Krupski:
cornwallav8r:
It's a quandary...a 4-cell AA battery is too high voltage for the 5v input, but almost too low for the 78M05 regulator input, adn will drop out fairly quickly despite battery capacity left. A 9v doesn;t have the endurance of multiple batteries.
I do see however there is a 5v switching arrangement on the Nano at least, to switch, from the USB 5v to the 5v main bus? Not sure that is an alternative...So what are people doing in this regard?
So nobody can blame me, I won't suggest that anyone do this, but I personally have run an Uno R3 board on 4 AA lithium cells in series. As you may or may not know, a brand new lithium AA cell is around 1.80 volts DC, which is 7.2 volts for 4 of them.
The 4 AA cells in series power the board's "5 volt" supply directly. It does not go through the regulator.
I've made several projects powered like this and they are running fine.
Well that is well above the absolute maximum Vcc voltage limit of 6.0 per the AVR datasheet. How do you account for not having a bunch of burned out boards? And it's magic is not a proper answer. 
Lefty