Laptop low screen brightness on battery

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http://crunchbanglinux.org – Hello!I have just install Crunchbang 11 (Statler) on my 64 bit. laptop. And I have problem. Every time I put my laptop on battery, system reduces my screen brightness.    I checked Xfce-power-manager and it shows that my monitor brightness level is 20% (on AC 80%). I tried to change these number but it didn't worked out. Next time I open manager it show the same numbers (20%).   I also tried modify xfce4-power-manager.xml file but it also didn't worked out.I was wondering maybe somebody knows how to make system not reduce screen brightness than laptop is on batter (HowTos)