Wawa:
For that reason (instability) it might be better to drop battery voltage with the divider to 1volt,
and measure that with the internal 1.1volt Aref enabled in setup().
A resistor ratio of 1:15, e.g. 2k2 and 33k, is good for ~16.5volt max.
Leo..
Note that the internal reference needs calibrating for each chip, the datasheet says it can be anywhere
from 1.0 to 1.2V. Careful with the ground wiring with such low voltages, do not share the voltage sense ground
wire with anything carrying current or you risk injecting noticable error voltages into the measurement.