After getting a new MacBook Air and deciding to try out OS X for a while, I started wondering why the battery would suddenly start dying really fast, but only when I was sitting at the kitchen table. That’s when I noticed the display brightness kept changing on its own… and realized the problem.
OS X, just like your phone, and any modern device, tries to figure out the brightness of the room you’re in, and adjust the brightness of the screen. If you are over in Windows, it’s called “Adaptive” brightness, while your phone will just call it “Auto”. While this technology is great in theory, I don’t really need max brightness all the time, and I like to control it manually to make sure I’m getting the best battery life.
Open up System Preferences, head to Displays. Then uncheck the Automatically adjust brightness setting.
The difference in battery life is enormous, so your best bet is to keep the brightness fairly low.