Charging depleted battery with solar panel?

Thread Starter


Joined Oct 29, 2009
I have a rather old and week solar panel. It is charging a 12v 5AH battery though a charger.

A combination of several days of bad weather and bad ground caused the battery became totally depleted.

The battery has been charging fine all along till this incident. But once I got the ground situation straightened out and the sun came out the battery still refused to charge.

The panel is still operating delivering 22 V without a load.

So I took the whole works back to my shop where I connected the battery and charger up to my bench power supply. The battery started to charge immediately.

I am not certain what is happening here but I will guess. As long as the battery is at some reasonable level, the internal resistance is low enough to allow my wimpy solar panel to maintain the charge on my battery.

When the battery became depleted, it's internal resistance when up. My wimpy panel does not deliver enough current to over come the internal resistance so it does not charge.

Another curious thing. When I have the battery hooked to to the panel (via the charger) the charging voltage never exceeds the battery level. As the battery charges the voltage rises to the charged voltage.

I was told by the tech at the company where I bought the charger that this is normal.

When I have the charger hooked to my bench supply, the charging voltage at the battery is around 13 V and then starts to drop a bit as the battery charges. This is what I expected to be happening, same as a battery in an automobile.

Is the same reason I am not seeing the charging voltage of 13V with the panel connected because of the wimpy panel struggling to overcome the internal resistance of the battery?


Joined Feb 19, 2011
Sorry if I miss understood the type of battery you are using, but is this a sort of lead acid battery for a alarm system? I'm wondering if your panel isn't providing enough watts the battery needs to charge. Also if I may ask, do you have a regulated voltage and a protection diode to keep the current from going from the battery to the solar panel?