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.
>>> >Pic12f675 code (XC8 compiler) <<<
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.
>>> Arduino code <<<