Most aftermarket D/C - A/C inverters will automotically shut off when the incoming battery voltage drops below some pre-set level. In the case of my unit it shuts off at around 11 volts. This leaves plently of go juice in the battery, not that the 12 volt needs that much anyway since it dosn't start the ICE. I just run mine with the key off and I have never had it shut down because of low voltage. Of course I only charge electonics (camcorder, digital camera, R/C car batteries, gameboy SP and such) on it, those aren't much of a real load.

Chris Brown