Weird. When I run solely off battery with energy savings on, it shuts down screen brightness to 0. First time is when battery is half charged, next when the battery is almost down to none (with 10-20 minutes to spare). The only way for me to fix it is to shut down the lid, let it go to sleep and wake it again. I HAVE tried resetting the PMU (command, apple, shift, on-button) and it still does it? Do you know of a way to fix this?