see this
http://www.apple.com/asia/support/macbook_macbookpro/batteryupdate/
Apple has recently discovered that some batteries used in its MacBook and MacBook Pro notebooks may have battery performance issues. Apple is offering a software update that is designed to address these issues.
The battery update is available immediately via Software Update or downloadable here.
Note: The factors causing the performance issues do not present a safety risk. You may continue to use your current battery.
This battery update should be run on all MacBook and MacBook Pro computers and extra batteries that were purchased between February 2006 and April 2007.
If, after you have installed the battery update, your battery has any of the symptoms listed below, please make a reservation to bring your computer with its battery to your local Apple Retail Store, or bring it to an Apple Authorized Service Provider (AASP), or call your local Apple Support Contact Center. If Apple or an AASP determines that your battery is eligible for replacement, you will receive a new battery, free of charge, even if your MacBook or MacBook Pro is out of warranty.
For MacBook and MacBook Pro systems with Intel Core Duo processors, this program extends repair coverage on the battery for up to two years from the date of purchase of the computer.
Identifying an affected battery
Affected batteries will have one or more of the following symptoms:
Battery is not recognized causing an “X” to appear in the battery icon in the Finder menu bar.
Battery will not charge when computer is plugged into AC power.
Battery exhibits low charge capacity/runtime when using a fully charged battery with a battery cycle count (as shown in System Profiler) of less than 300.
Battery pack is visibly deformed.
Note: If your MacBook or MacBook Pro battery does not have any of the symptoms noted above, your battery does not need to be replaced.