MyBattery uses a historical estimation technique to estimate the remaining useful time on your battery.
What does this mean? As you use your PowerBook, MyBattery monitors the battery level. As the battery level drops, MyBattery measures the amount of time it took, and stores this information away. MyBattery then uses this information to make the estimates.
When you first run MyBattery, it initializes the estimates with some default data. As you use your PowerBook with MyBattery, however, it will start basing its estimates on the data it has collected about your computer. Therefore, the accuracy of the estimates will increase the more you use your computer.
How accurate is it?
Well, there is no foolproof way of estimating the remaining time left on a battery. There are too many factors that can affect your battery life - temperature, disk accesses, screen brightness, initial battery charge, and so on. But once MyBattery has collected a full set of measurements, the estimate should be pretty accurate, and tailored specifically to your computer.
One advantage of the historical technique is that it allows MyBattery to handle any capacity or vintage of battery. For example, as your battery ages, its useful charge life slowly decreases. MyBattery will continue to update its estimates to reflect this change.
Estimation problems
MyBattery assumes that your battery usage doesn’t vary much from session to session. There are, however, two situations that can cause MyBattery to give poor estimations:
1) Inconsistent battery charging. If you sometimes charge your battery fully, and other times only charge it for an hour or two, MyBattery’s estimate will lose some accuracy. On general principles, it is always a good idea to fully charge your battery between uses.
2) Swapping between batteries with vastly different characteristics. If you had two batteries - the standard Apple battery that lasts 2 hours and a suitcase-sized battery pack that lasted 15 hours - and swapped back and forth between them, MyBattery would be quite confused. Its estimates would fall somewhere in between. MyBattery supports multiple batteries, so as long as you identify which battery you are using to MyBattery, this should not be an issue.
