DIY Spot Welder

#014 DIY Spot Welder

To safely connect cylindrical cell batteries into large packs (see right) one needs a spot welder to weld nickel strips to connect the batteries. After buying one that didn't work, I decided to make my own. Others should be able to use this as a guide.
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My semi finished test pack for my car. I later added the cell voltage wiring and replaced the copper tape with nickel for the p connections.

Step 1: Do your research

I am sure you are here because you were shocked by the price of a spot welder and thought "I can make that". I was inspired to do so after a comment on my reddit post recommended a Youtube video. Whatever the reason for your need, the basic premise is as follows: You use a car relay to short a 12 volt car battery, sending hopefully hundreds of amps through your nickel strip. You should use some sort of timer to turn the car relay on and off precisely. I bought a timer attached to a relay, but an arduino with a relay would work great as well. (SRD-05VDC-SL-C is a good one, and the one embedded in my timer)
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Step 2: Purchase the parts

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Here's what you need:
  • Car Starter Relay
  • Timer Relay*
  • Copper Electrodes**
  • Low Gauge Wire
  • Push Button (any button will work, not a toggle switch though.)
  • Car Battery (I don't have one to recommend, but just make sure to use a lead acid.)
* I have seen some designs without the timer relay. I would highly recommend including it. The car relay, although heavy and clunky, turns on and off with reasonable precision when combined with the timer relay. The timer is (in my opinion) the EASIEST way to get reliable welds.
** I originally used part of a copper bus bar and used some tin snips to shape it into a point. This did not work consistently. I ended up looting the electrodes from the previous failed spot welder purchase. I would either buy copper nails or purchase a pair of electrodes from Amazon. (That one is just an example, probably cheaper options out there.)

Step 3: Put it together already!

Follow the wiring diagram and put it all together. Do what you need to regarding the timer; depending on the CCA (cold cranking amperage) of your battery you can adjust the timing. Start low (50 ms is when the car relay actually had enough time to make contact for me)
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