According to Apple: "Battery Lifespan A properly maintained PowerBook or iBook battery is designed to retain up to 80% of its original capacity after 300 full charge and discharge cycles. You may choose to replace your battery when it no longer holds sufficient charge to meet your needs." "A charge cycle means using all of the battery’s power, but that doesn’t necessarily mean a single charge. For instance, you could listen to your iPod for a few hours one day, using half its power, and then recharge it fully. If you did the same thing the next day, it would count as one charge cycle, not two, so you may take several days to complete a cycle. Each time you complete a charge cycle, it diminishes battery capacity slightly, but you can put both notebook and iPod batteries through many charge cycles before they will only hold 80% of original battery capacity. As with other rechargeable batteries, you may eventually need to replace your battery." If you're running Tiger, open System Profiler (what? can't find it? Use Spotlight!) and look for the Power section under Hardware. The Battery Information section should tell you how many charging cycles your battery has gone through. Here's mine.