Weak cell?

[Dave (Boog)]

I recently built a 48v battery using 16 272ah Lishen cells. I finally got around to running a discharge test using my Growatt SPF 3000TL LVM-ES inverter with a 1600 watt heater as a load.
I let it run and monitored the cells on the JK BMS app. The test ended when just one of the cells, #5 caused a low voltage disconnect. The remaining cells had various voltage’s remaining but none close to cell #5. Cell #5 reached 2.722 volts and the highest cell voltage was #11 at 3.177 volts.
Any ideas on why this single cell #5 would have gotten so low compared to the others?
Just a weak cell? Any further tests I could do? Is there anyway to possibly save this cell and get it to be closer to the others or should I replace #5?
Any suggestions would be greatly appreciated.

 

[SeaGal]

Was the pack top balanced initially / recently?

 

[Dave (Boog)]

Thanks for your question. Yes the cells were top balanced. Just about two weeks ago.

 

[Brett V]

Double check all your cell interconnects and BMS sensing leads. How did you prep the connections? A photo might help someone to spot a potential problem.

 

[SeaGal]

IMHO, I wouldn't say cell #5 is particularly weak - there are already 6 other cells below 3.0V and once you get to the bottom knees, the voltage will drop off very quickly.

To me it looks like 1 to 8 are pretty well balanced with each other and 9 to 16 are balanced with each other as they are all around 3.1V. but there's a bigger difference than I'd expect between the bottom 8 and the top 8.

Are they all the same cells from the same batch, or did you join two 8s packs together at some stage?

 

[Dave (Boog)]

IMHO, I wouldn't say cell #5 is particularly weak - there are already 6 other cells below 3.0V and once you get to the bottom knees, the voltage will drop off very quickly.

To me it looks like 1 to 8 are pretty well balanced with each other and 9 to 16 are balanced with each other as they are all around 3.1V. but there's a bigger difference than I'd expect between the bottom 8 and the top 8.

Are they all the same cells from the same batch, or did you join two 8s packs together at some stage?

Thanks for your comment. You actually caught something I thought would not be readily apparent. Cells 1-8 are the Lishen 272ah cells, and 9-16 are Eve 280ah cells. I did notice that I only got 254ah out of the 272 that I had hoped for.
When I tested the Lishen cells individually I got between 275 to 277ah.

 

[SeaGal]

I recently built a 48v battery using 16 272ah Lishen cells.

Cells 1-8 are the Lishen 272ah cells, and 9-16 are Eve 280ah cells.

Yeah... so despite the mis-information, I solved the puzzle and got the correct answer what do I win?

 

[Dave (Boog)]

Double check all your cell interconnects and BMS sensing leads. How did you prep the connections? A photo might help someone to spot a potential problem

Rechecked all connections. I know that the wiring of the sense leads to the BMS and cells is correct. I did re-torque all the bus bars, they were already torqued to 4nm. The BMS ring leads were all crimped and soldered. Sense ring connectors are directly on top of the bus bars, and underneath the serrated flange nuts.