It's a battery capacity tester. It uses an Arduino nano on a custom PCB. It's built on the first three versions built by others; hence: 3.1.
Adapted from Hesham Moshiri’s code as modified by Debasish Dutta but still using Hesham’s PCB layout). Ported to lcdgfx for handling the OLED. To the best of my knowledge the original idea for the hardware, and the very first sketch, came from Adam Welch, who uses a Nokia LCD and some protoboard for it.
This sketch can be used with either the Arduino IDE (just open the .ino file inside of the CapacityTester folder – but you’ll have to read the #include statements and install libraries manually) or with PlatformIO (clone this repository, then execute pio run -t upload with the Arduino Nano connected to the computer via USB; libraries will be downloaded and installed for you). Before uploading it, you might want to use a multimeter to measure your power supply’s exact voltage and note that down under #define VCC in the sketch as that directly influences the battery voltage measurement. If someone wants to implement setting that from the Nano’s internal voltage reference, I’ll gladly accept a PR!