Hello, I'm suffering from brightness problem.

If I change power source (power adapter plugged in / out), brightness goes to 0.

However, after experiencing first changes, it doesn't happen again even though I plugged power adapter in / out multiple times.

Is there any way to preserve brightness level even though power source changes (AC <-> BAT) ?

Have a look at the System Settings -> Hardware -> Power Management

https://user-images.githubusercontent.c ... ba6c77.png
https://user-images.githubusercontent.c ... f00413.png
https://user-images.githubusercontent.c ... 934c62.png

Currently running Kubuntu 20.04, Plasma 5.18.5, Frameworks 5.68.0, Qt 5.12.8, kernel 5.9.3.050903-generic

Well, the very first item, "Screen brightness" is the place you can act upon. You can change the settings there...

What I want to do is not changing brightness level even though power adapter plugged in or out.

If I connect power adapter after boot, the brightness level goes almost zero and vice versa.

I don't know why this happens.

well, fix the level in the settings, according to your screenshots you haven't done that

If I fix the level (e.g. 70% with ac adapter, 50% with battery) and plugged out (previously charging state with power adapter), it changes 70% -> almost 0% -> 50%.
If I fix the level (e.g. 70% with ac adapter, 50% with battery) and plugged in (previously discharging state with battery), it changes 50% -> almost 0% -> 70%.

I don't want my brightness level changing even though I plugged in/out. I think there is no option like that.
And this happens only once after boot when I change the status of power plug. (plugged in -> plugged out or plugged out -> plugged in)

Please check again, as the default brightness level for the battery is at 50%, you have to actively change it to the same level as the AC Power to avoid screen brightness change...

Of course if you leave it at 50% it will change...

If you read your own post, you would see where the problem is...

FWIW: I set the screen brightness to 90% for both the AC Power AND the Battery, no change of brightness when plugin in or out,, no change either when restarting the laptop.

I think you don't understand my situation.

I found that some other users suffer from the same issue.

https://bugzilla.kernel.org/show_bug.cgi?id=208341

I see, well apparently this is a Kernel issue, so not KDE related.

FWIW: I can't reproduce this at all on my Tuxedo laptop, maybe this is specific to a particular brand or BIOS?

I found that after "org.kde.powerdevil.backlighthelper" activated this symptom doesn't happen again after boot.

Nov 17 00:11:23 eub-Inspiron-5505 dbus-daemon[773]: [system] Activating service name='org.kde.powerdevil.backlighthelper' requested by ':1.60' (uid=1000 pid=1623 comm="/usr/lib/x86_64-linux-gnu/libexec/org_kde_powerdev") (using servicehelper)
Nov 17 00:11:23 eub-Inspiron-5505 dbus-daemon[773]: [system] Successfully activated service 'org.kde.powerdevil.backlighthelper'

I think it is related to the behavior of powerdevil.
https://github.com/KDE/powerdevil

If I activate 'org.kde.powerdevil.backlighthelper' forcefully after boot, I may solve this problem. However, I could find a way to do that yet.