Backlight goes to max when I unplug the AC adapter, then dims down when I replug

[iram]

Function backlight keys work both when plugged/unplugged.
AC adapter seems correctly detected (battery icon displays when I unplug)
I tried with out success:
GRUB_CMDLINE_LINUX_DEFAULT="acpi_osi=Linux acpi_backlight=vendor"
and also
GRUB_CMDLINE_LINUX_DEFAULT="acpi_osi=Linux"

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: unity-control-center 14.04.3+14.04.20140604-0ubuntu1
ProcVersionSignature: Ubuntu 3.13.0-36.63-generic 3.13.11.6
Uname: Linux 3.13.0-36-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.14.1-0ubuntu3.4
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Sep 25 13:14:25 2014
InstallationDate: Installed on 2014-07-17 (69 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
SourcePackage: unity-control-center
UpgradeStatus: No upgrade log present (probably fresh install)
usr_lib_unity-control-center:
 activity-log-manager 0.9.7-0ubuntu14
 deja-dup 30.0-0ubuntu4

Additional info:
Trying to find on my own, I discovered that the luminosity slider in unity-control-center Luminosity and Lock applet is ineffective, either actuated with mouse and keyboard left/right arrows. The slider moves, but luminosity doesn't follow it.
I also noticed that the luminosity setting, when set with the Fn Keys (which make the slider to move), is recorded without control of the current power supply mode, which allows to store a lower setting when on AC than it is recorded when on DC.... which was the reason of my symptom.
Maybe a simple change in the window text would make users aware of what they are actually setting: "Luminosity for current ([AC|Battery]) mode"... without preventing them to experiment what was my issue, for any reason (test or any strange reason) they would like it. As I see the window is able to monitor and adapt its look to Fn keys events, it would be nice if it could also monitor and adapt the text above to power supply source (not only when the window opens, also when the power supply changes when the window is yet opened).