What kind of battery? Li-Ions (mod
batteries) don't have a 'memory' like NIMH (think rechargable AAs), so you can charge those whenever. But, keep in mind that
batteries do have a life and X amount of charges, so the more you charge them the less longevity you will get from them.
I believe the same is true for the 510 stick
batteries, as well as 808, 901 etc.