Why doesn't any of the guides mention you must disable the battery?!

Direct from User's Manual:

To disable the built-in battery, do the following:

1. Turn off your computer and disconnect the ac power adapter and all cables from the computer.

2. Turn on your computer. When the logo screen is displayed, immediately press F1 to enter ThinkPad

Setup.

3. Select Config ➙ Power. The Power submenu is displayed.

4. Select Disable built-in battery and press Enter.

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5. Select Yes in the Setup Confirmation window. The built-in battery is disabled and the computer turns off

automatically. Wait three to five minutes to let the computer cool.