Need info: I have a flashlight that uses size C batteries,
( 3 x 1.5v=4.5v ) to operate. What size(s) of Li-ion can
safely replace the (x3) C size batteries ?
depending on the battery tube ID, a single protected 26650 with custom spacer (adapter) would work.
Does it have to be a protected 26650 ?
most C-cell flashlights are direct-drive Incandescent, or 3C LED lights don’t have a low voltage protection in the driver (some don;t even have a driver, & only a resistor) thus risking over-discharging a non-protected Li-Ion cell.
AHA ! That’s the reason for the use of a protected and not an unprotected 26650 battery to replace
the (x3) C cells in the flashlight , which is a Defiant LED (HD15BF01).
You might consider NiMH as well. Flat discharge curve and safe.
You can make your custom spacer out of two x 3/4” pipe caps and a short piece of pipe or two x threaded caps and a brass nipple. You might need to file the OD of the threaded caps though or use 1/2” parts instead and in either case will need a solder bump to make contact with B-
What NiMH size suggestions that can replace the primary ’ C ’ cells ??
Hi
I use 18650 doing this…
But you can buy ready-made 
I’m not too mechanically inclined , but do you have any
pics of the parts needed. I could probably figure out how
to make your suggestion work. I have some ideas also.