When on battery power, the brightness of the Dell XPS 15z LCD screen becomes unstable. If you switch from a bright web page (http://www.google.com) to a darker web page (http://www.nasa.gov), it automatically dim a little. It gets brighter the other way. It's almost like screen flickering when you switch between windows of different brightness.
Apparently this is a feature by Intel to save power on battery. (Display Power Saving Technology)
"This feature extends battery life by lowering backlight intensity while minimizing impact to display quality. Select Maximum Quality for the best display quality or Maximum Battery to prolong battery life. Clear the check box to disable this option."
To fix it (if you find it annoying), go to Intel Graphics control panel and disable the feature.
Right click on some empty space on Desktop and from the menu choose Graphics Properties > Power > Change the pull down menu on power source from 'Plugged in' to 'On Battery' and uncheck "Display power saving technology". Click OK. Now the flickering should be gone.
I slid the bar to almost left (More towards 'Maximum quality' side) without disabling the feature and it also worked.