G'day Mate,
It does not matter what radio you are using, if the battery does not have enough capacity, or has not had a proper, balance charge & been cycled to get it up to full capacity, when it was new, the plane it is in will drain the battery & crash.
It's just that the DX7 is new, & some people think they are foolproof, well they are not, the RX still needs power, & so does the servos. Without that power, they will stop working.
End of story.
Check out this page for balance charging procedures.
http://members.aol.com/davthacker/balancecharging.htm