Top Balanced 8s Pack - High Volt on 3 Cells While Charging

[alferz]

I'm in the process of building an 8s2p LiFePO4 12.8 KWh DIY Powerwall and I have assembled my 230Ah prismatic cells first in parallel, and top balanced them to 3.65v with a bench power supply. After assembling in series I'm seeing good balance between cells of around .003, never over .01 while discharging at modest rates (1-15 amps). Discharging at higher rates I sometimes see delta of .01-.02 but not much more than that. I'm using a multiplus-ii to charge them individually for now at 70a, and one thing I noticed is that the delta grows larger, like .12-.13 volts at the top end as the pack approaches 28.8v. See screenshot. It always seems to be the last cells in the pack, cells 5, 7 and 8 in the screenshot below. Its not causing any problems with cell overvolt in the BMS but I'm wondering if this is problematic for other reasons.

My question is, how much of a delta is too much while charging or nearing the end of the charge cycle? What else can I check to bring them close together if the voltage is an issue?

 

[sunshine_eggo]

My question is, how much of a delta is too much while charging or nearing the end of the charge cycle? What else can I check to bring them close together if the voltage is an issue?

If you can charge to the desired absorption voltage without triggering charge protection due to a cell hitting 3.65V, then you're fine.

 

[Warpspeed]

I might suggest lowering your charging voltage to 27.6v or 3.45 volts per cell.

Around 3.45v is where the voltage begins to climb very rapidly. Above that there is very little extra energy being stored and you are risking going overvoltage. Higher voltages also shorten the expected lifetime.

Much safer to stay closer to the flat voltage part of the charging curve, rather than pushing too far into the exponential voltage rise area, where bad things can happen much more rapidly.

 

[mikefitz]

Your screen shot shows a fully charged battery with all the cells over 3.50 volts and 4 amps charge current. As suggested there is no need to exceed 3.45 volts per cell under charge unless carrying out a capacity test.
It seems the cell protection high volts is set too high allowing the cell to reach 3.668.
It's possible where you are seeing high delta at high current, cell to buss bar resistance is a factor

Mike