You have to be a bit careful with these, and make sure you have one that has a good algorithm.
Since they don't have all the pins (many batteries have a thermal pin and I can't remember the last one), it has to watch for voltage cut-off, when charging, so it doesn't overcharge your battery. The good ones are implemented well, I have an old one that's very consistent. Unfortunately, given all the differing pin-outs, having one of these that's universal and uses the thermal and other extra functions is largely impractical, particularly since they have to compete price-point-wise.
I think with a lot of the stuff out there now, you have to very carefully read reviews, paying close attention to the negative ones, to make sure they're not causing exploding or leaking batteries, as the result of overcharge.