question

Sounds as if battery A is faulty. You can test by removing the other batteries, applying load to A and measure voltage drop over time. Then replace A with another battery. Do the same test and compare.

Hi @Oleksandr_P

Even from new, batteries can arrive with differing resistance. So then you apply the recommended charge V, the ones with less resistance accept more current, get hotter, develop less resistance, then take more current again. This is usually noticed in practice by their Temp, and a touch-test can often help. Ease back on their Charge V and don't push them so hard under charge.

Hello To All.

It seems @JohnC was right.

I discharged all batteries together at 20% (SOC80%), then I charged every battery individually till storage mode and voltage V=13.25. The average consumption of the system is about 130W (max 200W), so the current is 130W/12V=10.8A, 10.8A/4 batteries=2.7 A per battery. But in reality, the discharge current from the batteries is distributed as follows:

Total – 10.8 A (100%)

Battery A – 0.7 A (6.5%)

Battery B – 3.9 A (36%)

Battery C – 3.4 A (32%)

Battery D – 2.8 A (25.5%).

After several hours of discharge (about 6 hours) I noticed that the current have been changed as follows:

Total – 10.8 A (100%)

Battery A – 2.7 A (25%)

Battery B – 2.8 A (26%)

Battery C – 2.7 A (25%)

Battery D – 2.6 A (24%).

As we can see the current is almost equal for each battery.

So I decided to investigate this situation for a few days:

I increased the loads up to 300 W and was discharging batteries for about 6 hours:

As we can see current was changing every hour and in the end, it becomes almost equal for each battery. Then I charge every battery individually. The next days I was discharging batteries with a load of 130-150 W. I noticed the same trend: at the start, the current is different for each battery but with the next hours it heading to equal for each battery.

Then I decided to charge batteries together. Right after the 6th hour of discharge the grid has been recovered and the batteries start charging (charge current in Victron Connect setup 37A) but again current is distributed unequally:

Total – 37.5 A (100%)

Battery A – 7.9 A (21%)

Battery B – 11.6 A (31%)

Battery C – 9.4 A (25%)

Battery D – 8.6 A (23%).

I am afraid that during discharge at power about 150-200W batteries B and C will fail faster than the other ones (in accordance with the datasheet the rated capacity is 20h, which means the discharge current should be not more than 90Ah/20h=4.5 A, but in my case, during the beginning hours discharge current of batteries B and C more than 4.5 A (6A and 5.3 A respectively for battery B and C)).

Maybe I'm worrying too much and this situation is normal?

@Oleksandr_P

Ideally you would want a much shorter bus bar and the exact same length cables to each battery connected to the bus bar.

And from the inverter to the bus bar the exact same length path.

How you have done it some will always be cycled deeper than others. It is quite typical of battery stacks to behave like that. You see that behaviour even in Pylontec stacks. I also dont like it

Thank you for the answer @Alexandra. Previously batteries were connected halfway via 35mm2 wires with almost the same length (see screenshot) but the problem was the same. I also switched batteries, but battery A always gave/consume less current. I do not think the shorter busbars will solve the problem.