PV Gap

Virginia Said:

How long does it take to charge a 12v, 200AH deep cycle battery with a 125 watt solar panel?

We Answered:

Hey Alip, Graham is on the right track, his math is correct, but there are some inefficiencies here, mainly the voltage curve of panels and batteries during charging, and the heat losses inside the battery as well. Fortunately they are easily calculated.

A 125 solar panel that is wired for 12 volts (as most of them are), actually puts out about 18 volts open curcuit, and only 7 amps short circuit. But your battery will not see these voltages, once you connect the two together, the panels voltage will droop to match the battery, but the 7amps will remain. So the problem becomes simply a question of Amp Hours. If your battery is completely discharged, you will have to put back the 200 amp hours it holds, plus about 10% more. On a clear day, you might have sun for 8 hours, but the first 2 and last 2 are not full production because of the angle the sun is to the panel, and how much atmosphere the sun has to travel through. In most clear dry areas, we consider a good day of solar collection to be around 6 hours of direct noon time sun over the course of the day. With 7 amps flowing into the battery for 6 hours, you end up with 42 amp hours to work with, but since the battrey uses about 4 of them heating up and boiling some fluid away, only 38 are stored. At that rate, you would need 6 good days of sun to bring that battery back up to full.

If you're considering hooking something like this up, you'll also need a charge controller since the panels out put is more than 2% of the batteries amp hour capacity (4amps). Charge controllers today are pretty efficient though, maybe 98% or so, so it isn't worth allowing for its losses in your system. If you had 2 batteries wired in parallel for 400 amp hours, you could skip the contoller, and spend that money on the second battery instead, then you would have twice the storage.

We live in a solar powered home, which happens to be powered by 16 125 watt panels. They run almost everything in the house, the lights, appliances and electronics. So you can go a long way with a panel like that if you learn to use your power judiciously. There are some great sources you can check out to learn more if you like, I will list some below. Take care Alip, Rudydoo

Connie Said:

Trying to charge a 12v battery with a solar panel?

We Answered:

I don't think you can build or find a solar panel that would fit in such a small space, even if you were dealing with very low current. The only workaround I can think of would be to build a foldable solar panel that would deliver enough current to charge the battery. And attach one end of it to the box lid. But you'd have to be careful to make sure that folding and unfolding won't break the cells. Or the weight of it folded up won't break it.

FYI, I have a very small and compact 6V solar panel that delivers 340ma. And it takes up more than 2x2 inches.