I have an old car that I rarely drive and the security system drained the battery. This allowed the battery to be fully charged overnight. It stopped by itself and wouldn't keep charging like those stupid chargers. I tested it with a voltmeter and an online manual and the charged battery was fine. At $20 this is an insanely good buy because it saved me from having to buy a new battery or have to keep changing it. Please note that the power cord is very short, so you will need an extension cord to reach the outlet.