M. Glavin stated
A safe no fly voltage for 4.8V batteries is 4.4V and 5.5 for 6V.
First of all let me state that I do not know a lot about electrical formulas. (I almost flunked math years ago).
Anyway....I am SURE Mike is right on with his figures. I have owned 2 analog, Hobbico loaded voltmeters and a loaded digital voltmeter I made using Red's web page. There was quite a difference between the 2 Hobbico units in a side by side comparison, and also to the digital amount. The point I want to make is this...... I don't trust some of the voltmeters out there, (especially the analog type). Sooooo, these units may be off enough that if you use it and go down to 4.4 volts..... it MAY be lower than that. I would suggest to compare your voltmeter's readout to several others. (the digital type). Then make your decision on your "safe fly" voltage based on YOUR voltmeter. Just my humble opinion.....lownslo...
Bob