This may be your problem. It affects the older MacBooks in the manner you said. Check and see if you have battery update 1.3 installed. Try these two locations for it. It will be one or the other.
MacBook/Library/System/Library/SystemConfiguration/BatteryUpdater.bundle
MacBook/System/Library/SystemConfiguration/BatteryUpdater.bundle
Leopard installs it. It is the MacBook Pro update.
If it is 1.3 drag it to the trash and then reboot. After you reboot you will be able to empty the trash. Once you have done that run software update and it should install Battery Update 1.2. You should reset the SMC (System Management Controller) after doing this.