I wonder if something is up with the wiring, or if sitting around for long periods of time with the dashcam plugged in (even with the preserver) is not great for the battery... Got this today...

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I'm pretty sure the radio does that all by itself if you leave it turned on while the engine is off.

Quite possible - though it was about a minute after turning the engine off. I'll check if it happens when the camera is disconnected.

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It's usually about 15 mins I think, though it didn't do it at all when I using it to watch a DVD last summer. 

I usually find about twenty minutes but I have done that three times before I start the car when using the TVs.