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Solar panels for battery charging

Forum members have occasionally asked about adding one or more solar panels to the roof of their RV to maintain the batteries in a state of charge. Here's some advice offered by Gary Brinck.

If you are just keeping the batteries topped off, a small panel, say 18-24 watts should do it if you have adequate sun when and where it is stored. The angle of the sun and haze in the sky makes a big difference, though. In northern climes during the winter, the sun angle is low and the sun "weak". And sometimes a moderately bright day may still be hazy and panel output is neglible.

The other question is how much parasitic load is on your batteries? Parasitic load is the drain from various appliances and components that occurs even when they are idle, and from sensors that function all the time. Examples include refrigerator, TV, VCR & stereo controls, clocks, LP & smoke detectors (if wired in). Just about anything with a remote control or clock is drawing a small amount of power when "off". In larger motorhomes parasitic load is often high, sometimes a couple amps per hour, but in trailers it is usually well under 1 amp. But even 0.25 amps/hour adds up when it is 24/7. You can measure parasitic load by disconnecting the battery ground cable and putting an ammeter between it and ground.

An 18 watt panel delivers about 1.0-1.25 amps to your batteries for as long as the sun is bright and more or less directly overhead. That may be only a few hours a day at some times of the year, so you get a few hours of charging versus 24 hours of discharging from the parasitic loads. To compensate, you may need to increases panel size so that a few hours of charging can replace 24 hours of discharge (load). And of course you can try to remove the parasitic loads by using a battery disconnect switch or perhaps by placing some appliances in a full off position (rather than stand-by) or simply unpluging them.

5 amps is probably about what you will get out of an 80 watt panel at max sun. The output is typically rated at peak voltage, which is usually around 15.0-15.5 for these panels (will depend on the brand). After regulation it will probably be a bit under 5A.