I have 2 identical 12 V 100 Ah lead-acid batteries in parallel. I used to keep them fully charged with a mains charger, one with a built-in algorithm for these kind of batteries.

I recently hooked up a 50 W solar (PV) panel to maintain them fully charged instead.

The PV panel is connected to a solar charge controller, the charge controller has a gel charging algorithm, claims overcharge protection, and is set to use PWM.

My problem is this:

Everyday around 09:00 AM the batteries get charged to 16 V+ and the charge controller displays an overcharge error. When I measure the voltage at the battery poles with a multimeter, it displays pretty much the same voltage as the one shown by the charge controller.

Now, I'm pretty sure that this is a spurious reading - when I disconnect the PV panel from the charge controller, the voltage falls to 13.3 V within a few seconds.

The batteries also seem to be quite happy - there is no discernable increase in temperature from the outside, irrespective of how long the overcharge voltage is measured.

Is there some explanation for this?

I'm thinking, maybe the batteries have some built in circuitry to prevent overcharge as well and disconnect the input, so that the charge controller gets confused?

Or could there be some obscure battery chemistry at work, perhaps on a very small scale at the poles?

  • \$\begingroup\$ If they are gel batteries, then spending any significant time at 16v will damage them in the long term (ie maintained fully charged), even if they are not getting hot now. It may even indicate that they are damaged now. You can charge them to 15v, as long as it's cyclic and not maintained. For the long term, do not maintain more than 13.8v on them. \$\endgroup\$
    – Neil_UK
    Commented Oct 15, 2018 at 10:27

1 Answer 1


Yes, a plausible explanation is that this buck converter or shunt regulator is damaged and through conducting. It can measure the overcharge but contoling the MOSFET switch does nothing, because it is probably conducting all the time.

It is normal that after diconnecting the charger the voltage drops, since this is the cell voltage. Even if you charge it with 20V, the battery cells can't be charged to that voltage, it'll just damage them.

You can measure at bright sunlight the panel voltage and battery voltage, and if both votages are equal, than it means that the charger is damaged.

  • \$\begingroup\$ Thanks. What I did was to disconnect both the battery and the solar panel from the charge controller and to measure both. The battery measured 13.8V and the solar panel measured 19.3V. So it seems as if the charge controller is doing something, at least. Also, when everything is connected, the charge controller will display 0 amps flowing, even if the voltage is high. \$\endgroup\$ Commented Oct 15, 2018 at 11:45
  • \$\begingroup\$ @PieterBeneke Actually you could write exactly which battery and which charge controller you use, to have a better idea what's going on. \$\endgroup\$ Commented Oct 15, 2018 at 12:42
  • \$\begingroup\$ It's a STECA PR 1010 charge controller. (steca.com/index.php?Steca-PR-en) The Batteries are 100Ah Gel batteries by a company called 'Enertec'. (enertec.co.za) I don't think they make the exact model anymore.They are about 5 years old, but they had very few deep (50%) discharges - not more than about 3. \$\endgroup\$ Commented Oct 15, 2018 at 13:56
  • \$\begingroup\$ @PieterBeneke Make a draw or sketch about conencted PV, charger, battery and loads. Maybe you did some wrong conenction that bypasses the charger. \$\endgroup\$ Commented Oct 15, 2018 at 14:51

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