I can’t find an active thinkpad centric community on here I am just going to post here. My Thinkpad T480s, 16 GB RAM, 238 GB SSD is randomly shutting off while at like 50%. When I try to open it back up it always is dead and I have to connect charger to boot it up again. Why is this happening? Is it battery issue? It complains about not being able to read temp2_input while shutting down and sometimes say PM usage count underflow. Not sure what that is supposed to mean.
Debian has a program in the battery-stats package that logs battery at a (by default) 30 second interval. That has a pretty graphing program too. I dunno if Arch packages that, but if not, not hard to roll your own.
That'll get you a capacity within ten seconds of the next shutdown. If you save that log, repeat it running until it dies a couple times, you can probably tell if it's consistently at the same capacity that it goes down.