Parallel connection problems

[tcparris2]

 

[tcparris2]

3.9 volts is too high for an LFP cell. 27.1 on the whole pack should only be 3.3875 on all 8 cells, so you have a serious balance problem. Use a good meter and measure the voltage at each cell's terminals. You may have a bad connection somewhere, or the BMS is not working correctly also. Most do not recommend going over 3.65 volts on LFP cells. If all of the voltages look ok directly at the cell terminals, then I would suspect a bad connection between cells, or a problem with a BMS lead connection. If you add up the voltage you read at each cell, it should be very close to the voltage across the entire pack. If the full pack is higher than the total of the cells while charging, it is a bad connection somewhere. And if the cell voltages do not match what the BMS is reporting, it is also a sign of a wiring problem, or a bad BMS.

Thank you for the valuable information! I hope I’m not coming across as arrogant. I know I’m dealing with fire literally and potentially 100’s of dollars. Now that I know the panels are decent I’ll set the BMS parameters such that the bad cell won’t go above 3.65. This brings the charging capacity of the pack way down and stresses the BMS but I know I won’t damage the cell any more. It’ll just charge between 20% and 70% in partial sun and BMS will cutoff charging around 8 A until I get the new cell.

 

[tcparris2]

So I actually have another 4 pack coming because three other cells are iffy too if you check this out.

 

[MisterSandals]

I bottom balanced them all and when charging or discharging that one cell has a huge voltage drop or gain.

I think the consensus for solar is to top balance LiFePO4 cells.
If you’re at the point of just waiting for a replacement call, it might be worthwhile to try a top balance.
Worst case you can get some experience to balancing.

 

[tcparris2]

I think the consensus for solar is to top balance LiFePO4 cells.
If you’re at the point of just waiting for a replacement call, it might be worthwhile to try a top balance.
Worst case you can get some experience to balancing.

Cool, thanks!