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.
There are lots and lots of possible reasons for this to happen, so people will need some more information to help you find the issue.
Are you able to boot it at all after it shuts down? How often does it "randomly" get shut down? How do you determine your battery has 50% before it shuts down? Can you check if it happens on other operating systems? What does "it complains" mean, is it a warning in journalctl? Also, please post precise outputs of any errors you might see in journalctl at the time it shuts down.
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.
I am using upower to determine the battery status. It happens once every day or so. It posts the messages in the tty as it shuts down. systemctl schedules the shutdown slightly ahead of time.
Here a picture of the screen I get before it shuts down.