Just received my air. All set up and it sounds great. I got 9 hour battery option. How do I know if battery is fully charged or running low on charge? I couldn't find anything in the manual.

You know the battery is fully charged when you can run the Air for 9 hours on battery.

A good clue that you are running low on charge is when you've been listening to the Air on battery and then all of a sudden you can't hear the Air any more.

Mmm...kay? (the one-two punch for BobKay)