I believe the issue is heat related or something like that.

I replaced my battery with a 2700mah Nimh pack. After getting a couple of flights in yesterday, I put the transmitter back into my case and went home. I apparently left the transmitter on. Tonight, I went to do some setup work on a model, and discovered that my 9303 is quite dead. I took the battery out and put it in my wife's dx7, and the battery still had power, around 9.8v. So the battery never went anywhere near "low voltage", but the transmitter was on inside the transmitter case for ~22 hours from what I remember based on when I left the field last night and when I grabbed to this evening.