Simple Battery Spot Welder




Introduction: Simple Battery Spot Welder

About: I'm a tinkerer and a DIYer. If it can be made rather than bought then I'll always have a go. A lot of the things I make simply can't be bought anyway!

As part of an electric bike project I needed to weld 70 18650 Li-Ion cells together. I could have just bought a battery welder but where's the fun in that? This is my build. Nothing complicated and it didn't need to last too long.

Step 1: Parts List and Tools Needed



  • Screwdrivers
  • Drill
  • Soldering iron (60W+)
  • Solder

Step 2: Wiring Diagram

This is the circuit we're going to make.

The timer relay can be set to be closed for 0.15s when the button is pushed. This in turn fires the starter solenoid, which allows current to flow down the electrodes, creating the weld.

We could do without the timer relay and just control the weld by varying the time we press the button for but it gives a lot more control.

Step 3: A Solid Base

We don't want the battery floating around the workbench so we need to construct a solid base for it.

Take some 3/4" MDF board and cut it to be 4" wider than the battery and about 4 times as long to give a good welding area in front of the machine.

Cut some 1" x 2" wood and screw it to the board to form the area where the battery will sit.

Step 4: Mount for the Button

Add some more 1" x 2" on the top of the enclosure we just made and cut a hole in it the same diameter as our firing button.

Step 5: Mount the Button

Mount your button in the hole we just made and feed the wires out of the bottom.

Step 6: Test Your Circuit

Wire everything together as shown in the previous wiring diagram and test fire it, making sure that both the relay and the solenoid fire as expected.

Step 7: Mount the Solenoid

We don't want the solenoid floating around so take some of the 10mm x 2mm steel bar and drill 2 holes in it. This will serve as both the positive contact and a solid mount.

Step 8: Electrodes

Take two of the copper nails and cut the heads off them. These will be inserted in to the choc-block to form the electrodes.

Step 9: Main Wires

Cut down some of the copper cable and mount some choc-block on the end. Insert the copper nails in to the other end and bend the wires until the electrodes are vertical and the wires are horizontal.

Screw one wire to the -ve terminal of the battery and the other to the output of the starter solenoid.

Step 10: Test

Grab a battery you don't care about and test it.

You will need to experiment with how much pressure to apply to the electrodes but with a little practice you will be able to do a consistent weld.

I used this welder to build the battery for my electric bicycle:

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    3 years ago

    I built my own and was just wondering about the relay it turns off after like 1.5s which is too much time and I’ve changed out the jumpers to the shortest time and turned the knob as far as it could go I was just wondering if you or anyone else was having this issue as well?


    Reply 1 year ago

    Depending on which kind of timer you have. I have a couple of "Baomain DC 24V H3Y-2 Time Delay Relay Solid State Timer 60S DPDT w Socket" and I believe I can pop them open and change the RC time constant by altering the timing Capacitor to something 50 time smaller and then it would become a 0 sec to 1.2 sec timer instead of the 0 to 60 sec timer.
    Another work around is to measure how much 12V current is needed to hold the starter solenoid closed, then select a resistor (of the correct wattage) to limit the current. For example if the coil needs 200 mA, the R would be 12/0.2 = 60 ohms with a power rating of 2*W = 2*(12*0.2) = 4.8 watts or more. Easy way to do that is six 10 ohm 5 watt ceramic resistors in series. Of course if you can find a 60 ohm 5 watt then just use that. The next task would be to experiment (or calculate) to determine what size Cap. (minimum rating 25 V ) will get to the point of 66% filled in the target time of 0.1 sec (estimate) when being filled at a 200 mA rate. The thing to bear in mind is that the holding current for the coil is LESS than the current required to close the contacts. As the Cap. fills the current is dropping and quickly falls below the holding current and the starter switch pops open - spot weld completed, hopefully. Everything is in series on the control side PLUS a discharge path (a diode and large R [12V/50mA=240 ohms minimum] in series with each other and Parallel to the Cap.) is needed for the Cap. so it can reset and be ready for then next button push. SEE THE LINKS BELOW
    Might be fun and profitable to use a modern LED for the drain diode on the Cap. That way you know the spot welding system is ready when the LED fades to dark. It is a Visual feedback.


    Question 2 years ago on Introduction

    I bought the same relay you are using, but it came with no instructions. I need to know the jumper settings you used (S1,S2,S4,S5). Thanks!

    Mike Albuquerque NM


    Question 2 years ago on Step 2

    In the pics you have the solenoid attached to the positive battery terminal but in the diagram you have the negative going through the solenoid. Please explain.


    Question 2 years ago on Step 10

    About how many welds do you get from your battery? Any issues recharging the battery?


    2 years ago

    Did similar using arduino as timer - not sure about timings, set at 0.15s, but is there a delay due to solenoid action? And are the electrodes critical, just using thick copper wire for mine, maybe there is a better choice and what about shape of ends?
    Cheers JP

    Global montage
    Global montage

    2 years ago

    At our blacksmith's workshop it is probably not quite enough, our welder is secured with 32 Apm and we sew 2 x 1.5 mm steel plates together for garden fence posts
    note :The picture shows how our poles look when they are put up in the garden welding company the company we are makeing the garden fence posts to :)
    opsæt havehegn i komposit og stål.jpg

    2 years ago

    Very nice. I am going to save myself the $100 for sunko unit and build this. Will use an arduino to control the 5v relay instead of the time delay unit. It should come in at well under $50. And, of course, is much much more fun. Beat the system, yeah!