When assembling a battery pack from scratch you can essentially do one of two things:

Solder the cells together
Spot weld the cells together

The first option is only practiced by makers at home, and there's a reason! In fact, the heat introduced into the cells by the soldering iron is enough to permanently damage the cells. For real. Not only that, but excessive overheating can induce a thermal runaway, likely followed by the explosion of the cell.

With that said, soldering is still used because a good soldering iron is cheaper than a cheap spot welder, and spot welding is also a finer art.

I will explain how to spot weld, how spot welding works,
common errors and their solution. If you want to see how I built the spot welder shown in the photos, follow me on Instructables and YouTube, as the build video and article are coming out soon.

Step 1: How to Spot Weld

From a purely operational point of view, the sequence of actions is as follows:

Put the strip over the surface to which you want to weld it
Place the strip in contact with the tips of the welder, applying (the right amount of) pressure
Press pedal (spot welders are usually triggered by pedal)
Check results. You usually want to apply a bit of force on the weld just to be sure it really worked

"The right amount of pressure" depends on your specific machine, what you are welding and on what you are welding it. There's really no way to know before, you'll have to proceed experimentally.

The same is true for pulse duration and number. Usually machines with large capacitors can get away with very short pulses. In my specific case I'm using 5 pulses of 75ms each, with a 15ms delay between each other.

The only way to determine the correct parameters is by testing a number of combinations. I suggest you start from 3 pulses of around 30ms. If nothing happens, increase the duration. When the pulse becames too long you'll notice, because you'll find holes intead of welding points. Now you can reduce pulse duration and increase number of pulses. Clearly you do all this until it starts welding correctly, you want to find the combination with minimum pulses and shortest duration.

You'll have to do this (ideallyI) for every new combination of materials that you weld, but once you find a combination that works, the other sweet spots are usually not that far away. As always, experience is key.

Step 2: What Just Happened

When you press the pedal, a series of short current pulses are produced between the two welding tips, as shown in the oscilloscope. Note that this dataset was taken with the tips isolated from each other, so that's the maximum voltage amplitude that my machine can produce. As you can see, it's a modest 4V. Is it too low? Not at all. Just consider that you'll be short circuiting the transformer over a strip of metal with an electrical resistance of virtually zero, so the current will be very high, no matter what's the applied potential.

The discharge current will split between the two metals that you are welding and only the current that goes from the strip to the substrate will be effective to the welding. In fact, the two materials will weld only in the points just below the tips.

Step 3: Advantages Over Soldering

The most obvious advantage is the little heat transfered to the cell. It's true that when spot welding you are melting the two metals together, but the energy transfer is so quick and so localized that the overall temperature increase in the cell in extremely small. This might seem not very important, but Lithium cells are not cheap and you want them to last. Heating a cell over 60/70°C will reduce it's overall capacity (Ah) forever, while a temperature of over 100°C is likely to make it explode (seriously, check on YouTube). When soldering the iron has a temperature of at least 220°C. Not good.

Another not so obvious advantage is that the union made with spot welding has way lower electrical resistance than soldering, making it ideal for high current applications. Spot welded connections are also more mechanically resistant, so they are safer to use in battery packs.

Welding Nickel strips cells allows you to reduce the volume of the system, if compare to soldering, which is way bulkier.

Step 4: Common Errors

The most common errors are:

Pulses too short: the metals don't reach the melting point. This usually results in the top strip being marked but the bottom one is left untouched. If you already reached the longest reasonable discharge, then probably the machine is not powerful enough for the materials you are trying to weld.
Pulses too long: overheating of the material and likely perforation. The metal is becoming liquid and usually flyes right into your face in the form of sparks. Wear eye protection!
Pressure too low: if the materials are not in strict contact when the discharge happens, the current will remain confined in the top strip, thus not welding anyting.
Pressure too high: the tips of the spot welder will move away the metal when it melts, making it thinner and reducing its ability to conduct current. If the pressure is extremely high or the strip is very thin, it will likely get punctured.
Dishomogeneous pressure on the tips: you'll usually see sparks when welding. Wear eye protection!

Step 5: How My Spot Welder Is Made

It's a really simple electrical circuit. The main element is the modified microwave transformer, in which I replaced the secondary coil with a few turns of a very thick wire. This reduces the mains voltage to a few volts (2.2V RMS according to my crappy multimeter) but is capable of delivering a very high current.

The transformer in switched on and off by an Arduino. Clearly you can't connect the Arduino directly to the transformer, but you have to use an SSR (Solid State Relay). The SSR allows you to control the full power of the transformer without any additional electronics.

The duration and number of pulses is set through the two dials (potentiomenters) positioned on the front panel.

Here you can download the Arduino script. For detailed instructions on how to build one, including the laser cut files, follow me here and on YouTube, both the article and video will come out in just a couple of days!

Spot_welder.ino
Download

Step 6: Send Comments and Suggestions!

I'm very interested to know you take on the subject, with observations, comments, suggestions, photos and also corrections! Please write a comment and let me know what you think, either here or in the comments section of the video.

In case you are just beginning to play with battery cells, let me link to you a video (and article) I made on the subject:

Arcticle -> https://www.instructables.com/id/How-to-Size-Your...