My mid-'12 MBA, when I connect it to my HP 2009m monitor, it won't work unless the MBA is on external power. Since the monitor has it's own power, the MBA should just be pushing pixels and not using a lot of extra power, correct? Is there an override? I don't see anything in Energy Saver. I like extending my battery life to the utmost and keeping the MBA on external power will shorten battery life. That was a $139.00 lesson I learned at the Apple Store. Edit: running Yosemite, but the same occurred under Mavericks and MLion.