I have installed Linux Mint 22 in a DELL laptop with a buggy ACPI implementation (the kernel complains about it during boot). The laptop hangs if it goes to sleep (I tried various Linux distros/kernel-versions, the result is the same).
Because of that, I have disabled SLEEP in the firmware (latest version for that laptop btw). So basically, when you close the lid, nothing happens (it just locks the screen).
However, sometimes you might be in a hurry and you close the lid to do something else, and then you forget about it. The result would be for the battery to run dry, which eventually destroys the battery.
My question is: what would be the best way to setup an audible alarm if the battery reaches 20%?
I would first attempt to fix the problem by flashing the bios with the newest version. What you are describing sounds a lot like a common issue with how deep sleep works. Its on the manufacturer to fix it which Dell is typically pretty good about.
If that doesnt work for some reason you can get an applet that will shut down your pc when the battery gets too low for you. Theres a few that work just right click the panel and search for battery applets.
It has the latest firmware, I wrote about it in the description of the post…
https://www.dell.com/support/kbdoc/en-us/000129781/steps-for-fixing-windows-computers-that-do-not-wake-up-or-resume-from-suspend-or-hibernate-mode-but-will-power-on
this link may help solve the issue even tho ur not on windows try these fixes