Convert Cheap Flashlights to Lithium Power

Over the years I’ve collected a lot of flashlights, including a bunch of free ones from Harbor Freight. We have them stashed all over in drawers and closets and even in our cars. The problem is that they run on three AAA batteries which tend to leak if they sit for a long time. I’ve used the popular 18650 lithium batteries in some of the portable electronic projects I’ve built but those are too large to fit these small flashlights. I discovered that the 14500 battery is AA size and physically fits the flashlight case lengthwise. As a bonus, they can also use the same charger as 18650 batteries. This Instructable shows how to recycle some common materials to make the Lithium battery fit perfectly in these flashlights.

Step 1: Materials Needed

- One small LED flashlight

- One 14500 battery and charger

- One piece of standard1/2-inch PVC pipe (about 2-1/8 inches long)

- Something to make a contact plate for the + side of the battery

Step 2: Battery Holder

All of the flashlights I’ve modified come with a battery holder and three AAA batteries that allows for up to 4.5 volts for the LEDs. The single14500 battery can be charged up to 4.2 volts so that is right in the ballpark. As mentioned above, the 14500 battery fits nicely lengthwise in the flashlight case but it needs something to keep it from rattling around side-to-side. I have various pieces of scrap building materials around from previous home projects and I found that some standard 1/2-inch PVC pipe fits inside the flashlight while allowing just enough room for the battery to fit on the inside of the pipe. The PVC is easy to cut and most of the flashlights I modified require a piece about 2-1/8 inches long. You can just stick the PVC pipe into the flashlight, put a mark even with the top edge, and then subtract a little more than the length that the flashlight cap will go in.

Step 3: Contact Plate

As seen in the psychedelic picture above, the inside of the flashlight has a spring contact for the + side of the battery. Unfortunately, the spring opening is a bit too large to guarantee good contact with the battery. To fix this problem, we need to make a flat contact plate that will go between the + side of the battery and the spring in the flashlight. The key is that it needs to just fit inside of the PVC and not be too thick. The first thing I tried was some flat metal washers. I found some that fit nicely into the PVC but the hole in the middle was too big. The picture above shows three different pieces of scrap material that I’ve modified to work. The first item is some heavy gauge solid copper wire that I coiled up. The second item is some thin scrap aluminum that I cut and filed to fit. The third item is some scrap printed circuit board that I trimmed to size and then soldered contact wires on both sides. Add a comment if you discover something else that works well.