This is the second battery fix released for the 2013 MacBook Airs, with update 1.9 fixing a battery issue that could cause the battery to unexpectedly cease functioning.This update is recommended for MacBook Air (Mid 2013) models.
This update addresses an issue which may cause the battery to drain faster than expected when the lid is closed.
The update can be downloaded from Apple's support site or via the Software Update tool in the Mac App Store.
Article Link: Apple Releases Battery Fix SMC Firmware Update for MacBook Air