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

Battery 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:

After removing the bad batteries

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

  • \$\begingroup\$ You don't tell what the capacity (actual, not the one printed on the battery) \$\endgroup\$
    – PlasmaHH
    Dec 25, 2017 at 22:23
  • \$\begingroup\$ The lower voltage on the battery does not mean that this battery is worse. It may be that you chose accumulators with less capacity and not vice versa. \$\endgroup\$
    – AltAir
    Dec 25, 2017 at 22:29
  • \$\begingroup\$ Battery capacity decreases over time. It is better to group the battery by the residual capacity. \$\endgroup\$
    – AltAir
    Dec 25, 2017 at 22:40
  • \$\begingroup\$ @PlasmaHH i bought them all at once and from same patch, they should be same capacity even when it decreases over time.. problems would surface on a somewhat balanced way and not in such a way. \$\endgroup\$
    – bakz
    Dec 25, 2017 at 23:42
  • \$\begingroup\$ @AltAir thats what i did as illustrated above, but the problem continued showing thar weird zig-zag pattern.. consider the numbers on the battries as IDs, also consider my comment previous to this. \$\endgroup\$
    – bakz
    Dec 25, 2017 at 23:44

3 Answers 3


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.

  • \$\begingroup\$ If u check the 2nd setup you will notice that i have removed the battries (2,3,5,6) which showed in my last test to have had low voltage, but when i tested the 4 battries that should be ok they showed the problem and at the exact spots! I am pretty sure i did not mix them up as i labeled them beforehand and kept a note of all. Also the battry is rated at 1200cycle, 800Ah is too much for me so I almost never used to hit the 50% margin a lot and had it at 70%+ a lot of the time \$\endgroup\$
    – bakz
    Dec 25, 2017 at 23:50
  • 1
    \$\begingroup\$ You are half way through the anticipated cycle count, so I expect that they are starting to lose capacity. Even with just a 20% capacity loss at life you will start to see the capacity alter significantly half way through. Remember that your batteries are made up of multiple 2.4 V cells so lots of cells to age. \$\endgroup\$ Dec 26, 2017 at 1:15

Capacity loss is individual for each battery. When two batteries are connected serial and one will throw the capacity a bit faster than the other. It performs a deeper charge-discharge cycle. Which leads to even more loss of capacity.

It is necessary to measure the capacities of the batteries, this will clarify the situation.

It is better to group the battery by the residual capacity.

It seems to me that batteries with a lower voltage (2,3,6,7) have a larger capacity and in a better condition.

  • \$\begingroup\$ I only needed around 3kw/day, i took 8 batteries (near 8kw) to be over twice my need, the batteries never went under 70% charge for the first year or so. Thus the batteries are rated at 1350 cycle at 60% (~4years at 1cycle/day). The decay is much more than significant, hence why I am here. \$\endgroup\$
    – bakz
    Jul 1, 2018 at 15:15

Capacitance reduces and ESR rises sharply below 10% SoC or ~ 11.5V ( ballpark) Since there is no evidence of active or passive balancer for charge or discharge useage, all batteries have reached end of life for this design.

Had you included an active balancer for each cell, it would have slightly extended your end of life charge cycle to beyond 20mo x 30d = 600 cycles ( full?)

Battery imbalance accelerates as it ages and one of the two in series will ALWAYS age faster.


simulate this circuit – Schematic created using CircuitLab

How much beyond 600 cycles , depends on many factors such as balancer capacity, charge rate and battery capacity and ESR balance when new.

Impressions: It seems like a major expense and a high risk venture without a good BMS and active balancer. Does it have 3 yr warranty? or only 1?

  • \$\begingroup\$ They dont really give warrenties as they say they are "afraid of customer's miss use which could cause it to fail in less than a year". Is there a way to reverse the aging process? \$\endgroup\$
    – bakz
    Dec 26, 2017 at 15:23
  • \$\begingroup\$ not likely......and they are correct in your case . strings and arrays need cell balancing to prevent your inevitable failure mode. Be glad they last that long. Get a decent battery tester from Cadex. cells need to be matched otherwise when depeleted <15% SoC, one have see >10x more dv/dt reduction in C[F] in shared current resulting in SoC runaway from accelerated change in voltage. \$\endgroup\$ Dec 26, 2017 at 17:36
  • 1
    \$\begingroup\$ In sealed AGM you can't get to individual cells. You can only balance at the battery level, in this case 6 cells. \$\endgroup\$ Dec 26, 2017 at 17:46
  • \$\begingroup\$ Correct. but at least internal cells will be matched better. The issue here is the S2P4 array has no balancer . Ideally cells need to balanced <<1% for capacity and ESR. Have you ever heard of a 10kFarad cap with 1% tolerance? If I had no access to an active balancer, I might have used resistance wire to bridge the 12V connections to limit current. \$\endgroup\$ Dec 26, 2017 at 18:42

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