I’m wondering if there is a decent affordable cell welder. Purpose is rebuilding tool batteries and that sort of thing on an occasional basis. K-weld has been recommended but is pretty expensive and might be overkill. https://diy500amp.com/ has a bunch closer to my price range but might be underkill. Any thoughts? Thx
I made my own out of a motorcycle battery (which I have lying around in abundance), a length of jumper cable, a starter relay off of I think a 50cc scooter with a random salvaged microswitch as the trigger, and two short lengths of sharpened 10 gauge Romex as the probes. It works a treat. I haven’t fried a single 18650 with it yet.
I think my total net bill of materials was about $12, not including the junk I already had lying around, the majority of which was the relay.
Interesting. I don’t have a motorcycle battery around but I do have one of those lithium portable jump starters. I wonder if that could work.
In theory, yes. But those usually have electronics in them in an attempt to save you from yourself. So it’s unlikely that the thing would fire when you wanted it to without being fooled into believing it was hooked up to a 12v battery in some way.
You can get off brand motorcycle batteries online for like $20. The quality is irrelevant for this application, especially since you probably won’t be welding hundreds of battery tabs in a day. The point is that they can reliably deliver enough amperage to weld the tabs while having a low enough terminal voltage to be unlikely to deal either yourself or your cells a mischief.
Fair enough. Jump starters often have an override button so they’ll still operate when the car battery is at 0V, but IDK how it works.
It’s worth a try.
Maybe later I’ll post a picture of my gimcrack contraption.
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 = 1/2CV2 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.
