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If you always work while connected to AC power, the battery doesn't discharge: its electrons stagnate, and the battery's life span is reduced. Make a habit of working on battery power at least once a month. It is a good idea to occasionally drain the battery all the way, then fully charge it before using battery power again. Putting your iBook or Powerbook to sleep for transport is another way to reduce battery life. It is better to shut down the laptop when it will not be used for several hours.
If you do use your laptop on battery power a lot, you may want to consider purchasing a spare battery. One thing to remember, though: batteries like to be used, not tucked away for months at a time. They will develop a diminished capacity if they are just sitting in your bag. Rotate your spare battery with your original one on a regular basis.
Most manufacturers recommend that you calibrate your battery every few months. This involves draining the battery until the laptop goes to sleep, then recharging it to 100%. For detailed instructions, see docs.info.apple.com/article.html?artnum=86284