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 122) with a higher value (3 kΩ for 400 mA is OK).
  Alternatively, look for a board that is already configured for a lower current (500 mA boards exist but are rare).
• Boost converter based on MT3608 or similar. A mini board with two jumpers for voltage adjustment is recommended.
• KDC11 toggle switch (15×10 mm)
• Two‑color (optionally three‑color RGB) 5mm LED with common anode (+) and a resistor around 1–2 kΩ. Two colors indicate charging status, the third color indicates power on.

Assembly tips

All components are secured with glue. Do not use hot glue for components that may get hot, especially the charging board.

1. Solder wires to the battery and connect them to the TP4056 board (B+ and B-).
2. Connect the output of the charging board to the input of the boost converter. Connect positive directly, and connect negative through the switch (switching the negative side is required for proper RGB LED indication).
3. Set the voltage on the boost converter:
   • Option A – If you don't want to modify the calculator board, set the voltage slightly above 5 V, optimally 6–9 V.
   • Option B – Set it to 5 V and bypass the HT7550 linear regulator on the calculator.
4. Connect the output of the boost converter to the calculator in place of its original battery.
5. Connect the indicator LED:
   • For a two‑color LED: solder instead of the original LEDs on the TP4056 board.
   • For an RGB LED: connect common anode (+) to B+ or OUT+ on the charging board (this is required for power on indication to work), connect the remaining color (the one not used for charging indication) to negative after the switch.

!!!Important: clearance and insulation!!!
Cut the middle button pins as short as possible, then add an insulation layer between them and the battery. Otherwise the pins will poke through the battery wrapper — not fun (the device will not catch on fire if you connect the switch the recommended way). Use some plastic film (e.g. from a PET bottle).