Even Simpler?
Thank you both for the circuits. I like that Simple Low Voltage Monitor - looks especially easy to build.
Ideally the current through this circuit would be minimal - well less than 10mA, so I don't discharge my battery while I'm measuring it!
I have realized that I may be overthinking: First of all, the circuit does not necessarily need to go high when the voltage is above 13 -- it could go the other way.
Any reason I couldn't wire from +batt, through a suitable resistor to give me, say, 2mA of current, to the digital + input, then to ground through a 13V zener? I think the circuit would be digitally high until the breakdown of the zener, then would go low above the 13 volts. Is that right?