Id suggest letting the battery drain completely, just leave your psp on with a game running, or music on repeat for example.
Then wait until the psp just dosent turn on, then once that happens, stick the charger in and leave it for 3 - 4 hours, check after that see what the percentage is.
Some batteries loose their memories in terms of juice left, draining them restores this, thats what i have heard anyways, worth a try.
Kev
