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kWeld – Next level battery spot welder

The story

If you want to make your own battery packs from LiIon round cells, then you have to solve the problem of connecting the individual cells. Soldering leads to them is not an option, because the required prolonged heat at the battery tabs tends to damage the cell's inner structure.

I faced exactly this problem, and learned that the most common method to interconnect the cells is to weld nickel or nickel-plated steel strips across them using resistance spot welders.

The working principle of these is to push electrical current through metal strip and battery tab. As the metals have an electrical resistance, this current causes the material to melt together. The molten metal should only occupy a small area to prevent damage to the battery. This is accomplished by using a high current, allowing for very short pulses.

As this is a challenging task to achieve, professional welders start at several 1000$. There are cheaper models from a Chinese company, but I decided against this option... As I am an electronics engineer with quite some experience in power electronics, I decided to develop my own welder. It should be affordable for DIYers, more powerful and robust than comparable designs, and simple to use.

And I didn't like the fact that seemingly all spot welders would use a preset pulse length to set the weld intensity. The flowing current typically cannot be regulated, instead it varies with the electrical resistance of the weld spot itself. And that depends on quite a few factors like contamination, corrosion, and the mechanical force at which the metals are pushed together. The result is a variation in the amount of energy (=heat) that goes into the weld, leading to inconsistent results. To mitigate this effect, modern welders use two or more pulses, in order to soften the metals and burn away any contamination.

But there is a much more straight forward way to obtain consistent welds: If the amount of energy that is dumped into the weld spot is kept constant, then the same amount of metal is molten each time. This is what kWeld does. There is no need to experiment with time intervals, or the number of cleaning pulses. You just dial in the desired energy amount, and start working. The unit automatically adjusts the pulse duration to achieve the same result with each and every weld.

This picture shows the result of welding 0.15mm nickel strips to 18650 cells (source: eevblog forum, user "romantao"):

With the right power source, the kWeld unit is capable to weld pure nickel strips of up to 0.3mm thickness. But it is also agile enough to weld thin copper wires at low energies. The following picture shows a thin copper strand welded to a 2mm steel rod:

Where can I get it?

The kWeld system is available in kit form for self assembly in my shop:

The assembly and operating manuals are available for download here:

The system firmware is upgradeable, allowing you to benefit from new features or improvements.

The latest firmware is available for download here:

This tool is required to connect kWeld to a computer.

The necessary power supply for the welder is not part of the kit. It should ideally have an output voltage of 5 to 15V DC, and it should be able to deliver at least 1500A of current when short circuited for a few tens of milliseconds. As this is quite challenging to achieve, the following list gives some recommendations:

  • Turnigy nano-tech 3S/5000mAh/130C Lithium Polymer battery (link). I have stress tested one of them for several complete discharge cycles, and it shows no visible swell thereafter. The measured current is 1300-1500 amperes.
  • Turnigy graphene 3S/6000mAh/65C Lithium Polymer battery (link). These do not show any swell after a few full power discharge cycles, but I have no data on their life span yet. The current level is comparable with the nano-tech model.
  • Ultracell UXL65-12 (link). According to feedback from a user, the current reported by kWeld is approx 1000A when tested with 0.15mm nickel strips.
  • Bosch SMT 31-100 (link). According to feedback from a user, the welding current reported by kWeld is approx 1400A.

WARNING: Lithium Polymer batteries are potentially dangerous. If they fail internally, they can spontaneously self-ignite. It is therefore strongly advised to permanently supervise them during use and also while charging them, and to store them in a fire safe container when not using them.

You are dealing with very high energy levels when using this system, which may result in personal injury or fire when handled improperly. Take appropriate safety measures and use this system with caution. Never leave it unattended while being powered.

This product contains small parts, keep out of reach of children!

This system produces significant magnet fields, do not use it when you have a cardiac pacemaker!

The specs?

Hardware features (new features are highlighted in bold):

  • Input voltage for welding: 4V - 30V DC, enabling the use of ultracapacitors
  • Extremely rugged MOSFET-based power switch and mechanical current bus design
  • Fused overvoltage protection of internal power supply
  • Maximum switch current: 2000A
  • Power switch internal resistance: 120µOhm
  • Pulse duration hardware watchdog: 250ms
  • Undervoltage lockout for power switch transistors
  • Input voltage, output voltage, switch current, and logic supply voltage measurement
  • Logic supplied directly from main power source - no need for a separate auxiliary power supply
  • Screw terminal for external trigger switch
  • Logic supply holdup capacitor during pulse firing
  • user interface via an LCD, a dial/encoder, and a beeper

Software features (new features are highlighted in bold):

  • Central weld control algorithm uses a Joule metering approach instead of a simple timer, eliminating the need for two-pulse firing and providing more consistent welds - the amount of energy that is deposited into the weld spot is always kept constant
  • Ability to detect a failed weld, and acoustic feedback to the user
  • Calibration procedure to cancel out electrode lead losses
  • Manual mode, triggered from external switch
  • Automatic mode, accompanied with a warning sound and triggered with an adjustable delay, once the system detects that both electrodes are in steady contact with the weld material
  • Audible feedback of weld process completion
  • Numeric feedback from an executed weld, assisting the user to achieve best results: pulse count, deposited energy amount, required pulse time for this energy, measured current flow, measured ohmic resistance of weld spot
  • Simple and intiuitive user interface - just adjust the desired weld energy up to 500 Joules with the dial knob; experience fine control from the use of an encoder
  • Configuration menus accessible via dial's push button
  • Overcurrent monitoring aborts pulse when triggered, protecting the power switch
  • Battery monitoring with adjustable warning voltage
  • Fuse fitness monitoring
  • Firmware update interface

Please also check out

as well as these videos discussing the evolution of this system:


  1. pcnac says:

    when you can actually send complete kit, housing and firmware update tool ?
    Well thank you!

  2. danys121 says:

    When you send the kit? At week 12 (19-25 of march) or in the 12 month of 2018?

  3. Frank says:

    It is actually week 12 of this year, 19-25 of March.

    • henningd says:

      When i got the welder after correctly assembling it I connected it mistakenly with wrong polarity.It blew some diodes even so it still worked.After contacting Frank he recomended to replace the blown diode with new ones if I was able to solder it.He offered to send me new diodes.He sent me another e mail warning me not to use the welder without the diode protection.The soldering was just to difficult for me , so I sent it back to Frank.It came back to me repaired at no extra cost apart from postage naturally!! That is what I call exellen t customer service !! all communication was always answered promptly and advice was always freely available.
      The welder works extremely well
      Thank you Frank

  4. argo says:

    Very nice product.
    Regarding the actual welds (say using the common nickel strips), what is the conductivity/resistance of each weld “spot”? Has this been measured?
    Thanks in advance.

    • Frank says:

      Thanks! The welder measures this with each pulse (it needs to know it to be able to do the energy metering), and the displayed values are typically between 1 and 3 milliOhms.

  5. mjaynes42 says:

    How much is it

  6. info6 says:

    Received My Kit today what a nice peace of engineering was welding 1/2 hour later Welds like a gem. Great instructions clear & easy to read even for dummy like me.
    Like to thank you guys will buy more as I will make A table setup for this unit as I have been repacking batteries for 35 years
    Shipping To Australia was so Quick Thank you again will be buying more of this item.Frank you saved me lots of heartache great Service.

    Peter Goulding

  7. larry.schack says:

    Hi Frank,

    I received my kWeld unit. I have a few questions. Can you please check your email?



  8. mohammadfarag says:

    Hi Frank,

    I would like to order the complete kit. when will it be available?

    Best regards,

  9. vincentgossuin says:


    je viens de passé commande, pouvez-vous me donné une date de livraison?
    Merci Vincent

  10. acvdmolen says:

    I have iedereen the complete kit andere housing. When van i expect IT and is there a track and tracé tot follow the shipment?

  11. acvdmolen says:

    Great tool, great design, great service, great manuals, workshop great.

    Thanks Frank foto tour support.

  12. RC Model says:

    Hi Frank. When do you anticipate having stock to sell? I will order a complete kit complete with housing and spare electrodes as soon as you can handle an order from Oz sir.

  13. Metrozmartinez says:

    What would be a better power source? A 3s 11.1v lipo Graphene 75c or a 14.4v 4s lipo 75c…?

    • Frank says:

      A 3S Lipo with at least 300A rated current should be used. This means, a 75C battery should have at least 300/75 = 4AH of capacity. The more, the better and the less risk of damaging the battery from the high current pulses.

      Please also have a look at the product page where I give some recommendations:

      4S also works and will deliver even higher current, but more voltage increases the likeliness of arcing and excessive electrode wear, and more current isn’t healthy for the battery.

  14. RC Model says:

    Hi Frank, My order has just arrived, not long to get here in Australia. Now to put it together and have a play. Thankyou sir. Nick

  15. bobmutch says:

    Frank for some reason your link to no this page is no longer is working and is wanting a forward slash at the end of the URL.

    However is working fine.

  16. robts says:

    Hallo Frank,

    Ich habe jetzt mein erstes Akkupack (zwar nur 4 Zellen) geschweißt, und das funktioniert einwandfrei, schnell und unkompliziert.
    Als Stromquelle verwende ich eine Autobatterie, die ich sowieso rumstehen habe.
    Das Gerät ist echt toll!


  17. tmcsystems says:

    Damn I wish the postage was cheaper too Australia , airmail is only like 9.80 Euro, I want the kweld but may have too settle for malectrics instead, hope u have cheaper postage one day

    • Frank says:

      I’m sorry but no plans at the moment. I was shipping via DHL before but ~5% of *all* international shipments never arrived. After losing a lot of money when resending orders, and disappointed customers (some parcels took 10 weeks to arrive), I dropped that and switched to UPS. Maybe worth to mention: I’m looking for a reseller in AU!

  18. John says:

    I was wondering if anyone knew how to connect a regular desktop computer atx powersupply to this rather than the server psu.

    I’ve haven’t done this before so I’m a little worried about how to do it. Or does anyone know of a guide online?

    • Frank says:

      You can use the kSupply module with any 12V supply, the server PSU is just the most convenient way. Please have a look in the kSupply’s user manual (link on product page).

      • John says:

        Hey Frank.

        Thanks for your response.

        Just to clarify, I would use the following from a regular atx power supply: +12V for the positive and the -12V for the negative? Also, do I do anything with the “ground” wires from the atx power supply?

        • Frank says:

          Hi John,

          if you would do that, then you would instantly destroy the kSupply module. Please don’t treat this as an offense, but your response suggests me that your electrical knowledge is limited. Building and using the spot welder system, and also dealing with high power Lithium batteries requires a good understanding of these topics, or dangerous situations can arise. For example, an improperly large welded battery pack can self ignite, even when used with a BMS.

          Cheers, Frank

          • John says:

            No offense taken, but yes. I am just starting to learn about this area. I’m just trying to get this up and running, so I can start with just welding tabs on first then working my way up to making my own battery packs. Any help (i.e. how to connect an atx power supply to the ksupply module) would be apprciated. If not, then no worries, I don’t regret my purchase either way. I’m looking to experiment, learn and grow. Just reading anything I can right now on batteries.

  19. Frank says:

    I appreciate your approach, learning by doing is great and we should have more people out there who dare doing this adventure. But I strongly encourage you doing this step by step. Take a multimeter and see what happens when you measure between the +12V and -12V wires of your ATX supply (you’ll then see why your question alarmed me, and you’ll also learn what the black wires are for). Use transistors to switch something. Learn how to program with an Arduino. Spot welding and assembling LiIon batteries is just not the right step, the welder (shortly) uses more power that your entire household, and LiIon batteries are extremely dangerous when handled improperly. Please watch this video, the only mistake that this guy made was to charge them inappropriately:

    • John says:

      I took a multimeter and I’m just glad I didn’t connect those two together. I’m currently going through my old text book and brushing up on circuits. Thanks for the heads up.

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