Distorted Computer Screen

I have a Dell Inspiron N7110 laptop with the latest drivers from the Dell website installed.  When I unplug the power cord from the computer to run on battery power I get a distorted screen.  Sometimes I can barely make out the shout down button.  Sometimes I have to plug the power cord back in to get the regular screen to operate correctly.  I have updated and installed the latest drivers as mentioned for my computer but, I am still getting a distorted screen when I disconnect the computer from the power cable.  Any thoughts on what could be causing the screen to become distorted?  Thanks in advance, James