I know with earlier Thinkpads that you require a good battery..although there are workarounds.

Can you update X20 BIOS with a dead battery WITHOUT workarounds?

If NO, will Boot disk trick work with X20?

Thanks

On page 37 HMM you find the layout for the battery connectors.
Use any working battery from another Thinkpad, find its pin-layout and connect just the positive and negative poles to the battery-slot in your laptop, using loose pieces of wire.