Consider that a normal car battery is rated, say, 61 A · h, then even if your alternator could poke it with 50A for a whole hour, it'll still be
left uncharged; in reality, 20A charges are often called «rapid», 10A is «fast», and the normal charges are more likely to be just 6A to 8A, which means that a fully discharged battery of a compact car will take something like at least 8 hours to reach the full charge!
However, if you
leave your Roomba unplugged and
uncharged for a long period of time, its schedule may slip out of sync and run at the wrong time, so be sure to always keep your Roomba charged when it's not in use.