you can check the battery info in Mac OS X using some progrems out there, or open the terminal and paste this command in:
ioreg -p IODeviceTree -n "battery" -w 0 | grep Flags
My, pretty good battery returns:
IOBatteryInfo" = ({"Voltage"=0,"Flags"=1,"Amperage"=0,"Capacity"=0,"Current"=0},{"Voltage"=12303,"Flags"=5,"Amperage"=1200,"Capacity"=2738,"Current"=2738})
if the capacity is really low, you can try the apple battery calibration:
http://docs.info.apple.com/article.html?artnum=86284
or it might be time to get a new battery