Lithium-ion (li-ion) batteries work best when you partially empty them and then top them off. They don’t like to be emptied every time you use them. There is one exception: You should run your battery all the way down to empty about once a month. This ensures that the circuit that measures the battery capacity has an accurate idea of what “full” and “empty” are for your particular battery, since charge capacity decreases over time.
When doing this kind of maintenance, most Apple laptop users simply use their laptop until Mac OS X automatically puts the computer into sleep mode. I’ve found that this doesn’t always leave the battery meter with an accurate reading of the battery capacity. However, I have found a way to empty and calibrate the battery more effectively.
I’ve noticed that my PowerBooks, when on battery power, run much longer when the iTunes visualizer is running full screen. When iTunes is in that mode, Mac OS X somehow seems to let iTunes bypass the usual automatic battery cutoff level, and forces sleep much later. To try this battery calibration method:
1. When you see a low-battery warning or when the battery level drops to around 5 minutes remaining, start iTunes.
2. Start playing your music library. It should be a long playlist (like the entire library) so that the playing time is definitely longer than the battery’s remaining time.
3. Make sure the Full Screen command is enabled under the Visualizer menu.
4. Choose Visualizer > Turn Visualizer On.
You’ll probably find that iTunes keeps playing many minutes longer than the remaining time indicated by the battery meter. Eventually, Mac OS X will sleep the laptop. As a result of the longer runtime, the battery meter should indicate a longer life than it would have if you hadn’t extended the runtime with iTunes. Using CoconutBattery, I’ve confirmed that the number of milliAmp hours assumed by the Power Manager is higher after this procedure.
I don’t know why Apple keeps two different cutoff levels for sleep when on battery. It’s a mystery.