I have been using one of those 10-watt Solar Panels sitting in the windshield area to trickle charge extra batteries when needed... When connected to my battery I can see my battery charge start out above 12VDC right away and will eventually creep up to 12.6-7 Volts
I think the MAX current is around 100MA or so which should not do any over charging of your battery...
This certainly makes a difference if you are sitting for a long period of time and not running the vehicle etc...
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Mine has these 2-pin connectors on it and I built up a lighter plug adapter so I can plug this solar panel into an always hot 12V Connector inside my truck... Great for trickle charging my truck start battery doing this...
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I'm thinking this may be a great PLAN B thing to have around...
You can pick these 10W Solar Panels up from AMAZON for around $40...
Roy Ken