Macbook Air SMC reset
It needs to be plugged into the mains to do it.


  To reset the SMC:
  
  
  Shut down the computer.
  Plug in the MagSafe or USB-C power adapter to a power source and to your computer.
  On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
  Release all the keys and the power button at the same time.
  Press the power button to turn on the computer.
  


Source : Apple KB - Resetting the System Management Controller (SMC) on your Mac