A member saw my wiring in a pic under my CocoMat thread and suggested I add this. For a couple of decades I have had cars that I did not drive very often, either a garage queen or a third vehicle for towing the trailer. I learned the hard way that modern electronics leave some drain on the battery. For cars in the garage, a float charger always worked with a connector wired to the battery. But for the truck in the driveway an extension cord was impractical so I learned a 20w solar panel on the dash was just as good. My GM trucks always had an always hot 12v power port to connect. A couple of years ago we moved to apartment living...no garage. My wife's car needed a solution and now my new Lexus. Most cars have the 12v port switched, that is off when the ignition is off. But you can buy an adaptor on Amazon that plugs in to the OBD port, which is always hot, and has a female cigarette lighter socket on the other end. Leave your solar panel on the dash when you park and plug it in, quick, no mess. The only lingering issue is how to route the adaptor wiring. I found a dash trim screw in a good location to mount a hanger for the adaptor cord and you can see the pic in the other thread.
Larry

 

This sounds like a pretty simple solution that would probably work for most cars. Not being an electrical engineer... how did you know if the 20w solar panel was going to be enough? And is there any possibility that the battery could be overcharged (if that is possible)?

 

You cannot overcharge the battery with a panel that small, in wattage, plus many of the pricier ones come with built in controllers that make sure of it, in addition to preventing back feed if you forget to unplug it before starting the engine. I determined that 20w was about right by measuring battery voltage about an hour after charging stopped(late in the day) for several consecutive days and seeing whether the voltage held at 12.6 or more, full charge. A 10w panel works most, but not all of the time. Anything smaller is a waste. You will never get a full 20w out of a 20w panel because the angle of the dash is not optimum relative to the sun. Plus there are clouds, etc. I would recommend any of the panels that are modestly expensive and advertise a built-in controller. I like this one: