Hi mate,
yeah i used a 300W inverter to power the pc, however the computer that i built only uses 70w(mini computer, based around the sony lx1)
i installed a relay so when i start the car the computer boots up, however when i swtich off the car itll turn off during whatever it is doing, also if i have the ignition on and am using the pc, then start the car, it will reboot, but, im about to go get an uninteruptable power supply, so hopefully, itll store a current so when i swithc the car off it will shut it down and when i start the car it should maintain a steady power supply.
Hope that helps
Dev
