So, I have solar power system and it is as follows:

  • 2 AGM batteries each 100 Ah
  • 2 solar panels, 24 volts, 250 watts each
  • a controller
  • an inverter

the batteries are connected in series to give a total of 24 volts and 100 Ah which in total is 2400 Watts. On the other hand, the panels are connected in parallel because they are already a 24 volt system.

Now when I first started using my system I noticed that the total daily charge is around 20-40 Ah which to me was not logical because I expect the batteries to charge around 60-80 Ah on a daily basis since I use the batteries as much as possible before my inveters beeps.

After a while I noticed an unexplainable degradation in the system performance so I did some measurements.

During the charging phase while in sun light I have noticed that the two batteries had almost the same reading which was around 13.8V. However, after using the batteries for about 6 hours, I have noticed that one battery was reading ~12.4V while the other was reading around ~8.2V.

So I turned off my inverter and after a while the first battery still read ~12.4V while the other read ~10V.

Now I want to know what caused my batteries to degrade and what can be done about it.

  • 2
    \$\begingroup\$ Might be a busted cell. \$\endgroup\$ Commented Apr 24, 2016 at 22:47
  • 1
    \$\begingroup\$ As Ignacio commented you may have one bad cell in the lower voltage battery. This one bad cell could also be reducing the overall charge current in the rest of the battery system. Be sure that your solar panel set is capable of supplying adequate voltage (more than 24V open circuit) into the charging system. Verify the needed requirements of the charging system, (what is the minimum input voltage/current, what is the recommended connection scheme for multiple panels, does each panel need a separate blocking diode, etc). Additionally, it may be best to buy at least one new battery. \$\endgroup\$
    – Nedd
    Commented Apr 24, 2016 at 23:15
  • 2
    \$\begingroup\$ 24 volts at 100AH is not 2400 watts, but 2.4kW hours. \$\endgroup\$
    – user86234
    Commented Apr 24, 2016 at 23:53
  • \$\begingroup\$ Yes, it is 2.4kW hours but I thought that it was implied \$\endgroup\$
    – Saleh Omar
    Commented Apr 25, 2016 at 8:43
  • 1
    \$\begingroup\$ Ok, per the Math: A 24V panel at 250W, 250W/24V = 10.42A each, so with 2 panels in parallel that's a max of 20.84A. Are you expecting more? With a bad cell the battery resistance could be high, so the charge current could be even lower. Worse yet, if the bad cell is partially shorted the other cells may overcharge and soon go bad too. \$\endgroup\$
    – Nedd
    Commented Apr 25, 2016 at 10:11

1 Answer 1


I use the batteries as much as possible before my inveters beeps.

You are running the batteries flat. This is bad because the cells won't all have exactly the same capacity, so some will run out before others and may eventually get damaged. You should never let the batteries run right down, but make sure they get a full charge as often as possible so that the cells stay equalized.

In full sunlight your panels can deliver ~20A of charging current, but to fully charge a 100Ah battery you need at least 5 hours at 20A (the 'bulk' phase), plus another 10-15 hours at reduced current (the 'absorption' phase). This could take several days.

One of your batteries may now be permanently damaged from over-discharge. To find out if it is recoverable, charge it by itself (with a 12V charger) until it can take no more charge (this may take 20 hours or more depending on charging current). Note how much charge it took. Then charge the other battery by itself until it too is fully charged.

After charging the batteries individually they are now equalized at full charge. the amount you can take out is determined by the weakest (lowest capacity) battery. You should take out no more than 80% of the weakest battery's capacity, then put back in more than you took out. That way the batteries should always return to full charge and stay equalized.

  • \$\begingroup\$ I agree with you except that what I meant by using the battery as much as possible is that I use it until my inverter beeps, the inverter beeps when the battery is at 28% capacity as a safety measure. \$\endgroup\$
    – Saleh Omar
    Commented Apr 25, 2016 at 8:33
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    \$\begingroup\$ @SalehOmar - "the inverter beeps when the battery is at 28% capacity" - unfortunately that is not completely true :-( Your 24V inverter does not know the individual 12V battery capacity (i.e. voltage) levels. It is measuring the overall 24V level. As you explained, your inverter continued running even though one of your 12V batteries (now likely damaged) was reading ~8.2V !! :-( In short: On a 24V system, using 12V batteries, something needs to monitor the individual battery voltages to avoid a much weaker battery not being noticed, due to the higher voltage from the stronger battery. \$\endgroup\$
    – SamGibson
    Commented Apr 25, 2016 at 9:48

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