To ensure the indication of remaining battery life is as accurate as possible, the battery should be fully discharged in your computer and then fully recharged. Simply use the PowerBook without the AC power connected until it shuts itself down, then plug in AC power and recharge the battery.
If this does not work then you may need to reset the power manager. Refer to Article 14449, "PowerBook and iBook: Resetting Power Manager".
For the PowerBook G3 Series (family number M4753 or M5343) or the iBook (family number M2453), refer to Article 60655 "Battery Reset 2.0: Read Me".