I have a RPi zero powered through a PP3 9v battery and a switcher to take the voltage down. I would like to monitor the remaining power of that battery by measuring its voltage. A quick and easy way to do this would be with a resistive divider connected directly to a GPIO pin.
From what I can see LVCMOS (3.3v) is specified to report low below about 0.6V and high above about 2.6V (quite a large range).
Has anyone done any experiments to see what the Pi does within this voltage range? For this use case, I would be happy if there was a known voltage at which the GPIO stops reporting high all of the time.