It has been stated by Dynon on a number of threads that the screen(s) will remain on battery (if equipped) if power is lost and airspeed is detected. I don't recall the threshold.
Yes, but did joe lose external power? Turning the screen off using button one is not the same as losing power. It is a direct command to "turn off", which can also be done if you lose power and are on the backup battery. I can't think of a time when I wasn't able to turn off a screen regardless of the airspeed being indicated, regardless of how it was being powered.
Joe said he was sometimes unable to turn off the screen "because" there was indicated airspeed (erratic), even though you were on the ground. That still seems very odd and is not my understanding of how the system is designed to behave.
You should ALWAYS be able to manually shut the screen off by holding down button 1 for the designated time until it turns off.
Dynon screens with backup batteries are designed to stay on when external power is lost, regardless of the indicated airspeed. What they do with the airspeed when external power is lost is not to then automatically shut off after a countdown on the backup battery power.
Again external power loss is different than holding button 1 down to shut down. To be clear, you can do that to manually shut down at any time, whether under external power or under battery power, in the air, on the ground, ... Literally at any time.
Joe, if you are having trouble manually shutting down your Dynon with button 1, regardless of the circumstances, I think you have a whole 'nother problem too. You should always be able to do that.