shows you something new.

As a sanity check, have you tried changing the order of your cells? Cell #1 for me seems to be the "most active" for most cells but i think i found one particular cell (in all 3 batteries!) that likes to be in that spot (the far negative end). Don't ask me why, it was pretty disturbing.

Edit: I recommend labeling your cells and keeping notes.

 

[Supervstech]

Many things can cause series connected cells to charge imbalanced.
First would be a weak cell, but also the possibility of oxidation or a loose connection preventing equal voltage reaching each cell.
3 volts isn’t bottom balanced, 2.5 is... but I don’t like taking my cells that low, so I top balance. Up to 3.65V until the amps drops to zero... then I know they are full, and ready to tie in series, discharge to 3, charge to 3.45, and be happy.

 

[FilterGuy]

For most of what we do with solar set-ups, top-balance is more appropriate than bottom balance.
* many of the BMSs only balance at the top.
* Our batteries tend to spend much more time at the top.

 

[mkpieters]

thanks everyone for your input. i have a string of cells in parallel charging to 3.65V. we'll see what happens