It's not actually a problem, it's a feature. It changes the brightess, as stated, according to whether AC power is applied or not.
The only way to change this is if your dell BIOS has support for changing it. Boot up and tap F2, or F10, or whatever the hell key it wants you to tap to get into BIOS/SETUP. If it's not in there, and you have the latest BIOS revision, then you can't change this feature.