Upon powering up your Fly6, an audible battery status will sound, alerting you to the current battery level. This also occurs as you power down so you know if you have enough battery for your next ride. See below for audible meanings:

Should your battery deplete to beneath 5% capacity during your ride, 3 long beeps will sound. This indicates your camera has turned off, however the flashing of the tail light will continue for a further 1.5 hrs (approx.) so as to keep you visible for the remainder of your journey.