depends on the battery. i have had some that can charge separately and others that do not. normally, the high ouput ones can be charged on their own. they are usually the ones that don't fit flush into the laptop but rather stick out some for the extra stack of batteries.
you will need to look at the specs to see which way it is. the last hp laptop i got one for would charge on its own and even had a charge meter to look at and verify the charge level. i don't recall seeing any that fit flush into the laptop that charged separately but then again i have not really looked in a long time.