my laptop does the same, the charge voltage is higher than the normal battery voltage (it has to be or the lectrikery runs out of the battery...not in
), so running on mains you get a
slightly brighter display, the difference should not be huge, have you tried making sure that you have the brightness turned all the way up?, on my Acer you hold down the blue fn key and press the left/right cursor keys to change the panel brightness, or maybe you have a dedicated management tool supplied with it, some program on the desktop or under the makers name in the start menu?
Windows: 32 bit extension/graphical shell for a 16bit patch to an 8bit OS originally coded for a 4bit CPU, written by a 2bit company that can't stand 1bit of competition, now available in 64bits.