Totally different brand, but my Dell works just like the empeg does. It knows whether AC power is connected and just sets the backlight brightness to what the user previously set manually. i.e. it's possible to have the backlight brighter on battery than when on AC.

So unplug it and adjust the brightness using "Fn+XXX" keys with it running off battery.

I think on the Dell it's also in the BIOS but there's no need for you to enter the BIOS to adjust it though.