If a flashlight will take a protected 18650 (a 18700) and will also take 3xAAA, then will it necessarily take a 21700?
i tried several of mine and 21700 fit.
Length could be an issue.
I know for sure that somebody will say: but there are programs for it on the net. Aren’t there? Yes there are!
But I am an oldfashioned techie who used to calculate things without an electronic device.
So what size should be the diameter D of a hole where you can put in three (equal sized) smaller cylinders (batteries).
Using a ruler and a pair of compasses I come to: D = d * (1+2/√3) ……or…… D = d * 2.1547
where d is the diameter of the smaller circle/cylinder (the AAA battery measures 10.5mm or 0.41in)
So I’m pretty sure it will fit. (after removing the sleeve 
)
fit fine in my “green ring” zoomie and ultrafire g700 type zoomie.
its tight lengthwise but fits.
Answer for ‘A’ is 10.
18700 + (3x(A^3)) = 21700
21700 - 18700 = 3000
3000 / 3 = 1000
Cube root of 1000 is 10.
Solved :student:
Yep I don’t know what I’m talking about 
ROFLOL!
btw i tried in a hausbell tulip/x700 zoomie and while it works its a bit snug the last thread or so.
i didnt want to crush/dent the cell so i stopped.
o ring is well engaged so no problem not bottoming the tailcap.