Maximum brightness change when plug in AC to laptop

I am on a Dell XPS 13 7390, shipped with Ubuntu, now on version 20.04.4 LTS.

I am pretty sure that maximum brightness setting, isn't really setting it to max.

I have never tried Windows on this machine, but I have noticed that when I plug the AC power, the screen become brighter, not by a lot, but certainly by a noticeable amount. Regardless of my actual screen brightness level, I can't reach the same brightness as when I am plugged in.

I know there is a pretty similar post here, but not only I already tried the proposed "solutions", even if no one was marked as "solution", but I can't even comment on it as I have not enough "reputation"... Hence why I am creating a new one.

Now, speaking about what I've already tried:

1. Updates
2. BIOS update
3. Firmware update
4. Grub edit, as proposed by the other post (basically broke brightness control level, its fixed to maximum)
5. I have no light sensor, so nothing to try.

Speaking of what I won't try:

1. Upgrade to latest Ubuntu. Last time everything broke and need to reset it to factory default.

Any help is welcome. Thanks.

2 Answers

In the Dell bios, there is an option under the screen menu to change the maximum brightness on AC and on battery. Drag the battery slider all the way to max and the maximum brightness should be the same on both AC and battery power. At least this is how it is on my Dell Latitude.

What solved the problem of brightness not changing was:

sudo gedit /etc/default/grub

Now looking for the line

GRUB_CMDLINE_LINUX_DEFAULT

And add "acpi_backlight=native", it should look like this

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=native"

At this point just Save and Exit Gedit, then

sudo update-grub
reboot

Just for reference, I also tried before all other grub setting in the "GRUB_CMDLINE_LINUX_DEFAULT" line such as:

• quiet splash (and nothing else)
• acpi_backlight=vendor
• acpi_backlight=vendor acpi_osi=linux

But they didn't worked for me.

As for what user Waldemar proposed, yes, there is that option in the Dell BIOS, but for me was set basically the opposite. When on AC 50% and when on Battery 100%, I don't know why. Anyway, I have set now both to 100% brightness, not sure if that has an impact or not. IMO it is just the brightness that automatically sets when plugging in and out from AC, but should not prevent nor force any max cap on brightness, at least that's my understanding.