Two or three years ago I started getting together items to rebuild a new battery bank. One of the items I got was a couple of these 12VDC 10WATT Solar panels...
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I started to plugin this into an always hot 12V socket in the truck that would trickle charge my Truck Start Battery...
This has really prolonged my truck start battery... Every time I look at it is at the 12.5V or so level... The charge rate is in the low MA's that will not be enough to overcharge anything...
I lay this small footprint solar panel on the inside dash where it gets sunlight most of the time... Seems to be just enough trickle charge to give my truck start battery a slow charge...
I see these small 10 WATT solar panels at the auto parts places and of course AMAZON has it as well... I leave it plugged into a always hot 12V socket that is in the back seat area. My front 12V socket is under control of the ignition key...
Roy Ken