Introduction: DIY Maglite USB Rechargeable 18650 and LED Upgrade

About: Just another tinkerer

Hello everyone,

A few years back I was lucky enough to get a bunch of Maglite 3 D-cell flashlights from my local metal recycler, these are the models before LED's became the norm and while I absolutely love using them having to replace the batteries so often led to them gathering dust in my closet.

Fast forward a few years... I started using rechargeable Li-ion in all of my projects and when I needed a new flashlight instead of spending money on a new one I decided to start retrofitting all of my Maglite's with some 18650 Li-ion batteries complete with USB charging and a bright new diy LED bulb.

Let's get started!...

Step 1: What You Will Need:

In order to convert your flashlight you will need the following:

- Maglite or similar flashlight

This conversion will work on any 2-7 D-cell Maglite and will also work with C-cell Maglite's if the holder diameter is reduced.

- 18650 Li-ion batteries

The amount of batteries required is up to you, personally a single battery gives me more than enough work time and reduces the recharge time.

I recommend only using trusted brands of batteries like Samsung, Sony, Panasonic etc.

Amazon - 18650

- Li-ion charging + protection board

Amazon - TP4056

- 3.3ohm 1W Resistor

Amazon - Resistor Kit

- An old Maglite bulb

- 1w LED

Amazon - 10pcs 1W LED

- A salvaged 8mm diameter capacitor

- Access to a 3D Printer

- Soldering iron + Solder

- Kapton tape or electrical tape

- 5/64" / 1.98 mm Hex key

- About a meter of ~24AWG wire

It's a good idea to have one half of the wire red and the other half black to not accidentally confuse the polarity.

Step 2: Battery Holder:

To hold the battery and charging module in the body of the flashlight I decided to print a holder that is the same diameter of the D-cell battery so that it can slide into place where the old battery was without having to change anything to the body of the flashlight.

I designed the holder in Tinkercad and made one available for a single or double battery.

For C-cell Maglite's you will need to reduce the diameter of the holder in order for it to fit into the flashlight.

During charging the TP4056 charging module does heat up quite a bit so I do recommend printing the holder in either PETG or ABS. You can also reduce the heat generated by changing the current setting resistor on the module but this will increase the charging time. TP4056 INFO

I printed mine in ABS and have had no issues with the standard 1Amp charging current.

Step 3: Assembling the Battery Holder:

Now that you've got the printed holder we can start to assemble it.

First we need to add leads to our battery, to do this you need to pre-heat your soldering iron to make sure it is nice and hot.

On 18650 batteries your positive terminal is the side that is slightly raised with vent holes around and it has a slight ridge around the top and the negative is the plain flat part. If you are unsure you can google the model number on the battery to confirm.

- When using a single battery you only have to solder about 15cm of wire to both the positive (red/yellow wire) and negative (black wire) side.

Only strip the one side of wire that you will solder to the battery to avoid shorting the other ends, to be extra careful I like to use some electrical tape on the other end of the wire.

- When using two batteries we want to connect them in parallel, I stripped the positive wire about 3cm and pre-tinned it, placed the two batteries side by side positive facing up and then soldered the wire across the two positive terminals. I then "folded" the two batteries together so that the positive terminals are pressed together with the positive wire coming out between them.

For the negative I just soldered a wire from the one negative side to the other negative and then soldered on the negative that will go to the charging board.

Now you can secure the wires with some kapton or electrical tape.

Feed the wires through the holder as shown in the picture and press the battery into the holder, it should click into place.

As per picture we now need to solder on our charging module:

- Positive from the battery gets soldered to B+ on the board

- Negative from the battery gets soldered to B- on the board

- a ~20cm red wire gets soldered to OUT+ on the board

- a ~20cm black wire gets soldered to OUT- on the board

- Feed the wires from the board through the holder so that it exits on the other side and then press the board into the holder, the holder has a pillar that will stop the board from pushing in too far.

Lithium battery safety:

Lithium batteries are generally safe and unlikely to fail, but only so long as there are no defects and the batteries are not damaged. When lithium batteries fail to operate safely or are damaged, they may present a fire and/or explosion hazard. Damage from improper use, storage, or charging may also cause lithium batteries to fail. Testing batteries, chargers, and associated equipment in accordance with an appropriate test standard (e.g., UL 2054). NRTL certification (where applicable), and product recalls, help identify defects in design, manufacturing, and material quality. Damage to lithium batteries can occur immediately or over a period of time, from physical impact, exposure to certain temperatures, and/or improper charging. Physical impacts that can damage lithium batteries include dropping, crushing, and puncturing. Damage to all types of lithium batteries can occur when temperatures are too high (e.g., above 130°F). External heat sources (e.g., open flames, heaters, etc.) can also accelerate failure in cells with defects or damage from other causes. Damage to lithium-ion batteries can occur when the batteries themselves or the environment around the batteries is below freezing (32°F) during charging. Charging in temperatures below freezing can lead to permanent metallic lithium buildup (i.e., plating) on the anode, increasing the risk for failure. Charging a device or battery without following manufacturer’s instructions may cause damage to rechargeable lithium-ion batteries. For example, some manufacturer-authorized chargers will cycle the power to the battery on and off before it is fully charged to avoid overcharging. Since ultra-fast chargers may not cycle power, do not use them unless the manufacturer’s instructions include them as an option. Heat released during cell failure can damage nearby cells, releasing more heat in a chain reaction known as a thermal runaway. The high energy density in lithium batteries makes them more susceptible to these reactions. Depending on the battery chemistry, size, design, component types, and amount of energy stored in the lithium cell, lithium cell failures can result in chemical and/or combustion reactions, which can also result in heat releases and/or over-pressurization. In chemical reactions, by-products from the electrolyte solution and electrodes can increase the pressure in the cell to the point where the cell walls expand and by-products leak out. Chemical by-products usually include carbon monoxide, carbon dioxide, hydrogen, and hydrocarbons. In many cases, the by-products are also combustible and could ignite. In combustion reactions, a thermal runaway releases byproducts that may ignite to cause smoke, heat, fire, and/or explosion. The by-products from a lithium battery combustion reaction are usually carbon dioxide and water vapor. In some lithium batteries, combustion can separate fluorine from lithium salts in the battery. If mixed with water vapors, fluorine may produce hydrofluoric acid, which is particularly hazardous because workers may not feel its effects until hours after skin exposure. Prevention Workplace injuries from lithium battery defects or damage are preventable and the following guidelines will assist in incorporating lithium battery safety into an employer’s Safety and Health Program: Ensure lithium batteries, chargers, and associated equipment are tested in accordance with an appropriate test standard (e.g., UL 2054) and, where applicable, and certified by a Nationally Recognized Testing Laboratory (NRTL), and are rated for their intended uses. Follow manufacturer’s instructions for storage, use, charging, and maintenance. When replacing batteries and chargers for an electronic device, ensure they are specifically designed and approved for use with the device and they are purchased from the device’s manufacturer or a manufacturer authorized reseller. Remove lithium-powered devices and batteries from the charger once they are fully charged. Store lithium batteries and devices in dry, cool locations. Avoid damaging lithium batteries and devices. Inspect them for signs of damage, such as bulging/cracking, hissing, leaking, rising temperature, and smoking before use, especially if they are wearable. Immediately remove a device or battery from service and place it in an area away from flammable materials if any of these signs are present. If batteries are damaged, remove them from service, place in fire resistant container (e.g., metal drum) with sand or other extinguishing agent and dispose in accordance with local, state, and federal regulations. Contact a local battery recycling center for disposal instructions. Follow manufacturer’s guidance on how to extinguish small battery fires, which could include using ABC dry chemical extinguishers, Class D fire extinguishers (for lithium-metal), dirt, or sand."

Step 4: Modify the On/Off Switch:

Now we need to disassemble the flashlight.

- Screw off the front reflector housing and the back battery cap.

- Pry out the rubber switch cap, it should pull out relatively easily.

- Now you need to slide the hex key into the centre hole of the switch and loosen the grub screw at the back, there are several useful videos available on youtube if this is your first time disassembling a Maglite.

- If your flashlight hasn't had any battery leaks or corrosion the switch should drop out the back with just a slight tap on a table. If not you can gently hammer it out from the front.

Once disassembled we need to modify the switch a little as it will no longer press against a battery for a electrical connection.

- On the bottom in the middle of the switch there will be a spring that made contact with the positive of the old batteries, to this spring we will solder on the resistor and then the positive wire from the charging module on the other end of the resistor. If you have heat shrink tubing you can cover it with that or just tape up your connections with electrical tape.

-Then we need to solder the negative wire from the charging module to the metal plate attached to the nut where the switch grub screw goes through, in order for the wire to fit I melted a groove into the switch housing below the metal plate with the soldering iron.

Step 5: Disassemble the Old Bulb:

Now it's time to start making the LED bulb.

First we need to disassemble an old bulb to get the metal casing. I recommend wearing safety gloves for this part.

- With a hot soldering iron loosen the connection on the side of the bulb and the bottom point.

- Wiggling the glass a little should loosen it from the adhesive.

- Pull out the old glass bulb from the metal casing and then using a screwdriver clear out the remaining adhesive in the metal casing.

Step 6: Making the New LED Bulb:

This is the easiest way for me to make an led bulb without having to make changes to the flashlight.

- Find an old capacitor from salvaged pcb's it needs to be 8mm in diameter to work, open it up from the lead side and remove the innards. We just want the aluminium case.

- You'll need a small <3cm piece of wire that's less than 1mm thick and has a decent amount of copper wire.

- Make a small hole about 1-2mm from the top in the side of the capacitor. I used a drillbit and just drilled it by hand. The hole needs to be big enough for the wire to fit through.

- Now using thermal glue or CA glue stick the LED on top of the capacitor shell making sure the positive leg is facing the hole.

- Feed the wire through the hole and solder it onto the positive leg of the LED.

- Put some thermal glue or CA glue around the base of the capacitor and push it into the old bulbs metal casing (rotate so that negative leg is in line with the groove in the metal casing) with the positive wire coming out of the bottom hole of the bulb casing and solder the wire in place leaving a nice blob of solder for good a electrical connection.

- Solder a piece of wire (I used the excess that was left over from the resistors legs) between the negative leg of the LED and the metal casing.

And that's it now you have an LED bulb that fits into any normal bulb socket.

Step 7: Assemble Your Upgraded Maglite:

Now all that's left to do is to reassemble you flashlight.

I used some adhesive backed foam on the sides of the battery holder to stop it from rattling in the flashlight.

You will need to plug your flashlight in and let it charge the first time using it to reset the protection circuit.

In the design I made loop where you can put a cable tie through as a pull tab for when you want to remove the battery holder.

Step 8: Enjoy!

Now you can enjoy your new upgraded flashlight and never have to worry about having to replace the batteries again!

With the 1 watt LED I get about 8 hours of runtime with a 2 hour charge to full.

If you have any questions please leave a comment below.

Happy making!

Step 9: Updates:

While making this Instructable jgbianchi discovered his Maglite had different internals and dimensions than the ones I have used.

Using Fusion 360 he altered the design to fit his flashlight and was kind enough to share his files with the Instructables community.

"Hi guys! thanks, JGJMatt for the guide! was super fun to make them... I manage to get x2 two cell Maglites for this mod. I download the stl file and print it. Unfortunately, the Charging board didn't fit on it... I am learning Fusion 360 so it was a good opportunity to remodel it and make the adjustment I need it. here is the file if you have the same problem light output is really good for the use that I intended to give to them (home stuff and once in a while some outdoor use) I now have two friends with their old Maglites waiting for an upgrade.
Thanks again Matt for sharing!"

Thank you jgbianchi

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