quick dirty capacitor spot welder?

[coppercone2]

I have 2x 2000F/2.7V super caps, is this enough to make a spot welder for those standard lithium cells in laptop battery?

Capacitors shorted into the circuit by a timer through some kind of reactor?

what should my wave forms look like?

esr is 0.5 miliohms per cap

[coppercone2]

Is there a peak current limit that the super capacitor needs to be designed with, or is it OK to short them out and rely on ESR?

[coppercone2]

in short, how do i de-rate this stuff?

I figure I can control voltage to control the weld with fidelity rather then using a pass switch and just short it out with something like a copper rod I bang into a plate

I think so long the surface is good clean and degreased with its fixtured there is no reason that more advanced control then capacitor voltage is necessary?

[cur8xgo]

in short, how do i de-rate this stuff?

I figure I can control voltage to control the weld with fidelity rather then using a pass switch and just short it out with something like a copper rod I bang into a plate

I think so long the surface is good clean and degreased with its fixtured there is no reason that more advanced control then capacitor voltage is necessary?

Okay heres what I know. I'm answering because nobody else has, although I don't know how much this will help.

I needed a quick and dirt li-ion tab spot welder, years ago. I found this site and built it accordingly. It worked great for hundreds and hundreds of tabs.

https://www.philpem.me.uk/elec/welder/

That said, it was only 0.6F total. Yours would be 4000F?

I think the esr of my caps is probably about 10m ohm and I used it at around 10V?

If your cap ESRs are 20 times less and your capacitance is 6000 times more, I think there may be an issue here with too much energy going to the weld, since our end-applications were the same.

Here I'll take a stab at making it all equal:

Energy mine dissipated at 10V = 0.5 * 0.6F * 10^2 = 30J

To get your 4000F to the same energy..   30J / (0.5 * 4000) = voltage ^2 = 0.122V ?

And maybe we can just say the ESR doesn't matter because the time scale is very short for the weld to take place and the weld resistance dominates anyway (hand waving here)

.

[cur8xgo]

You know..on second thought. I think you probably can't just set your caps to 0.122V and get the same energy because the resistance of the weld isn't going to be so low that the ESR of the caps dominate. You may need to use smaller caps (to limit the total energy) at a higher voltage (to drive enough current into the weld for I^2R heating).

[coppercone2]

Ok, do you know if the caps can get hurt?

I don't mind it being a unit for thicker welds (psuedo bus bar) but I just am scared that the capacitors will experience destructive forces and should be current limited.

[MosherIV]

  Ok, do you know if the caps can get hurt?

Probably yes. Super caps are not designed for rapid discharge, they mainly for supplying voltage over a short period, ie battery backup.
You are better building a bank of capacitors. Cheap high values ones will work. Do not worry about esr, just get as much F as you can get.
Anything above 12V will work, the higher the voltage, the higher the current pulse!

[MarkF]

Here is a DIY one:   https://www.instructables.com/id/Capacitive-Discharge-CD-Welder-for-battery-tabs/

[james_s]

I built one I've been using for years. I used a bank of 22,000uF capacitors adding up to about 1F and a variable 24V power supply built around a LM317. For triggering I used the biggest stud mount SCR I could find for a few dollars on ebay. Total cost to build the thing was under $100 and it works great.

[cur8xgo]

Is there a peak current limit that the super capacitor needs to be designed with, or is it OK to short them out and rely on ESR?

Have you looked at the datasheet? Is this the one you are using?

https://www.mouser.com/datasheet/2/257/Maxwell_K2Series_DS_1015370-4-1179730.pdf

Maximum current = 680A

Short circuit current = 7700A (Current possible with short circuit from rated voltage. Do not use as an operating current. Just listed for information.)

I don't know if maximum current = peak pulse current. They talk about a non - repetitive peak current in their product guide but the word "peak" doesn't appear in the datasheet.