USB powered li-Ion charger for 18650 battery

thingiverse

It's a basic enclosure for 18650 batteries. The charging circuit is based on the TP4056 chip, which comes in different models - opt for the cheapest one. To find a supplier online, search for "TP4056 USB Li-ION Battery Charger." A small spring was salvaged from an old battery holder, and there's a tiny slot to secure and block it. For the positive side, strip 2/3 cm of wire insulation, shape the wires into a small ball, apply solder to it while hot, and press it firmly onto the plastic. The SCAD file is available if you want to adjust some dimensions.