That's been requested multiple times in the past. If I recall correctly, none of the iFly devices have an internal voltage sensor so that information is not available.
One alternative is to have the battery plugged into ship's power and charging, then have the iFly device plugged into the battery for use (if the battery you're using is capable of being used while it's charging). I used to do that with my 720; I still essentially do that with the Android tablets I use.
Since I'm going to be looking for a place to set down pretty quickly after suffering a complete loss of power, at that point there's little risk that I'll be in the air longer than a fully-charged battery will last.