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Most cars have a lighter socket that's , which is no bloody use, so you need to wire to the battery direct or come up with some other scheme. I rigged up a cable to route mine through the ODB2 port, which is usually unswitched, but I don't know if you can buy such a thing.
Solar panels output orders of magnitude more power if they get direct sunlight, so ideally you want it in a south facing window and don't park under trees / buildings / etc. Inevitably they work less well in winter.
And if you're not using the car for long periods, disconnecting the battery is a better bet, electronic tantrums permitting.
(You might get better answers in the car thread)
Has anyone ever used a solar powered car battery trickle charger? Was thinking of getting one and wanted to know if there was any gotchas or if they actually don't work or anything like that.