Power bank

Summary

This is a small power bank made of two 18650 lithium batteries. In this page there are files and instructions for building it.

Schematic

The design incorporates a small ready-to-use board and only requires a minium amount of soldering. The board uses a 134N3P IC that steps-up the battery voltage to a stable 5V output. The two 18650 batteries are soldered in parallel and then to the board's B+ and B- poles.

Batteries in parallel

Before soldering batteries in parallel it must be made sure that they have over-current protection or that they are outputting the same voltage. This is important since when connected in parallel, the emptier battery is charged by the other one until the two output the same voltage. This charge current is not controlled and could exceed the battery's maximum current ratings if the batteries are not protected.

Connecting the batteries through a current limiting resistor will cause them to balance while keeping the current low.

Most 18650 batteries are protected by an embedded over-current, over-voltage protection board that should prevent such mistakes from damaging the battery.

Circuit

The soldered components are embedded in the casing as shown in the above image:

1. Lanyard hole
2. Bottom part of the casing
3. 2x 18650 batteries
4. 134N3P powerbank board
5. Top part of the casing
6. Notched geometry for tying the casing together
7. Some foam or other elastic material placed there will ensure that the batteries will not rattle in the casing.

Electrical tape should be used in between the top portion of the batteries and the board to prevent any short circuits.

Casing

The casing is designed to be 3d printed. It is composed of two parts - the bottom and the top halves.

The two halves of the casing are to be fastened together with a decorative cord or rubber bands.

The STL files for the casing can be downloaded here.

Leash

There is a hole for attaching a leash on the casing. It can be helpful since the powerbank could be worn on the wrist while it charges the phone in hand. A leash can be made by cord or an old leash can be used (e.g. from an old handheld gadget, flashlight or console).