Convert 3-AAA Flashlite to Lithium





Introduction: Convert 3-AAA Flashlite to Lithium

About: I like to tinker and experiment with electronics, robotics, programming, and photography. Along with my latest interest in Steampunk.

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.



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22 Discussions

The lithium cell shown is a common lithium ion 14500 Ultrafire that charges at 4.2 volts. The 18650 you mentioned will not fit the flashlight and can't be used. Only use a 14500 size cell. When in use, the cell voltage will be about 4 volts.

9 replies

Why replace the AAA with a battery that has less voltage and less storage capacity?? I use rechargeable Sanyo E's which have a voltage around 3.75 and a reserve of 2400 ma. Your conversion is 3.7 to 4 volt with 900 to 1400 ma reserve.. Did I miss something???

Actually AAAs are are pretty horrible when you consider they have about 1/3 the capacity of AAs and cost EXACTLY the same. All but the tiniest flashlights made with AAAs are a crime against all that is good. Any attempt to removed a AAA powered device from service should be lauded.

Actually, yes, you did. The absolute LARGEST capacity AAA NiMH cell I have seen available is 1350mAh, and that is unlikely to be a credible rating. Most are something in the 1000mAh range, and in particular the Eneloops. In my experience, the charge life of the lithium cells eclipses even alkaline AAA batteries, and the higher potential and current provide a bit brighter performance, without burning out the LEDs. 14500 lithium cells are also available in the 2000-2200mAh range.

I think you also missed something. He is replacing (3) batteries with (1). My Sanyo have a 800 ma rating so you times that times (3) batteries and you get an easy 2400ma. Alkaline have even higher rating so I get back to my original question. Why replace with a lower power and lower reserve battery??

You can't just add the mAh rating of 3 AAA cells together. Since the current is in series, the mAh rating would be no better than one cell. The best you can get from a single NiMh cell would be 600 to 800 mAh for a AAA cell. With a good quality 16500 lithium, you can get 600 to 900 mAh and have 4 volts to start with. So in terms of power, there is an advantage. The Ultrafire cell shown is probably one of the worst 16500's around with at best, 300 mAh of capacity. I only got it to try this little experiment out. With only dealing with one battery, its easier to change out the battery instead of messing with 3 cells in a holder.

Oops, 14500 cells. My mistake. One thing I wish is that Instrucables would allow the author to make edits on their own posts.

Ah Ha-- The right answer! Thank You Sir.. Now it all makes sense to me...Forgot that series doesn't compound the ma!

While the 18650 won't fit most of these cheap lights, I really like using the 18500 size. They have a larger charge capacity, somewhere between the 14500 and 18650, but fit nicely inside. They are commonly available in four-packs for about $10, for use in solar pathway lights.

The flashlight I carry these days came with a holder for 3 AAA batteries, with a spring contact on the positive end and another spring contact on the but cap of the flashlight to contact the negative end. I did a bit of trial and error, and found that a 18650 fit just fine in that flashlight lengthwise, (but would rattle side to side.) The length is not right for some other flashlights, so I may look into the 14500 for some of those, I just don't have any at the moment. To handle the side to side rattle I measured the diameter of the battery (18 mm by definition) and length (also 650mm by definition) then measured the size of the battery space in the flashlight, and then set the positive end with a smaller diamateropening both to remind me which end of the battery went where, and which end of the adapter went into the flashlight first. I then modeled the idea in Tinkercad and started tweeking things till I had an adapter that fit the flashlight and that the battery fit in. Printed up a few on the 3d printer (breakage you know) and carry some spare 18650s that are charged for when I see that the brighness is going down so that I can switch them out as needed.

1 reply

Great idea using a 3D printer. I you got one use it.


1 year ago

You might also want to add some protection circuit to the battery. Like one of these:

4 replies

The Utiltech flashlight I use an example, will start to dim around 2.7 volts and become obvious charging is needed. I don't think it has any voltage conversion circuity inside. Some flashlights can still be bright at really low voltages that could be a danger to the cell. So a protected cell is a good idea. I just charge my cell every few weeks to keep it up.

I have a special charger that holds two 18650 cells and has built-in protection, so I don't need the protective circuit.

That is good for you. Can you elaborate.

What about under voltage protection in the device? You can ruin a lipo by letting them discharge too far.

Very good idea. I have used this method with success. Please note that Li Ion replacement battery is 18650 and charge limit is 4.2 volts. I use 14500 (AA) size in miniature flashlights that use 1.5 volt or 1.2 volt rechargeable batteries. I find that these batteries charge to 4.2 volt limit in a cheap dual AA chargers. Example Use a blank short circuit instead of one battery in series. The older Li PO4 type 14500 batteries are charged to 3.2 volts. Be careful not to overcharge the batteries. May overheat, leak, explode or become non rechargeable.

2 replies

If you've got two 4.2 v cells in series, you're using 8.4 v (at max charge) in a flashlight designed for 4.5 v (with new cells). I'm sure that makes the light a lot brighter, but it may overheat the LED and cause premature failure (as in a few minutes operation instead of years).

Sorry for my omission. Yes in this case use 14500. My flashlight is older type, larger with aluminum body. I adjusted the length by pressing down on negative terminal extension, then it fit, used a thinner insert. Another flashlight type 1200 lumen has a 15 mm screwed extension that allows for 18650 Li Ion battery or 3 type AAA cell cartridge.