My husband groaned when I ordered this..."one more piece of junk" were his exact words. You see, we have tried two or three of these in the past and each has failed to live up to expectations. One was roughly the same price and two were substantially more costly, so when this inexpensive little device came up for review, he was less than enthusiastic. I thought "hey, what do we have to lose" and went for it anyway.Glad I did because this one - despite the low price - delivers! In fact, by the next morning the spouse came to breakfast with this thing in his hands to show me all the reasons it was such a nice little device. We have a travel trailer and like many people, have to constantly mess with the battery..or worse, we forget about it. Yes, we have a battery tender and usually just end up taking the battery out of the travel trailer, hooking it up in the garage and leaving it there for ages OR forgetting and letting the battery run down more than it really should (the fate of our last battery). Of course, the price of batteries makes this less than desirable especially when having to shell out a couple hundred dollars each.Our current battery is an AGM so this was tested on that. It was down about 15% at the time this was tested. The day was sunny but had some clouds. We are in Florida so even during winter have a fairly long day of sunshine. We were both rather surprised how fast this managed to charge the battery...no, it's not as fast as hooking it up to electricity but def had close to full charge by that evening. We didn't really keep a close eye on it but the next morning when went to take a look it was already fully charged.It should be understood that this is a trickle charger NOT something you will use to jump-start a car. If the battery is really low (not suggested especially for AGM batteries) then it could certainly take much longer to restore. We are using this to maintain a charge in AGM batteries for a travel trailer and as such, it is just about perfect!The suction cups on each corner make this easy to attach to a window in the travel trailer for maximum sunshine then simply run the cables and viola'...done! Love the fact that it is then all but maintenance free. To be on the safe side, we went ahead and connected our usual battery tender to assure no overcharging. No more forgetting, no more hauling a battery in/out, no more nothing. Simple but effective.