About the worst battery I had it on was an ATV battery that drained out before I got the Battery Minder, but was only a couple of years old, so it probably wasn't the best test of the charger.
I did keep one of my truck batteries going with the Battery Minder, but after doing a complete draindown on the battery one day in the parking lot, I bought new ones because it wasn't too convenient trying to hook up the charger every night, especially for 2 batteries.
It seems to do a great job of keeping all of my part time use batteries in tip-top shape, but that's with alternating the charger between all the different batteries every month or so. They make these cables that you can permanently mount to the battery in the vehicle, then you just plug in the charger without having to pop the hood, cover, etc. I only have one of those, and haven't decided which battery to hook it up to. I really should get a Battery Minder for each and every battery in each vehicle, toy, etc. since many of them sit for periods of time, and leave them all plugged in all the time when not in use. The charger will not overcharge, and will keep the battery in new condition, fully charged, ready for use, no matter how long something sits.
And, if nothing else, the charger is very small, so it's a lot easier to use than a standard charger. I haven't used my standard battery charger for a few years, since I got the Battery Minder.
