If the MacBook Air's sleep indicator light flashes 5 times at power on and the pattern repeats after a brief pause, the computer's battery may require service. If the power adaptor is present and connected to a working power source, the MacBook Air will continue to start up normally; however, the battery should be serviced by Apple (or an AASP) at your earliest convenience.
This 5-flash pattern may also occur when waking the computer from sleep if the power adaptor is connected and the battery requires service. This is intended to let you know that the MacBook Air's battery requires service but will still accommodate continued operation until it can be serviced by Apple or an AASP.
Important: It is important to note that the "Safe Sleep" function may be unable to recover unsaved data if the MacBook Air's power is interrupted while in this state.