The PowerBooks use Lithium-ion batteries.
Apple recommends this:
Recharge Lithium-ion batteries when they become depleted. Although they are not subject to the memory effect, it is suggested that you let the battery drain fully before recharging.
This means let the PowerBook run until it goes to sleep (battery reports 0% time left), then plug it in and let it recharge to full. The battery includes a microprocessor that reports back to the laptop its status. By letting the battery run down, it actually drains to a predetermined minimum power level that will protect whatever is still occupying memory. At that point the microprocessor records this power level as its minimum and starts calculating its full power level as it recharges. This is called reconditioning the battery, and should be done periodically whenever the time remaining displayed in the menu bar appears to be incorrect. The microprocessor in the battery is calculating its time remaining based on the laptop's power drain, so the time remaining will go up and down depending on your usage.
After conditioning the battery, it's perfectly OK to recharge your laptop anytime you're through using it. This is what I do. When I'm done for the night, I simply plug it in.
There is a shareware tool for checking on the status of your battery called XBattery that's available at
http://www.versiontracker.com/dyn/moreinfo/macosx/12649
One final thing: If you elect to purchase a replacement battery later from a third party (non-Apple), read the fine print. Some of these batteries do NOT perform this convenient "go to sleep when power is low" feature, and will simply drain themselves dry, deleting whatever is in RAM, and effectively performing an ungraceful shutdown of the Mac.