Dual Fuel 21700 Flashlight Design


Newly Enlightened
Aug 27, 2022
B.C. Canada

I recently came up with an idea for a 21700 flashlight that could take 6 AAA's in it as a backup power source. Similar to 14500 flashlights being able to use AA's as backup. Are there ways to improve it? I appreciate any feedback.

If you have a use for this, feel free to use it! I'm not going to do anything with it, although if you ever make anything from it, some pics would be cool. :)

The idea is to basically use an inner and an outer tube for the body of the light. The inner one can be threaded into the outer tube from both directions making the flashlight longer or shorter.

There would be 2 bundles of 3 AAA's, one on top the other.

Three AAA's together in a bundle almost fit in a 21700 body, but they are a little big. If the inner tube was hollowed out in 3 lengthwise channels, the AAA's would fit perfect and both bundles would be aligned with each other. (diagram 2)

The inner tube would have three sets of threads on it. One about at the 2/3 mark and one on each end for the tailcap. The threads on the middle are spit in two parts with a ridge in between them. That way the tubes can be tightened together. The threads on both ends are recessed so that the tubes can come apart without catching on the middle threads.

The outer tube just has the one set of threads on the inside at the tail end.

Here are some basic diagrams to try to explain better. The dimentions are way off, Really, it's just a diagram to show the connections.

Red - Threads
Yellow - O rings
Black - Outer tube
Green - Inner tube
Blue - Tailcap

Flashlight Diagram 1.png
Flashlight Diagram 2.png