As stated above, after the upgrade to 11.10 my laptop no longer dims the screen when I go on battery power. Why?
Go to the below Screen setting while on Ac & uncheck the "Dim screen...' box
Now unplug power from the laptop & move the slider back & forth a bit, then set it to the brightness you wish for when on battery.
Plug power back in, hopefully it should return to full brightness, unplug & again hopefully the screen should dim.
If all that works you can check the 'Dim ..' box or just leave unchecked
The whole system doesn't quite make sense, I'm thinking the check box relates to Ac only