One of my favorite PowerBook applications is Christoph Sinai's excellent CoconutBattery. Version 2.0 is a major update that adds a dashboard widget, preferences menu for custom values and it's FREE.

coconutBattery is a tool that reads out the data of your notebook-battery (iBook/Powerbook). It gives you live feedback, what’s going on in your battery (not only a snapshot like other tools).

It shows the current charge of your battery as well as the current maximum capacity related to its original. If coconutBattery detects that you have connected the wrong charger (i.e. an ibook-charger plugged in a powerbook) it will warn you! That’s a cool feature!

Of course you are able to save the current maximum capacity of your battery - with just one click! coconutBattery uses Apple’s new and really powerful technology named “CoreData” to realize that.

Finally coconutBattery calculates the age of your Mac exactly! So you can derive the age of your battery. And last but not least: coconutBattery is Universal Binary (you can run it on Intel-Macs! - cool!) and it is complete localizable!