Power/battery problem

My iBook G4 has recently started powering off abruptly, when the battery is half full. There is no warning or anything. It just turns off. This so far has only happened when I'm running on battery power. I looked around, and found the PRAM and PMU reset, and I did both of these (I think, there's no confirmation). No luck. Maybe I'm not doing it right.

I'm supposed to hold ctrl+command+shift+power button, right? Am I supposed to do this while the computer is starting up, or when it's running? When I do it, it turns the computer off, but even after I've done it I'm still having this problem. Any ideas?

I had one doing this, and ended up replacing it.

Does anyone have any advice here? Should I replace my battery? Am I doing the PMU reset properly? Bueller?