18650 to Double C and Double D cell adapter

thingiverse

Adapters to allow the use of 18650 Li-ion cells to replace double C or double D batteries is flashlights and other applications where the minor difference in voltage (3.7 nominal, fully charged 4.2 vs. 3 volts) is not a concern. In most flashlights this will result in a brighter beam, but in some applications the voltage difference may be a concern, so be sure to check your device. I designed this initially to replace C cells in an otoscope/ophthalmoscope with rechargeable 18650 cells, and decided to make a version that would replace double D dells as well. Two versions are included. One version is a solid shell with a built-in spacer, the other version has holes spaced around the sides of the shell to allow any heat to escape as well as reduce the amount of plastic needed. In addition to the batteries you will need M5 bolts (or smaller bolts with a spring) to insert into the shell. The 18650 batteries are supposed to be 65 mm long, but some have flat terminals and others have a raised positive terminal that adds 4-5 mm. C cells are 50 mm long (with raised terminal), so 2 cells together are 100 mm. You will then need an M5 bolt approximately 35-40 mm long (or a smaller bolt with spring of that same total length). D cells are 61.5 mm long (with raised terminal), so 2 cells together are 123 mm. You will then need an M5 bolt approximately 60-65 mm long (or a smaller bolt with spring of that same total length).