ThinkPad p43s does not start when internal battery connected

[u12345]

I recently tried to "reset" my Lenovo ThinkPad p43s laptop by removing the internal battery to get rid of the dreaded warning/error on boot:

The connected AC adapter has a lower wattage than the recommended model – Please connect the AC adapter which was shipped with the system for best system performance

https://support.lenovo.com/dk/da/soluti ... -p320-tiny

It did NOT fix the issue and as a result of my tinkering the laptop will no longer boot when the internal battery is connected. If I disconnect it and connect the AC power adapter it starts fine. This could mean that I have somehow broken the battery or the connector when trying to detach/attach the battery.

Am I missing some easy tricks to get it to work?

I am also considering buying a new battery but before doing that I have ordered a Multimeter (Uni-T UT125C) to investigate the issue a bit further since it might also be cased by a lose connection or something else than a dead battery. Any hint on how to debug this are highly appreciated!