Excellent in theory. Doesn't work in reality. Once your voltage drops to a certain level it will cut off 12V. Only to turn it back on when power is being used (headlights or brake lights). Well, if I ever had a short circuit, which essentially drains power, wouldn't the short circuit turn it back on? Obviously like that. Worked for about a week (although I'm just guessing it took that long to run out of battery).