If that panel is from VW it is indeed a trickle charger. They do a halfway decent job of keeping a car battery around 12.5 volts, which satisfies our delivery requirments. As far as output the best I've seen them put out is about 300 milliamps. There is no regulation but I'm sure no battery has ever been overcharged by one.
I've got 4 or 5 laying around and have been comtemplating wiring them together, mounting 'em on a piece of plywood and setting it on top of the AC unit.
Good luck, Tom.
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