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Cell welder advice?
(lemmy.ml)
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this post was submitted on 07 Apr 2026
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A bit delayed, but I also built my own with a starter solenoid and a used car audio capacitor. I use a bench power supply to change the capacitor and made a diy foot switch to trip the starter solenoid.
This approach is pretty cheap, very repeatable, and easy to control.
Changing the bench supply's voltage lets you dial in the amount of energy delivered per weld. Just keep in mind that energy stored = 1/2CV^2^ so don't jump from say 5 volts straight to 10 volts thinking that you're doubling the power - that's a 4x increase.
Wow I didn't know about car audio capacitors. What capacitance did you use? What voltage, typically? They are on the expensive side, but impressive. I bet they could also jump start car engines. I'm not keen on using a lead-acid battery so capacitors seem like an ok alternative.
I'm using a 1 farad Rockford fosgate capacitor that I paid $31.51 for on fleebay about 5 years ago. One of the threads on the unit I bought was goobered, but I just drilled and tapped it for the next size up and all has been fine ever since.
This unit looks very similar to the one I purchased. Ideally look for an older unit that's purely passive. I see a number of cheaper used options available from other manufacturers, but I am not in a position to be able to recommend any of them.
Using a capacitor seems like it would be cheaper than using a lead acid battery unless you already have a spare battery on hand. As an added bonus, capacitors are pretty shelf stable so you won't have to worry about caring for it when you're not using it.
The voltage needed for spot welding depends on the thickness of your nickel strip, the thickness of whatever you're welding to, and the overall resistance of your spot welding setup. Somewhere between 7 and 8 volts was enough for what I was doing. Start low and work up, you don't want to blow a hole in a battery cell.
I'm not sure that a capacitor could jump start a car. That takes a stupid amount of current to get the motor turning over, something like 200 amps iirc. Then you need to keep the engine turning long enough to fire. A 1 farad cap charged to 14.4 volts stores 103.68 joules of energy. That sounds like a lot, but when you convert it to watt hours it's only 0.029 watt-hours. So unlikely to start a car, but it sure can deliver a metric ton of current for a very short duration without having to worry about the longevity of the cap.
Thanks! There is a Torque Test channel video where they tested Milwaukee and Ryobi jump starters and found they delivered around 4WH and 3WH of energy respectively. They use a tool battery to charge a capacitor, then use the capacitor to jump start the motor. I thought I had seen a teardown of the Ryobi once, that showed the capacitance, but I don't seem to have saved a link. But let's see, E= 4WH = 14.4kJ = 1/2CV^2 where let's say V is initially 14.14 volts so 1/2 C V^2 = 1/2 C 200 = 100C = 14400J. So C = 144 farads, can that be right? They are supercapacitors after all, and the jump starter is a fairly large box of them.
Here's the jump starter test vid: https://www.youtube.com/watch?v=z4N5txe7oZA
This claims to be a 100 farad 13.5V capacitor bank (5x 500 farad 2.7V in series): https://www.aliexpress.us/item/3256804444528644.html
The jump starters are a fair amount bigger than that bank.