Daniel, could you do some tests and report here?
Do
sudo showkey
to see what keypresses are being generated when you press Fn+Brightness.
Do that for the first laptop both before and after pulling out the power. In
other words, both when the buttons are doing nothing and when they work.
I
At any case, the result is always the same:
keycode 225 press
keycode 225 release
or
keycode 224 press
keycode 224 release
depending if your are lowing or increasing the brightness.
With some video cards it is possible to set brightness via a file named
brightness below /sys
It contains a number 0-100
