Latitude E6500 Dim Screen on Battery

Hi:

I was using my latitude E6500 laptop un-plugged, and all of a sudden the screen went dim.  When I plug it in, the screen goes bright again (although it 'seems' a bit dimmer than it used to be, I think ...).   I changed the power plan, but it still goes dim on Battery.  the Dimming controls do work.  Disabling the ambient light sensor does not fix it either.  

Wondering what part I need to replace to fix this?

Thanx