I say run the relay off the 12 volt PC power supply molex so the batt will only charge and be tied intot he circuit when the PC is on.
-This eliminates potential hazards of using a Ni-Cd in the car.
-It's been tested and works.
-8 Diodes is alot. How many PC's you running? lol
My Setup survives cranks and I don't even have anything except for my stereo amp 1 farad cap. Woo hoo. I run everything off my PC PS, screen etc... and it's the 250watt Keypower.