Power management kinda doesn't work on Linux, outside of ACPI. There are very minimal rules once you disconnect from AC, and most of the time you'll switch into high-usage mode when it isn't needed. There's not a clean way to fix this, to my knowledge; the Linux scheduler stack is just kinda based around desktop and server hardware.

That being said, I also use Linux on my Thinkpad anyways. There are decent enough workarounds that I can keep my system up for 5-6 hours when away from AC:

- Switching into battery-saver mode will keep clock speeds down, which generally reduces power usage (as long as you aren't slamming the cores)

- tlp can help if your hardware has power-draining characteristics (I don't use it, my defaults are good enough)

- Using an auto-nicer can keep your system feeling responsive in low-resource environments: https://github.com/pop-os/system76-scheduler

So... caveat emptor, YMMV. Linux is far from the most efficient OS away from the wall, but with a little bit of configuration I feel like my system does indeed work as a "normal laptop".