Lenovo ThinkPad T420 4236 User Manual page 132

3. Align the Extended Life Battery with the bottom side of the computer as shown in the illustration below
and firmly attach the battery.
4. Slide the battery latch to the locked position.
5. Turn the computer over again. Connect the ac power adapter and the cables to the computer.
To remove the Extended Life Battery, do the following:
1. Turn off the computer, or enter hibernation mode. Then disconnect the ac power adapter and all cables
from the computer.
2. Close the computer display, and turn the computer over.
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User Guide