2011-10-18 14:36 - edited 2011-10-18 14:37
The E51 is quite an old model now, is the battery also old?
Eventually batteries no longer hold as much charge or perform as well as when they were new. If the battery is a couple of years old it's likely that it just needs to be replaced with the correct official Nokia battery for that model.
According to Nokia the correct battery for the E51 is the BP-6MT.