Mismatched Voltage Cells

[NaughtGeek]

I had to replace 2 cells in my 16S4P system. Since they were new, their SOC was off from the bank they're in. I charged the entire system and then discharged until the new cells hit the low voltage cutoff (2.5) Now I'm changing the entire pack at 40A (10A per bank) 52.6V(3.29 x 16) to slowly bring the SOC back up. Is there anything more/better I can do to bring these 2 cells in line with the rest of the system or if I just leave it once the inverters feel the pack is charged will they eventually even put? I don't have panels yet so this is only serving as a UPS for my house. I have a Batrium BMS but once charging stops I don't know (understand) if the bypass function will do the job.

4 MPP Solar LV6548
64 EVE 304AH cells 16S4P
Batrium Watchmon Core with 4 Cellmon K9s
Solar Assistant for Comms With Home Assistant

 

[sunshine_eggo]

The bypass function will do the job, but it takes time. I run an older WM5, and it has about 0.5A of passive balance in bypass.

Something got out of whack with my cells, but it brought my pack in balance:

Before:



mAh burned off per cell:


dV trend:


Note that this is a 14S NMC battery - not LFP.

You may find it easier using the Autolevel function rather than relying on the bypass voltage in the cellmon hardware screen.

 

[NaughtGeek]

Thanks for the reply, that's a LOT of bypass mAh! I thought I understood it was a bad idea to use the Autolevel function on the LiFePO4 chemistry.

 

[sunshine_eggo]

Thanks for the reply, that's a LOT of bypass mAh!

450Ah, and somehow two of the cells were completely wonky - one higher voltage and the other lower voltage. I put the two in parallel, and it transferred about 50Ah between them over the course of an hour. This is what it looked like BEFORE I started using the Batrium to balance:

I thought I understood it was a bad idea to use the Autolevel function on the LiFePO4 chemistry.

You use it when you need it, and it sounds like you need it.


The key is to ensure that it's only happening above 3.40V.

Once it's brought into balance, you resort to the standard method.

 

[NaughtGeek]

Much appreciated. Cells are currently hovering around 3.34(low cells 3.3) so it will be a while but I'll take that direction and see what happens.

 

[sunshine_eggo]

Common for imbalanced cells to read nearly identical in the operating range. Balancing should not occur below 3.40V. As you drive to higher SoC, you'll see what's up.