A0 is not meant to measure battery voltage directly, it's to read the analog out of sensors (which have an output impedance low enough to not be affected by the 10 µA leakage due to this resistor divider).
Knowing these values: if adding a 100k resistor between batter and A0 you would be able to measure up to 4.2V. For safety, I'd use 120-150k there - max voltage 4.4-4.5.