This update addresses an issue which may cause the battery to drain faster than expected when the lid is closed.
Note: Your computer’s power cord must be connected and plugged into a working power source, or your battery must be at least 25% charged. When your computer restarts, a gray screen will appear with a status bar to indicate the progress of the update. Your computer’s fans will run at full speed during the update but will return to normal speed once the update completes. Do not disturb or shut off the power on your computer during this update.
• OS X Mavericks 10.9.2 & later
• OS X Mountain Lion 10.8.5 & later
MacBook Air SMC Update v2.0 is available via Software Update and also via standalone installer.
More info and download link here.