Introduction: Convert a 3xAAA Flashlight to a Lithium 18650 Cell

This may not pertain to all 3x AAA flashlights, but with some calipers and common sense, you can probably check for yourself.

Step 1: The Background

I bought a cheap UV flashlight the other day to augment my pet detective toolset.

The initial problem was this: the flashlight is supposed to take 3xAAA batteries. Harummph, I say. Completely useless. I bought it anyway, figuring I'd modify it to take a useful battery type.

Step 2: Battery Carrier

This is the battery carrier. It holds 3 x AAA alkaline batteries in all their overpriced, anemic glory.

Whipping out the calipers, I found the battery carrier to be 52mm in length and 22mm in diameter. So the idea was to find a rechargeable lithium battery that would fit within those dimensions.

Step 3: Lithium Cells

Conveniently enough, lithium cells are labeled with their dimensions listed in millimeters.

Example:
One of the most common cells found in laptop batteries is the 18650 cell.
This means the cell is 18mm in diameter and 65.0mm in length.

Having a bunch of these, I tried to jam one in, but sadly it wouldn't fit. It was just too long.

So I went down to my local lithium battery super store, right down the street. Ermm, j/k. I went to the online electronic crap superstore based in Hong Kong. And I searched.

They had a 25500 lithium C cell, which was almost perfect. Well, just a tad too fat. I checked the internal diameter of the flashlight tube to be sure, and 22.5mm was the max it would accept.

After a few minutes it became apparent that the largest lithium battery that would fit is a 17340 cell. That would have left a horrible waste of space. I wasn't going to pay good money then wait 3 weeks for shipping time for that bad a solution.







Step 4: Making Space

The bottom cap of the flashlight contains a spring and a plastic spacer. I removed the spring by twisting it out while pulling on it. It came right out. I tried to put it back in for the picture, but it didn't want to cooperate. So be forewarned that this might be the point of no return.

Step 5: Dremel

I gutted the plastic to hold the battery. There is a little metal contact at the bottom, which I was careful not to maul.

Step 6: Done

When it's done, the laptop cell fits right into the hole, buying enough space to reassemble the flashlight.

Step 7: The Result

It's a perfect fit. The parts screw down all the way. The cell does not move when the flashlight is shaken.

Step 8: Afterthoughts

There was one small hitch. The plastic dust from the machining worked its way into the switch. I removed the rubber dust cover over the switch in order to shake it out. I destroyed the seal in the process. Oh well.


Step 9: Other Thoughts

I'm disappointed so far with the UV LEDs for my intended purpose. I think they emit too much visible light (as compared to a fluorescent blacklight) to be of much use in forensic work. So for now, the mystery of the unusual pet odor goes unsolved. But the investigation will continue.