[iBook] calibrating my battery

[ibooklist2 at matthurst.fastmail.fm]

I've had my G4 iBook about 18 months from new, and the battery life is
tailing off a bit. Nothing massive, just not the four and a half hours
or so I got at first, more like nearly three.

Thing is, I didn't do the battery life calibration thing when it was
new, so I don't know if the meter on the menu bar is accurate (and
presumably, neither does the iBook. I could have hours left and it shuts
me down...)

Is there any way of recalibrating it now?

Matt