An extension for gnome shell that will change the screen brightness when the machine is running on battery power. By default, this extension will dim the screen to 50% brightness when running on battery, and restore to 100% brightness on AC.
git clone git://github.com/nailfarmer/gnome-shell-dim-on-battery-extension.git
cd gnome-shell-dim-pn-battery-extension
cp -r dim-on-battery@nailfarmer.nailfarmer.com ~/.local/share/gnome-shell/extensions