ESP32 ADC measurment with variable input voltage?

[noobmastha]

@orangedays22

Do I even get an accurate voltage measurment with option 1 where the batterys voltags, and thus also the voltage input of the ESP, varies between 3.2 and 3.6V? Or is the reference voltage also varying so that it makes it impossible to measure accurately and I have to use option 3?

What level of accuracy is ok for your needs, what is ADC used for?
One tip, ESP32 internal ADC attenuator is very nonlinear, so if you want accuracy use 0dB attenuation in code settings (in this configuration, max input voltage is about 950mV, so you need to reconfigure external attenuator).