Question Regarding Cell Imbalance

[Sergio Veltri]

Hi!

I’m currently using one the SBMS120  and I’ve noticed something in the monitor screen that I would like to ask about. I’m attaching a photo of the screen for reference.

As you can see, the battery voltage is 24.06V and the charge level is at 39%. However, I’m concerned about the cell voltage difference (Cell Δ), which is reported at 390 mV.

Is it because levels are too low or the cells internal resistance is high?

It is dark and rainy today, thats why little solar imput.

As a reminder, a couple of years ago I had to replace the shunt amplifier and also removed the balancing resistor of Cell number six. I’m not sure if this could be affecting the current readings.

Any advice or suggestions? The cells are 100A each and I've had them for 3 years already.

I also changed the dynami cut-off in Victron from the voltage values in the picture to lower values 23.2 22.8 21.5 and 20.5 so that the Multi didnt turn off.
Thank you for your time and support.

[Dacian Todea (electrodacus)]

Sergio,

All other cells except cell 6 are very close to 0% SOC while cell 6 is at around 39%

Not sure when you removed the cell balancing resistor on cell 6 but likely a few years ago in order to get at this sort of cell delta.

In any case the only thing you can do is to manually discharge the cell 6 so that is much closer to the others. And you may need to do this once a year or have the cell balancing fixed on cell 6 if possible.

Here is a graph with typical SOC vs cell voltage for LiFePO4

You can see you cell 6 will be around 35% based on this graph the 2,5,8 around  3% SOC and the rest of the cell 1 around 2% SOC so cell 6 is the only one that is not balanced thus highest SOC.

That also means that due to imbalance you can only use about 60% of your battery capacity and so you need to manually balance (discharge) cell 6 to get it in line with the others.

I will wait for better weather conditions as now your battery is fully discharged 2% SOC at the moment and then use a load to discharge about 140 to 150Ah from cell 6

So say you can have some load on cell 6 that draws 1A you can leave that there for 140 to 150 hours to get it in line with the others. And if you do not fix cell 6 balancing you may need to do this once a year or so.

[Sergio Veltri]

Thanks for the quick feedback and recommendation! 

Some time this year I will get the new SBMS0 and solve it.

Thanks again!