Why not just wire the Linux box to the car battery? Most after market dash cams do this to capture accidents during parking. Certainly a low power device is enough ?
You would be surprised how even the most innocuous electrical systems can draw down a car battery if the car doesn't move for extended periods, especially in cold weather and with a battery that's already degraded or only gets charge from short drives.
I have an old car (with a modern <2 year old battery BTW) sitting in the driveway that rarely gets driven. For a long time, every time I wanted to drive the car the battery charge would be so low it would fail to crank the engine enough to start it. I would jump-start it and drive it for at least an hour, and if I would drive it again within a week or two it would be fine, but after ~3 to ~4 weeks the battery would be dead again.
When I finally got around to diagnose the problem and measured leak current while removing the fuses 1-by-1, I found out that the tiny light in the glovebox compartment was not turning off because the lid switch did not engage properly. The current was something like 100 mAh but it was apparently enough to drain a less-than-full battery within only a few weeks...
But that makes sense, right? 0,1 Amps = 2,4 amps/day, many cars have a ~60 Ah battery, so that would be empty in 25 days (and probably in less than that doesn't have enough power to start the engine).
