I think the built-in battery should be temporarily disabled from the BIOS before opening up the T450s:

Before replacing any FRU, ensure that you have disabled the built-in battery by doing the following:

1. Disconnect the ac power adapter and all cables from the computer.

2. Restart the computer. When the ThinkPad logo is displayed, press F1 to start the ThinkPad Setup program.

3. Go to Config ➙ Power ➙ Disable built-in battery. [Right arrow to “Power”, down arrow to “Disable built-in battery”]

4. Press Enter. A Setup Confirmation window is displayed.

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.

If you’ve done it right, pressing the power button will not turn on the laptop now. (Also, remember to take out the SIM card if you have one.) When you’re done, plug it into the AC power supply to re-enable the built-in battery.

(via the T450s service manual)