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Posted by David Sherman on Saturday, June 27, 2009 at 16:04:24 :
In Reply to: OT - Solar charging 12V battery posted by Joe Cimoch on Saturday, June 27, 2009 at 09:28:03 :
The small "battery maintainer" solar panels don't need a charge controller. Even under the sunniest conditions, they can't damage a normal-size lead acid battery. A cheap panel with an area of 2 square feet will put out only a little over one amp in full sun. You can generally put 5 amps into a car battery indefinitely without any damage, though you might eventually need to top up the electrolyte. At 1 amp, even that is unlikely. Given that the sun is only directly shining on it for part of the day, an amp or two for the sunniest part of the day won't hurt the battery. It's only when you get into the larger installations (say 50 watts and up) or you have a small battery, like the sealed gel-cells used on motorized gates and fence chargers, that you need a charge controller. For just compensating for self-discharge of a car battery, none of the smaller panels need any sort of charge controller.
Lead-acid batteries, especially flooded ones, are wonderfully resistant to electrical abuse, and solar panels put out fairly mild current, so fancy electronics are overkill in most cases.
They do definitely help, though I find that disconnecting the ground cable is enough to keep a good battery alive over the winter most of the time. It's amazing what minor loads there are on most vehicles that will eventually run a battery down even though you have everything turned off.
The main thing with using a solar panel to keep the battery charged is to put it where it will get lots of sun, and won't have a shadow across even part of it during the sunny part of the day. Any shaded part of a solar panel causes an inordinate amount of leakage. Of course the panel should also have a blocking diode in it to keep battery current from flowing backwards in it when it's dark. Most of them do now, but I wouldn't take that for granted.