DIY Super Cap Battery Spot Welder - Real Deal or Fake Clickbait? (4K)

[tsbrownie]

There are lots of videos on the internet about making a battery spot welder using only a 500 F super capacitor and some copper electrodes. Is it real or is is clickbait?

WARNING: Welding is dangerous and carries the risk of burns, fire, shock, electrocution, etc. Welding batteries carries additional risks of toxic chemicals and explosion. If you do not know, do NOT do it!

[DIYer]

Thanks tsbrownie! We've added your Mini Spot Welder to our Welding category,
as well as to your builder page: tsbrownie's Homemade Tools. Your receipt:

Mini Spot Welder by tsbrownie

tags: spot welder

[Claudio HG]

I wonder how much current is required to weld that thin seet of metal, is it 100A, 1000A ? I have a bunch of capacitor laying around that I would like to put on service for a mini spot welder one day.

[tsbrownie]

I wonder how much current is required to weld that thin seet of metal, is it 100A, 1000A ? I have a bunch of capacitor laying around that I would like to put on service for a mini spot welder one day.

It has been a long time since I learned about this in shop class, but if I recall correctly there's a lot going on. Total amps are important, but amps per square area (in the welded spot) is more important to get the metals to melt in that spot. A bigger spot / tips requires more amps total (and it increases as pi * r squared). That's the basics for welding stuff like cars, tin boxes, etc. When you start welding batteries and power carrying connections there's another consideration that's very important. The total area of those "spots" would have to be at least equal to the cross sectional area of the other conductors or the spots would heat / burn.
I tried making the spot smaller, and got a couple of welds that "took", BUT the spots were so small that in moving current they would not pass 1 amp without burning, let alone 10-20 amps. That's why in the video I said something about needing 12 welds per battery end to get an effective (power carrying) connection.
I'm sure it can be done / is done and can produce a good spot weld, just not like the many videos on the internet with one 500 farad cap.

[Claudio HG]

Thank you mate. Yes cross section is really important, in fact I assumed the amperage related to a given section.
I do agree that many videos present dubious results.
But 500 farad is a pretty large capacitance. I am gonna think the capacitance could introduce some delay to the flow of current due to internal inductance, or (and this could be even more likely a critical point) the fact the circuit is closed directly on the electrodes that could cause some loss in current over time because the contact is instantaneous on a very tiny spot.
Let me elaborate better the point.
When you touch the metal to weld, one very tiny spot at the electrode tips (due to irregularities of the tip surface) enters in contact causing the majority of the current to flow through it. This melts the metal and weld it, but in a very tiny spot. Fraction of seconds later (you are still pressing the electrodes against the metal to weld) the contact is enlarged and the current can flow even through this area. However the quantity of charges stored into the capacitor is no longer the same as the one that was there at the very first instant of contact, thus the current is much lower, causing some metal to melt and some other to simply heat up but definitely not to melt.
This is because the following law: Q = C * V
where C is the capacitance, in farad; V is the voltage; and Q is the quantity of charge which in turns is given by the following: Q = I * t
where I is the current, in amps; and t is the time. Remark: Q is measured in coloumb, symbol C (not to be confused with capacitance).
Therefore the previous one can be rewritten as follows: I = C * V / t
As you can see the current is inversely proportional to the time, as the time goes the current become less and less intense.
So suppose your first contact is on a surface (cross sectional area) of say 0.2 sqmm, at the very first instant for a length of time of say 1 millisecond.
Now I do not remember in the video the voltage you used to charge the capacitor nor the quantity of charge, so suppose it is 50V and charged at 1A for 600s (10 minutes) or 600C (coloumb), this gives us the following:
I = 600C / 0.001 = 600000A (!!)
That current wouldn't flow of course because of resistivity from both inside of the capacitor, the wires, electrodes and -the highest- the welding point.
Say the resistivity is in the order of 0.01 ohm, so the current would be limited by the voltage drop: 50V/1e-2 ohm = 5000A
After this first burst the remainder charge would be 595C; for the following 10 milliseconds you enlarge the point of contact by suppose 1 sqmm for 100 ms; assuming the voltage and resistivity is not changed (it is pointless to be too precise as this is just to give an idea of what could happen) an other 500C of charge are eaten up so the remaining charge would be 600 - 5 - 500 = 95C.
When you finally establish a large enough spot the amount of charge is reduced by 84%, so when the contact area reach suppose 19 sqmm (from a round electrode of 5 mm in diameter) the current would be 95C * 50V / 1s = 4750A which should provide a good weld on that 19 sqmm but because the surface is enlarged heat can move easier away from the spot reducing the ability to melt the metal.
And your capacitor now is completely discharged.
As I said all these numbers are to give just an idea of what could happen, I neglected to consider the drop in voltage at the capacitor after each instant the contact is made larger and larger while you press the electrodes against the metal to weld, the variation in resistivity due to heat, and parasite inductances.
Maybe inserting a switch circuit using some MOSFETs in parallel that could be turned on when you already have established a good contact or maybe a mechanism that provide a twitch to the movement of the electrodes, could provide an improvement.

[tsbrownie]

Yes, that all makes sense and matches what I see. There is an initial very hot spot, then a larger spot where there's no welding (but lots of deformation and discoloration). I mentioned that smaller points might make a difference, but then current through those really small spots would burn them. Of course LOTS of small points would compensate, but at some point it's just not practical. Thanks for the great info!