18650 Container Bottles for 28mm PET Caps

Design

Project objectives:

• A basic container to protect common types of lithium-ion cells for storage or carrying in a bag, etc. as short-circuits with lithium-ion cells are no joke. (Never carry a bare cell in your pocket next to your keys! Not personal experience, btw.)
• Re-use 28mm PET bottle caps.
• Develop my earlier design of parametric bottles for re-used 38mm caps, refining with smoother details and improvements to the source code.
• Make it easy to print.

Flat-top and a button-top 18650 cells are both accommodated with two different bottle heights to ensure a rattle-free fit. I've are also some variants for other types of cell, as well as an example of a larger bottle that can be made by altering the parameters in the OpenSCAD file(s).

Any 28mm cap taken from a normal PET beverage bottle should work fine as they are made to similar standards (PCO28 1810, etc.) The thread on my design isn't strict to any standard as it doesn't need the details preferred for carbonated drinks - they are not needed here.

If you have a non-standard cell with a protection or USB charging circuit added, then make sure to measure it and see if it fits - otherwise alter the parameters to suit.

Cells with a very thick shrink-wrap might fit snugly in the bottle's tube. Though I have tested iterations of the 18650 bottles with several different cells and all of them now fit without jamming.

Parameters

The design can be altered to generate a wide variety of simple bottle shapes that deviate from the types this project was made for, although I've not gone far off the principal design yet.

My code is a bit buggy, but works (I'm learning) - the connection between the bottle cylinder and the thread area (using the “anschluss” module from UB.scad) can be tricky to customise  for different bottle diameters without an error being reported, but you can change some parameters on line 37 such as “rad”, “grad”, etc until the errors stop. Also changing the “ch” (connection height) parameter in the main module “bottle” may help.

I recommend using the latest (2023) version of OpenSCAD, as the 2021 version returns warnings when used with UB.scad.

Printing Notes

• PLA works fine, but I think PETG may be a more durable material for this purpose. I have not tried others yet.
• Variable layer height works well if you want the threads resolved a bit better, but I find that 0.2mm works very acceptably.
• Three perimeters (setting on included .3mf) is better for strength, but two would work if printing with a 0.6mm nozzle. if you want to try two perimeters with a 0.4mm nozzle, then edit the scad file's parameter “wt” (wall thickness) to 0.9 and render a new .stl.
• If the bottle has a very tight cell fit then make sure to check your flow setting, as if it's too high then the walls will print thicker than they should.