Also.. if you're getting whining or popping while starting your vehicle.... well keep in mind there's a REASON why factory radio systems shut off when you start the vehicle. It won't hurt the computer if you have a proper power supply (m2atx or the like) though. If your carpc is shutting off when you crank, and you have a DC-DC psu, then there's either something wrong with it, or something wrong with your electrical system.
Edit: Whups, I had my math inverted. That actually works out, 80F at 20amps is 0.25V/s, which would give you at least 5-6 seconds of power before it dropped to cranking voltage. Who knew.
On an amusing side note, a spare battery is orders of magnitude cheaper you know :P
"stop with the REINSTALLS, what do you think we got some lame-o installer!!!" - mitchjs