How do I tell when a car battery is fully charged and can it be over-charged?

Question

I am using a lead/acid 12 volt battery made for riding mowers to power various electronics while camping. At night It powers a fan and some lights and during the day I recharge it with a 5w solar panel.

I used a multimeter to check the voltage before and after a night of running the fan for 5 hours and the volts started at 12.60 and dropped to 12.35. I left the battery sitting there without charging it for a couple of hours and the when I checked it again it had climbed back up to 12.5 volts.

I have read that a battery of this sort should be somewhere around 13v when fully charged but I am not sure if I need to unplug the solar panel when it gets to 13v or if it is safe to leave it on all day. When I tested the solar panel output with the multimeter i got 23 volts in direct sunlight which is higher than an alternator puts out so i am worried that it may overcharge the battery. how do I know when the battery has full charge and is it bad to leave the panel on after that?

Answer 1

It can be over-charged and that would damage the battery, cause gassing, and possibly a fire or explosion if it's overcharged too long.  This is why the created smart-chargers.

But it sounds like you are only using a solar panel?  At 5 watts a solar panel (usually these are used to continously maintain batteries on less used ATVs, boats, and such) shouldn't have too much danger of over-charge, 13v isn't too bad.  Most cars charge up to 14v continously.  If you get more panels that push more than 1 amp (12 watts) you should consider getting a charge controller that will monitor the power but I don't think it's needed if the solar panel is the only thing charging the battery.

You may want to get a deep-cycle battery if you only use it for powering those lights and fan.  These are designed to put out low power for a long time (amp-hours) whereas a car battery is designed to put out very high power (engine cranking amps) for a very short time.  The other option would be a sealed lead-acid (or gel) battery because those won't leak if they tip over...whereas a car (or lawnmower) battery or deep-cycle wet battery can leak dangerous acid if it leaks.