I have an ESP32 board powered by 1000 mAh li-ion battery like to measure the battery voltage, to be able to inform the user of "Low battery".I think it can be done by doing a "voltage divider" to have a voltage between 0V and 1V (instead of 0V to 4.5V) and feed this into one of the device's ADC. Question: could this "battery measurement" be done without any external resistor, and just the ESP32/ESP8266 (+ cables between pins, if needed)?
Get method to get battery level without external resistance