# solar battery charger

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#### induction1

##### New Member
i need to charge two 12 volt, 2 amp batteries (hooked up at 24 v) with two solar cells rated at 9v, 60 ma each. Is it better to hook up the solar cells at 18v or 120 ma? any other ideas? also, the batteries will power a 1 amp device; do i need to limit the current to protect the device?
thanks

#### hamfiles

##### New Member
If you intend to charge both batteries, I would connect the panels in series to get 18 volts, charge going into the 24 volt battery set. If individually charged, connect the panels in parallel to get max current. Probobly more panels would be better, because 60 ma is a very small charge current, and from my experience, solar panels rarely put out the rated voltage and current, even in direct sunlight.

If you power a 1 amp device, this simply means that it will draw 1 amp of current from the voltage source. The battery voltage rating means how much current the batteries are able to supply. The actual current flow is determined by the resistance in the connected device. Current equals voltage divided by resistance. If your device draws 1 amp of current, that is how much current will be drawn from the voltage supply. You could connect it to a 1000 amp generator without damaging it. As long as the voltage level from the generator is the required amount, the device will only draw 1 amp of current. So no current limiting is needed. In fact, you always want your supply to be able to deliver more current than your load will draw.

I don't know alot about chargers, but here is a link that describes some:
http://www.solorb.com/elect/solarcirc/

#### Dean Huster

##### Well-Known Member

I don't see how you can charge a 24 volt battery with only 18 volts. You have to have more poopie from the charger than will be available from the battery, so either you need a third solar panel in series for 27 volts or use the two you have with a switcher that will bump the voltage up for some decent headroom. But 60 ma isn't a heck of a lot to work with in any case. What're these panels made from -- old solar-powered calculator panels?

Dean

#### hamfiles

##### New Member
That's true, I don't know why I didn't think of it. With 18 volts going into the batteries, the highest charge they could go up to would be 18 volts. Hardly a decent charge for a 24 volt source.

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