The DX7 has a low battery beep. Ileft mine on overnight once and she was beeping away at a voltage close to 9.0 but Idont remember.
I got fedup of the performance and switched to a Turnigy 3 cell, 1800mAh, 5C LiFe pack and it will last a month of 20 -30 flights every week.. Fits perfectly.
It will sit at 10.4V fully charged, 10V after a few hours and stay there for 1 or 2 weeks, then drop to 9.9/9.8V for another week. Usually I charge the pack on my battery charger after it is at 9.8v for that last week.
http://www.hobbyking.com/hobbyking/s...4_TX_Pack.html