This step comes right from the service manual which is available right here for you to download.
Power system checkout To verify a symptom, do the following: 1. Turn off the computer. 2. Remove the battery pack. 3. Connect the ac adapter. 4. Check that power is supplied when you turn on the computer. 5. Turn off the computer. 6. Disconnect the ac adapter and install the charged battery pack. 7. Check that the battery pack supplies power when you turn on the computer.
Give it a try and see what you get. Good Luck