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?