Sounds like there's something wrong with your battery, but it could just be its internal computer being confused. Will it actually run for a long time, or does it shut down quickly?
Have you tried "conditioning" the battery, to re-train its computer? You do this by draining the battery completely--leave it runing until the computer goes to sleep and refuses to wake (when you push the button on the battery, the botom light should be flashing)--then plugging it in and letting it charge all the way up. This (in theory) recalibrates the battery's internal computer, and gives you a more accurate number on how much power is remaining.
There was also a "Battery update" for the 12" BPs that Apple released quite a while ago. I have no idea if it will help you at all, but I tried installing it on an early model 12" G4 PB and it did indeed increase the battery's effective life. (I'd think it wouldn't be required for 10.3 or 10.4, but it did install under 10.4 and there's no note on the Apple site saying not to do this.) If it's compatible with your model, you might give it a try.
