...from: http://www.macworld.com/article/134496/2008/07/mbpbadbattery.html
MacBook Pro Battery Woes
Mac 911 Tip of the Week
Another year, another iPhone.
Reader Bill Grant has a problem with power, specifically the battery in his MacBook Pro. He writes:
"Recently, my MacBook Pro has been behaving strangely. I charge the battery over night, yet the battery menu constantly flickers between estimating a fully-charged state and telling me that the battery isn't charging. When running from battery power, I see a readout of about half full and then the computer shuts off abruptly. Any idea what's going on?"
It's quite likely you have a bad battery. From the Apple menu, choose About This Mac and then click the More Info button in the resulting window. When System Profiler appears, select the Power entry and eyeball the Health Information entry. If, under Condition, it says "Check Battery," that battery is probably on the fritz.
Fortunately, these batteries are under an extended replacement program as they're defective. Apple has issued a document that outlines symptoms a defective battery might exhibit as well as a link to a software update that can fix some battery issues. http://www.apple.com/support/macbook_macbookpro/batteryupdate/
If the software fix doesn't work, you should follow the document's advice and contact Apple about having the battery replaced.