I tried to point out a simple method to measure the battery, flag it's output and be a 0 drain load until enabled circuit. I can provide a schematic If you are interested. This isn't rocket science at all just an "Open Collector NPN switch that pulls a little current from the base of a PNP to turn it on and send power to another PNP and a resistor or two and the TIL431 If the 431 is turned on enough (the battery is high enough to turn the TIL431 on then the last PNP's collector goes high and you have a "Good Battery Flag". Test for it and then set the controlling port pin low to turn off the Batt Test enable. later today I will install Fritzing or something close and create a schematic for you. If I can find it I will try to simulate it in MultiSim 11. This is in order to verify something I have used many times in the past. As I remember there is a MAX666 or something close that can give you a linear low current regulator and a low battery flag... all for 10 uA overhead... less than the current required by the Arduino linear too. Interested?..
Doc