$20 lawn tractor battery isolated by a $5 30A relay powering a $39 400W inverter which powers a free 120 watt computer power supply. Works great, immune to crank, cheap, it wont drain the main battery if the computer stays on too long, it's nice to have a 110V outlet in the dash, and when I want to work on the carputer in the garage I just unplug it from the inverter and plug it into an extension cord. Now I know this setup is inefficient and that troubled me very badly. I crunched all the numbers and this gross inefficiency affected my gas mileage! My actual miles per gallon went down by .000173% or in actual distance, a whole three quarters of an inch! I can live with that.