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.
That sort of sounds like whatever is displaying battery remaining is somehow getting things wrong, assuming that the battery is actually discharged. I can't think of much that would prevent you from at least being able to power up to the BIOS if you honestly had battery charge.
You can see what the kernel is telling userspace in:
/sys/class/power_supply/BAT0/energy_full
And
/sys/class/power_supply/BAT0/energy_now
Other things that might cause it…there is some software that will auto-hibernate or similar when your battery reaches a certain threshhold. If you consistently have the thing go down at 50%, that could do it. But I don't know why that would prevent you from booting the thing without charging it. You tried holding the power button for seven seconds or so to make sure that the thing is really powered off and not in some suspend mode or something and ignoring taps on the power button?
Oh, wait. 50% and Thinkpad. I used to have a Thinkpad with two batteries.
https://www.ifixit.com/Guide/Lenovo+ThinkPad+T470S+Batteries+Replacement/140443
That looks like the T470s has a dual battery, and unlike mine, they look like they're the same size. Does your T480s have dual batteries?
EDIT: The T480 apparently does. Does the T480s?
https://www.reddit.com/r/thinkpad/comments/a4iben/t480_confused_on_how_to_charge_2nd_battery/
EDIT2: I don't see anything that looks like a second battery myself in this:
https://www.ifixit.com/Guide/Lenovo+ThinkPad+T480s+Battery+Replacement/144009
So I'm guessing not.