Spot welding method qustions
I'm going to be building my own spot welder and after watching countless YouTube DIY videos on the subject I have some questions. And let me say that I'd like to see if I can't come up with a way to use copper, either solid or braided, for the cell to bus bar if possible and certainly for the bus bars themselves. None of the videos seem to give a definitive voltage and/or amperage at the welding electrodes that works best. And for this let's assume pure nickel strips which will give me a starting point. But if anyone has these parameters for copper to nickel or copper to the steel used in 18650 cells, I'd love to hear them. I realize that these parameters will likely be higher for copper since spot welding relies on the resistance in the metal to produce the heat needed and of course copper has significantly less resistance. I will likely use tungsten electrodes as copper electrodes would be just as likely to melt as the copper material being welded. Possibly even carbon electrodes. And I'm going to attempt to design and build a wand type electrode holder rather that the more typical stationary type that you press the cells into. But I plan on making the electrodes spring loaded into the wand to hopefully eliminate the problem I've noticed in several videos of unequal pressure of the two electrodes when triggering the weld. Any information regarding pulse duration would be appreciated as well.
The second question involves the pros and cons of having both electrodes make contact with strip as it lies on top of the cell versus one electrode contacting the cell (- or + end) and the second electrode contacting the strip as it lies on top of the cell. I can see theoretical arguments for either method, but have only seen a small number of videos showing it done the second way. But in those rare ones, I've never seen a case where it was easy to separate the strip from the cell. And I've certainly seen several video examples where the first method resulted in a poor weld allowing easy separation of the welded pieces. I think the possible reason for this is that, in the first case, if both electrode points aren't being pushed solidly at the two contact points, the current may only be passed through the strip itself and the cell metal won't be heated sufficiently to create the required bond. Whereas the second method better insures that the strip is in contact with the cell metal, because if it weren't, no welding current would flow at all.
Keep your leads as short as possible because feedback can damage your controller when the magnetic field collapses. Some sort of snubber circuit should prevent this. Two turns of super flexible 4ga like this one https://www.amazon.com/gp/product/B01KP374TQ/ref=oh_aui_detailpage_o05_s00?ie=UTF8&th=1 through a large microwave transformer secondary will provide the correct power for welding. Three turns will provide more.You could use an arduino based controller. Micah (forum and company owner) is an electrical engineer I believe and could provide much better advise.