Lifehacker - Top 10 Battery Hacks, Tips, And Tricks - Batteries: "1. Recalibrate a laptop battery to regain life
It's a shame, but laptop batteries can lie to you about how much juice they have, or can really hold. The New York Times explains in a Q & A (look halfway down the page) the most straight-forward means of getting the real truth. Turn off all your interrupting apps, like screensavers and the like, put your computer to sleep, and plug it in until you know it's good and charged. Then turn it back on, make sure your power settings are such that the system won't try to sleep or hibernate, then run your computer all the way down on battery power. Charge it back up one more time, and you'll know whether you really need to start shopping at Laptop Battery Express, Laptops for Less, or check with your manufacturer to get a new lithium stick."