I will try to sum everything up and keep it as clear as possible.
I have a solar system:
- inverter: 1500VA 24V system
- batteries: AGM 100Ah (20 months old daily cycle)
- controller: semi-MPPT 80A 24V
Setup:
Suddenly my system started shutting down quickly because of low battery, after testing I noticed that batteries (2,3,6,7) were 10.5V while the rest were 12.5V.
I took them all out, balanced them all and returned in the exact same order and were charged to 100% (27V,13.5V/Bat.), within 48h the problem occurred again, so I decided to take the 4 bad batteries out and balanced the good batteries voltage, this is how the system looks like now:
Within 24h and after fully charged and used I tested it and found out that batteries (5,8) were 10.5V while the rest were 12.5V
What could be the cause? This has confused me a lot! Has anyone seen or heard of this phenomenon?
I don't really know how to approach or diagnose the problem
Load: max estimate is 1.2Kw used since sun down until tests time
Best Answer
If the batteries all the same model, and you are doing a complete charge/discharge each day then you've done about 600 cycles.
At this cycle life age I'd expect the batteries are starting to show significant capacity differences. This will mean the terminal voltage during discharge will vary and hence the re-charge be uneven.
You could extend the battery life by putting active balancing in. I'd suggest that you should connect 1,3,5,7 and 2,4,6,8 in parallel and then the two strings in series. Then you only need one active balance circuit for the whole group of batteries.
Even if you put active balancing in it's only looking at the 12 V level and not at individual cells, so variations in capacity/age will still occur. This means you will slightly overcharge some batteries (AGM is very tolerant of this) and undercharge others. Overall it's still better to have the batteries paralleled at the 12 V level to reduce differences as the fully charged terminal voltage will be the same regardless of the battery capacity.