Battery charge limit on KDE (Power Management → Advanced Settings) works fine, but resets every time the computer boots.
It must be specific to your laptop.
Neither my Lenovo Legion nor my Framework16 offer Charge Limit settings via KDE.
Both use Terminal commands to adjust the levels and both trigger a BIOS change.
I am not a Linux Expert (just a User), but from my limited knowledge, you will have to supply far more info to receive meaningful help.
Hardware info: Minimum Make/Model
BIOS version and what type of BIOS in use.
@amosbarsinai can provide most of what is needed by installing inxi then running the command inxi -ezxx and posting the output as preformatted text here for us to look at.
It should be in the KDE settings under “Configure Advanced Settings…”. On my Legion it’s just a checkbox, and on my Thinkpad I can set the percentage for both stop charging and start charging again. On both devices it survives reboots (but BIOS updates seem to reset it).
Sorry, this is just an “It works on my computer” reply… 
Which flavor of Fedora do you use - is it one of the immutable ones, and could that be causing this?
/Jaybe
On the new Framework 16 Ai9 HX 370 with the build in 890M GPU, I run: Fedora Linux 44 (KDE Plasma Desktop Edition) x86_64 with the current Kernel: Linux 7.1.8-200.fc44.x86_64.
So a rolling release that is pretty much bleeding edge.
Framework offers its own tools via Command Line to modify battery charge limit settings for the battery.
The command(s) look like this: sudo framework_tool --charge-limit 70
Where the “70” stands for 70% SOC (State of Charge). This number can be adjusted by single percentage, as if I wanted to I could set it to 83% or any number.
The 5 year old Lenovo Legion 5 17IMH 05 runs SuSE Tumbleweed KDE which is also a rolling Distro.
The 5 year old Legion has only 2 settings in its BIOS and when I was using Win11 via Lenovo’s app Vantage could either be set to SOC 60% or SOC 100%. Nothing in between.
Again, charge limit settings can be set via command tool (Konsole) by changing the command to either “1” or “0”.
But none of my Laptops can be set via KDE “Power Management”.