Как измерить уровень заряда аккумулятора. WeMos, Arduino, ESP8266

How to measure battery charge with Arduino, WeMos, ESP8266 A / D converter. I am conducting an experiment to find out how the battery is discharged, how the voltage at its terminals changes. I am talking about mathematical methods that will allow you to find out the percentage of battery charge, knowing the voltage at its terminals. I am making a voltmeter from an analog-to-digital converter based on Arduino, WeMos, ESP8266. Code and initial data taken on the test bench: https://www.2ram.ru/l/35 Text version of this video: https://www.2ram.ru/l/36