Since the transmitter stops working at 3.5V the battery cannot be over discharged while driving.

However, I forgot the transmitter turned on for a few days and it did over discharge the battery. Twice. (Yes, I know it's nothing to brag about...)

The battery didn't it go up in flames or even puff and I was able to bring it back to life both times. I am still using it today.

Of-course I don't know how other batteries will react and I'm only reporting my own experience.

I believe that leaving a battery connected to anything (external cut off, regulator, BEC, etc.) will slowly discharge it. So I'm not sure there is a 100% safe solution.

Avner.