Try reconnecting the battery connector to the logic board. If that doesn't work, try resetting the system management controller (SMC) with the instructions from http://support.apple.com/kb/HT3964
1.) Shut down the computer.
2.) Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
3.) On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
4.) Release all the keys and the power button at the same time.
5.) Press the power button to turn on the computer.
Note: The LED on the MagSafe power adapter may change states or temporarily turn off when you reset the SMC.