Re: reverse polarity
Drop under 9-10v and no computer car is reliable.
That said DO NOT ever ever ever start a computer car with a charger attached unless designed for computer systems (none from HF). If you need a boost use a jump box and not a charger.
Most do not put out 12v but rather a half wave rectified mess at sorta approximates 13 or 15v DC & the noise will drive a computer stark raving bonkers.
I have seen cars not start or start and run very rough with even a 10A charger attached.
Also many of the Clover Trail (last year's Windows tablets) run on 12v and people ask about connecting directly. I tell than that unless they want to chance a brick to get a 150-180w cigarette lighter inverter and use the wall wart that came with the computer. They have internal switchers and filters that provide what sensitive electronics need.