Welcome to Nokia Support Discussions! Here you can share advice and tips with thousands of other Nokia users around the world in English. Many Nokia employees also follow and participate in the discussions, see our guidelines for more information. Everyone can search and read the discussions, but to post your own question or reply to others, simply sign in with your Nokia account. If this is your first time here, you can choose an alias to represent you. And if you don't have a Nokia account yet, please register.
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.