I like having a small flashlight with me when working. Especially one with a nice pocket clip, but don't like those with 3 AAA cells. After noticing a 14500 lithium cell is very close in length to those 3 AAA cell holders, It got me to think if can I make work with it. A 14500 cell is close in voltage to 3 AAA cells.
Step 1: Compare Sizes
If you remove the battery holder from the flashlight (this one is a Lowes Utilitech), you will see the sizes are almost the same. Just the diameter of the AAA holder is larger.
Step 2: Make a PVC Sleeve
To compensate for the larger diameter of the AAA holder, cut a piece of schedule 40, 1/2 inch PVC pipe. The length is just a little over 2 inches. Try 2-1/16 inches. The 14500 cell will slip inside the PVC pipe. You may need to adjust the length to minimize rattle.
Step 3: Add a Piece of Shrink
To help enlarge the sleeve diameter, shrink down a piece of heat shrink tubing over the PVC tube. You could just add a layer of tape instead. This will prevent the tube from rattling inside and center the battery better.
Step 4: Place Inside Flashlight and Test
Slip the adapter and battery inside your flashlight. Remember to face the Positive End toward the front of the flashlight. The flashlight should work now. When using lithium cells, try not to over discharge the cell so as not to damage it. Charge the cell before it reaches 2.5/2.7 volts. Using cells with built-in protection is recommended.