Soldering with a spot welder...

Given the attempts to find a way to get copper strip to work well as a DIY-able battery connect it got me wondering the question:

What would happen if you place a wee blob of solder paste between cell and copper and then zapped with the spot welder?

An initial google throws up someone who tried for other reasons with Nickel and had promising results: https://electricbike.com/forum/forum/main-forum/diy-discussion/37105-homemade-spot-welder-issues-turned-into-soldering-device

If this were viable then it could potentially offer a strong set of advantages - short amount of heat on the cell but with wide contact area, but most of all it would potentially allow welding of much thicker copper strip as the melting point of solder is so far below the melting point needed for a spot weld, even with the thermal conductivity of copper it would seem to me that there would be enough heat generation to flash melt the solder.

Thoughts?

The positive nipple of the 18650 cells is fairly robust and has a couple layers of components before you reach the "jelly roll". So, the positive end can take some abuse. The negative end is sensitive to heat, so that is the weak link in any connection plan.

Do you think that it would be significantly different to spot welding nickel direct? I know even this isn't 'ideal' - but by my thinking if you used solder under copper strip and hit it with a spot welder - if the solder flowed and made a good join then it's superior contact with a metal that has a higher thermal conductivity would actually subject the cell to less total heat.

The solder paste will explode and the tab won't bond. Have a go yourself, but wear safety glasses.

jonescg said:

The solder paste will explode and the tab won't bond. Have a go yourself, but wear safety glasses.

Click to expand...

Well damn.... what if you reduced the joules to a level that was melting the solder but not flash frying the flux?

The problem might be time, I'm not sure we want the solder to have enought time to flow, that means more heat

It's an interesting idea and not a million miles from resistance brazing I've seen done on some motors with a piece of braze foil.
The physics of heating the solder will differ quite a bit from welding:

The metal to metal contact of copper to nickel-coated steel would be reduced, so potentially there may be less heat generated, due to less current flow. or it might explod
The heat may not be concentrated in the solder - getting something that has the right resistance to heat as nickel does might help a thin solder paste covered piece of nickel might work

The temperature needed to melt solder is less with typical melt temperatures of around 185 Deg C (365F) this helps

The base metal doesn't melt with soldering - I suspect this doesn' help as there won't be a local spot with the molten bit being squished to the sides and carrying impurities (oxide layer) away.

The problem might be that the flux has to get rid of the oxide layer on the metal and then, after that happens it must be displaced by the solder as it melts and the voids fill with capillary action