Defective cell?

[Xavier Alexandre]

Hi all, 

2 nights in a row now, I see my system in UVLK state. I know that the MOSFET supposed to kill consumption is not operational right now. But what troubles me is that this is a 400Ah battery, which was close to 100% the night before and that the only consumption was a 3A fridge. 

In the morning, the battery is at 11.5V and one of the cell is below 3. See attached. I am thinking this cell is likely dead. Any other suggestion?

Thanks in advance,

Xavier

[Dacian Todea (electrodacus)]

Xavier,

Your battery is fully discharged not just that one cell. The photo's you provided show a 48A charge current (assuming the current shunt is set correctly) and at this particular moment total battery voltage is 12.8V while you mentioned being 11.5V.  

I can asumme that lowest cell was at around 2V or even below and that is not good and will damage the cell so you need to fix the EXT IO3 to allow SBMS to control all your loads ON/OFF.

So with the assumption that low cell was 2V then 11.5V - 2V = 9.5V / 3 cells = 3.166V and that means the other 3 cells where probably at around 5% SOC so not much difference between the lowest cell and the other 3 cells.

You need to fix the battery protection both for under voltage and over voltage as if the SBMS0 can not have full ON/OFF control of all Loads and separate for all charge sources it can not protect your battery and you will damage the battery.

Then it seems the SOC indication is incorrect and that is likely due to current shunt being set incorrectly and or current shunt offset being incorrect likely due to the fact that current was flowing trough the shunt when you last powered ON the SBMS0 due to SBMS0 not being able to turn OFF all loads during power ON thus it considered whatever current was flowing trough shunt at that time as zero and thus the reason SOC is always incorrect.  SO your battery was nowhere close to fully charged as you where thinking and thus ended up with a fully discharged battery.