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Battery packs won't last forever. They probably need replacing every 3-5 years depending on use.
Battery packs need to be re-calibrated every once and a while too. You basically turn off energy management and let the laptop die and then re-charge completely. This will re-calibrate the battery "gas gage".
Here's a blog entry that gives detailed instructions on reconditioning your MacBook battery (letting it die then recharge completely). If you don't do it periodically, your battery will gradually lose its ability to hold a full charge.