This voltmeter is build with a pic microcontroller with I2C code to control the OLED. A boost converter is
use to power the circuit. Only batteries less than 5 volt can be measured. The code uses ADC to determine the voltage.
The importance here is to use your own datastructure and let the sensor run its serial data without
interruption. It can't handle delays. The capacitor in the circuit is just for decoupling.