This is a charger for lithium ion batteries which takes its power from the USB port of a computer.
It uses the MCP73861 or MCP73863 Li-ion battery charger chip manufactured by Microchip.
Step 1: The USB power connector
A piece of edge connector hacked off an ancient ethernet board serves as the power connector. To make it, cut off a piece that includes four edge fingers, and then file to make it fit inside the USB connector on the PC.