So here, for your review and comment, is an approach using three parts-bin parts, to do an under-load test of a 9V battery pack. I think this one would work okay.
A 5.6V zener diode drops the B+ voltage into the safe range for analogue input. The analogue in reads the remaining voltage, if above the zener voltage. If the battery voltage is below the zener voltage, you get nothing. Load on the circuit is provided by multiple output pins driven low, through a 220R resistor. With a 9V battery and 5.6V zener diode, the remaining voltage drop is 3.4V and a 220R resistor will draw 16mA, within the specified limit of 20mA/pin. Two, three or more pins may be used to add more load, up to a chip maximum of 100mA total. If the pins are switched to inputs, insufficient current will flow to allow zener breakdown, and analogue input will be effectively floating. By driving more or fewer pins low, you can adjust he load level on the batteries to, perhaps, get a better picture of charge state than with a single fixed load.
