Almost every day we're asked how to extend or prolong the life of your laptop batter. You should calibrate your iBook, PowerBook, MacBook, MacBook Air, or MacBook Pro computer's lithium ion battery for best performance.
The battery has an internal microprocessor that provides an estimate of the amount of energy in the battery as it charges and discharges. The battery needs to be recalibrated from time to time to keep the onscreen battery time and percent display accurate. With all iBooks and PowerBook G4 computers, except the aluminum PowerBook G4 (15-inch Double-Layer SD), you should perform this procedure when you first use your computer and then every few months thereafter.
You can read the entire article by logging in to Apple's Knowledge Base then enter "HT1490" in the search field. (if you don't have an Apple ID, you will need to create one)