These things aren't really designed to "charge" your car battery. They're designed to provide enough supplemental current so that you can start. You almost always have to have a depleted battery with some juice left, and these are just boosters. Often when I can't start a car, the lights will still turn on but the starter just makes this clicking sound that indicates there isn't enough current to start.
There are those things that plug into the lighter outlet. Those attempt to charge the battery. They have to do that since enough current to start an engine would blow a fuse. You could probably use a jump starter like that, but that's not the normal way to use one. If you're charging a battery, you'll typically want it to have a higher voltage than the nominal battery voltage - maybe over 13-14V.
I've got an older Schumacher jump starter. Mine is getting old (still works great) but I'm thinking of getting an IP-125 and leaving my current one just for my wife's car. I've used it for friends, relatives, and total strangers. I like these because they don't need an adapter (I lost one for a Vector jump starter that didn't work well anyways). You will need a way to connect to the recessed plug. I use a 3-prong adapter and a cheap extension cord. I know some think they need heavy duty cords, but they charge at maybe 1A household current, which is really low and a cheap extension can handle easily.