Introduction: User Constructed Lithium Battery Powered Soldering Iron

About: Technologist (41st year)

Recently, I found a surplus source for Weller(r) BP1 Battery powered Soldering Tips.

Soldering Electronics sometimes requires a site repair visit and field tools can be a challenge.

I often build my own tools, finding off the shelf solutions too costly.

I had an old burned up Sears? Home Works Power Screwdriver from the late 1990's

and decided to gut that case and retain the two way momentary rocker switch.

None of the Ni-Cad batteries that power my older tools have survived beyond 10 years.

I have a number of recovered 18650 2.2AH Lithium Ion cells and

I decided to put it all together to build my own battery powered Soldering iron in one Sunday afternoon.

The image shows a older case of a 1990 power screwdriver that had two Ni-Cad cells, coaxial recharge jack,

two way toggle switch and a nice round hole at the screwdriver end for a RCA female Jack.

The user can create ANY pen-like case, and another post on shows a similar project in a metallic ALTOIDS can.

Step 1: Explanation of the Circuit

In the schematic shown, read from LEFT to RIGHT.

USB recharging comes from ANY USB port to a MINI-B, into a TP4056 charger.

The charger output is connected to the Lithium battery TANK. I utilized a lithium battery from a Dell laptop, but any 18650 can be used,although I suggest the 2 AH capacity as practical.

To the right of the battery is the momentary push button to allow current to flow to the White LED and the Tip. That is the switch you press to make the TIP heat up.

Between the Battery, Switch and RCA connections to the TIP, I used 14AWG (1.6mm) wires to handle the current of up to 1.8 Amperes.

On the right side is my own representation of the Weller BP1 tip found here The White LED is in 'parallel' with the TIP.

I used interconnections or RCA jacks and plugs to 'extend' the hot tip out from the case as the tip gets hot. I also like to make my tools "serviceable" in the field, so the RCA connections help with exchanging tips.

I *soldered* AWG14(1.6mm) wires directly to the battery but the user can use 1S 18650 size battery holder if the user is uncomfortable with direct soldering to an 18650 without tabs. Warning: 18650 cells can be dangerous if there is an accidental shorting: only experienced CET's / EET,sshould solder directly to a cell; 18650 types can be obtained with solder "tabs" on the ends.

Step 2: Build Sequence (suggested)

(1) Solder AWG14 wires BLACK to Negative and RED to Positive end of 18650. No more than 30 seconds per side. use pre-wetting/tinning and its likely that you will need to use 63/37 type LEAD solder for this task. (sorry RoHS).

Professionals: using SAC305 will be brittle; I see lots of cases where portable tools fall apart over time with Lead-Free RoHS complaint material used in joints.

(2) Cut length and solder RED AWG14 to one side of momentary SPST Switch.

(3) Cut length of RED AWG14 from other side of momentary SPST Switch to center of RCA TIP connection point.

(4) Cut longer length of BLACK AWG from battery negative all the way up to the RCA barrel connection point.

(5) Solder attach smaller gauge wires from White LED "in parallel with" RCA connection points, ANODE to positive, CATHODE to negative. (images of LED polarity are )

At this point, pressing the switch should illuminate the LED and you can test the TIP with caution.

(6) Cut and Solder attach the small gauge wire pair from BAT+ to the positive of the battery, using pre-wetting/tinning method if necessary. Solder attach the TP4056 board BAT- pad to the negative of the battery.

Plug in a USB power source to the USB Mini-B jack at the end of TP4056. Observe Red LED as solid, indicating charging. Cutoff voltage across battery should be around 4.10 - 4.25 Volts DC. Nominal voltage for an 18650 should be 3.6 - 3.7 Volts DC.

The LED polarity image is from

Step 3: Operation

Depressing the Momentary switch causes current flow from the 18650 battery, to the White Led
and onward through the BP1 soldering tip via the RCA interconnections. heating up should occur in 8-10 seconds from 25C all the way up to 385+ C. I avoid heating for more than 30-45 seconds as that is hard on the 18650 cell; I like to heat in 30 second intervals with about 15-20 seconds OFF state. I only use this tool for short field tasks.

I had noticed that the outside (low Winter) temperature will effect the operation. I tried soldering in -10C cold Automobile and had to reach for my Gas Butane after the 20th connection or so.

Soldering operations should be performed indoors in a properly ventilated room. Solder fumes are a known health hazard; always use ventilation to exhaust the fumes. Obvious cautions for Eye and Skin protection are assumed by professionals in this field; wear Eye and Skin protection during soldering to protect against splatter, just as you would for Welding.

The first photo illustrates the complete Soldering Tool assembly.

The second photo shows how I re-purposed a dried up "Sharpie"(r)(c) Dry Marker pen cover to cover the tip of my tool while it is transporting in my tool case. This is optional and up to the imagination of the user/creator. I recall my experiences with Butane Gas Pens, burning sensation as I fumble in the toolbox for another item, and the lasting pain, as the explanation for my personal need for a (dummy) cover.

Step 4: Bill of Materials

The Bill of Materials include

(1) A case of some sort, likely cylindrical that is comfortable like a pen

(2) BP1 Weller Tip RCA female jack affixed to one end of cylindrical case (above)

(3) White LED, maybe the 5mm size , T1-3/4(0.2")

(4) large diameter wire (1.6mm, 14AWG) short lengths of RED and BLACK (or Blue and Green for Europeans)

(5) Momentary Switch, Single Pole Single Throw, capable of 2A

(6) 18650 Li-ION battery, like the types that are recovered from laptops (separate handling precautions required)

(7) TP4056 charging board, that manages the 2.5 to 4.2 Volts DC charging of the 18650


(8) 18650 single cell "holder"

(9) Cap or cover for TIP while transporting your tool

Weller(r) sells the BP1 at for $8 and I see the part on ebay

The TP4056 is very widely available on ebay, Amazon and Alibaba sites.

The momentary Switch may a bit more effort to source, but many enthusiasts should be able to obtain it locally.

I show a C&K tactile Momentary only because these are solidly built switches and last a long time.

I would NOT recommend a closure SPST switch that locks in the ON position as this may lead to TIP burnout.

(I will experiment with one tip over 30 minutes to see and report).

Step 5: Details

The components in my list are readily available on a global basis.

TP4056 boards are great for just charging one 18650 Cell right up to 4.2 Volts.

An 18650 has more current and capacity than three 1.5V Zinc or Alkaline cells that were used in the original design of BP6xx Weller tools.

A small momentary switch is needed for switching on the current and thus, heating the BP1 Weller tip.

The Tip normally runs from 4.5V but that triple cell pack soon drops to 3.5 Volts and still heats up that 2.4 Ohm tip quickly. [3.85V nominal, 1.8 peak and then 1.6A steady after reaching temp, around 2.4 Ohms, about 6 Watts]

This particular BP1 TIP is unique because the connection barrel will fit into a standard size RCA female jack.

In comparison another TIP known as the BP10 conical or BP11 wedge (6V) is slightly larger on center pin and does not fit.

I decided to make a housing with a female RCA jack to allow quick field TIP replacement.

I had a few metal barrel "joiners" or two back to back female RCA jacks in the shop. The tip heats up to near or over 400 degrees Celsius, so using metal jacks helps to dissipate the heat of prolonged operation. I fixed this joiner to the end of my case.You do not need to use the joiner as I did if your case is metallic; you may implement a RCA female surface mount jack with solder tabs. I used an RCA MALE plug on the interior connection to the inside end of the RCA joiner.

I did not put a drop resistor in the parallel WHITE LED circuit. 47 Ohms in series would have been a prudent choice, however, most White LED's work nicely from 3.25-4.25 Volts DC, and a 47 Ohm resistor is unnecessary. Other color LED's will have lower forward voltage and will burn out at 4V in this circuit. The LED illuminates the TIP area and it tells the operator that current is applied to the TIP. I drilled a hole in the end of the tool, hot melt glued that LED, and I "aimed" or pointed the LED light at the TIP area. Hint: Do not solder in the dark. please.

I soldered my 18650 ends to large gauge wires 14AWG (1.6mm), to handle 2 Amperes of current while heating the tip. With the momentary SPST switch in series, current flows to the internal RCA. This extra plug could be eliminated and a RCA female could have been mounted on the case, but I wanted to be able to service that RCA joiner if it melts in future.

Lastly, I found a TP4056 board in my shop and glued that to the end of the case I re-purposed for this project. Any long case that looks like a solder pencil will do. BAT+ connects to the positive end of the 18650 and BAT- connects to the negative end. I see that my 18650 charged up from almost dead 2.6V to full in 5.5 hours but each Li-ION model is different. That TP4056 board has two LEDs on it and BLUE seems to be the indication when the charger has reached full cutoff. I looked at the USB current and it was running at 500mA normal maximum (actually at 570mA), but this TP4056 board would handle 1000mA charge rate if I plugged to a wall power supply capable of that higher current supply. The point is that the TP4056 board will not allow a Li-ION to "over charge" "over cook".

The tool can be recharged from a USB portable Battery "bank" but many of these "Banks" cut off output at 1Ampere, and that is not enough current to heat a BP1 tip with the USB current alone. Besides, I did not want a tethered tool.

Now I (the brain) have to remember that the tool needs to be charged *before* I head out the door to a remote repair.

The TIP is manufactured by Weller and Cooper Tools and the November 2017 image shows a Tool found at a local surplus store for $20 Cad. UPDATE JAN 11,2018; I tried to fit a Li-Ion 18650 into the store bought Weller; !it wont fit!

Step 6: More Details on Use (this Is Just Extra Reading)

I have purchased several of the BP1 tips, and I have constructed another much more compact version with the same schematic where the RCA female was soldered right to the 18650 and the whole thing fits into a "magic" MARKER body with a flat momentary switch. This is my pocket design although I will not carry around an 18650 in my pants pocket; I have experienced a touch of a melting car key when accidentally shorted across an 18650. I enclosed the pocket pen version in a plastic marker case. I like to make tools out of discarded items.

A note: BP10 and BP11 tip models need slightly higher voltage (6) and current (1.8) and although those tips do get hot, the do not fit into RCA jacks at all.And these two TIP are more expensive than the BP1.

In a recent urgent job connecting 9 Wires to a PCB, I had noticed the case of the 18650 was slightly elevated at around 34c (warm) so this tool should be used for mobile work. I find this tool is much safer and more controllable than my Gas Butane powered iron, which is far too hot even after the job, with the safety cover over the tip.

I had recently searched, and three other posts mention the original Cooper/Weller BP645 on which this project is based, without real technical specification, and without a proper bill of materials or component sourcing information.

I also have the original Weller BP6xx and BP8xx tools but find that I forget the Alkaline Cells and get to a task with a half dead battery set. Although quick to replace AA batteries , I desired a Li-ION powered rechargeable solution.

Feel free to ask me questions on *this project* please. I hope my post has been clear.

The secret to Soldering successfully lies in the knowledge of the materials you are bonding, with special scrutiny of the eutectic properties. I have *rarely* revealed this secret to Students over these past 38 years, but Pro's know this.

I posted the BP1 specification sheet from the Weller/Cooper Tools web site for information. I measured an average of 2.4 Ohms resistance between connection tip and return ring, on four BP1 TIP's that I have purchased.

Step 7: Soldering SAFETY

Do not touch the TIP when heating. Avoid skin burns. Do not wear shorts while soldering to avoid molten dripping or accidental burns.

Wear EYE PROTECTION. Insist on Eye Protection for visitors of your bench. I wear eyeglasses and I have had solder splashes come close, hitting my cheeks and forehead.

Use proper POSTURE while seated at the bench. OK, so I have soldered under a Car, careful to avoid any molten solder dripping on my skin or clothes.

Wash your hands after handling solder and flux. Do not hold solder in your teeth or mouth. Use a 'third hand' or bench vice to hold the work and the solder if necessary.

Ventilate the work area or use a bench-top filtration fan. There are Instructable plans for activated charcoal fans to remove poison from fumes. I use a Kitchen Hood re-purposed with modifications to increase airflow and Oreck(r) Air purification. I also use a AC equipment fan to move air across my bench top and use the vent filter on the other side of the airflow. Do not lean over the top of the work as the fumes will enter your lungs through your nose and mouth.

Some of these precautions are obvious and I can think of perhaps another half dozen "don't" points.

Step 8: Improved Design (added January 10, 2018)

Added a Power MOSFET N-channel; reason= momentary switches get hot with 2Amperes.

Any N-Channel MOSFET will work. The MOSFET switches the negative of the Load (BP-1 tip).

Series White LEDs are wired in parallel with the BP-1 tip for indication; with 33 Ohm drop resistor, illumination is really weak at 6V combined voltage from two series 18650 cells.

Added dual 18650 power: faster warming and better operation in Cold outside temperatures (-30C at times; Canada)

Smaller Case, external 18650 charging, dual White LEDS, RCA female port that can be extended with RCA male to female extension cable.

See the attached PDF for all detail in one sheet. First image was my prototype; Second image is the current build in 'dollar-store-case-for-rubber-bands' and I have made a custom extension cable RCA male to RCA female.

Using this tool: I am getting faster warming and about 300 joints. A recent task involved Auditorium Speaker cables. I managed to do all 16 Speakers 14 AWG cables (32 joints) in -10C outdoor venue, with plenty of power left over. Cable ends were 14 AWG stranded to Pin Lugs.

Bill o Materials in version 2:

MOSFET N-Channel; Wow I found lots of these on DELL desktop motherboards from the late 00's . Mostly 40T03. IRF N-channel also work. These things are MAGIC and easy to hook up.

Momentary switches like the expensive C&K one in version 1 get warm with repeat use. So I use MOSFET with 10K ohm resistor between gate and drain to prevent latch up. I use a low resistance like 470 or 560 to activate the gate to positive. Wow , a lot of current can flow without heating up the n-channel MOSFET.

I used two White LEDS (3.6 V-forward) with a 33 Ohm drop in the middle (thanks gm280 for suggesting that) because of the 8.4V total voltage but the two tend to dim even when there is plenty of power left after heavy use.

RCA jack will get warm with repeated use; I made myself an extension cable "pen" and plan to make a remote momentary switch on that "pen".

The idea was to repurpose laptop parts; 18650 cells are plentiful , MOSFETs are all over most modern "green" motherboards for "sleep" modes, White LEDs are now everywhere, carbon resistors , those small momentary switches, and so on.

I use a 2S 18650 "holder" because I have an dual cell external 18650 AC mains charger. Now all I have to do is remember to *RECHARGE* my tools after each site task.