Resistor calculator 18650 mod

## Bottom case for resistor calculator to use 18650 battery

**Bottom case for resistor calculator** (from AliExpress and similar resellers) for powering from an 18650 li-ion battery.
Of course, a flat Li‑Po battery would be more convenient and easier to fit. But I have a bunch of spare 18650 cells lying around, so I used those.
Make sure you have the correct version of the calculator — they can vary slightly.
You can print with any plastic you like; I used black PLA. Supports are not required, but adding a brim is recommended (mouse ear is sufficient if your slicer supports it).

## What you'll need

- One **18650 battery** (recommended with pre‑welded tabs)
- **TP4056 charging board** with protection. It is recommended to use a board with a **reduced charging current (500 mA or less)**.
Most of these boards come with 1 A charge current — the board gets very hot and can melt the plastic case or soften the glue.
To reduce the current, replace the SMD resistor (usually R3 marked