I have been interested in getting one of the little spot welders I have seen online for a while, I have one made from a microwave transformer, but I designed that for thin sheet metal, it is a bit too unwieldily and overpowered for battery tab and wire welding.
I finally saw one of the little handheld battery spot welders for less than $20 from the usual marketplaces. And by battery spot welder I mean it is for welding batteries, and runs on one. These are simple devices, they have a single large low ESR LIPO battery, similar to the kind used for battery jump starters. These cells can very briefly provide the huge current needed for spot welding a small item. Other than that all that is on these is a small board with a big MOSFET and a microcontroller.
You can set the welding power (duty cycle) and toggle whether or not the the welder will automatically trigger when it detects the circuit is closed. This is a nice feature absent in my homemade sport welder.
Overall this is great value for such a tool; it works well welding battery tabs. I am not sure it is entirely safe; it is clearly stress that cell very hard, but nothing with the name “welder” in it is entirely safe, and if you are spot welding batteries you should be ready for a single cell to catch fire.
I want to be able to spot weld thin wire for another project. This requires a jaw mechanism that can pinch the wire between the electrodes, not the roughly parallel needles that make contact with the top of the battery tabs that are built into this unit.
So I bought a set of small spot welding jaws, it is amazing what you can just buy for very cheap.
However, it didn’t work, it could not get any power into the work piece. The thing with the lithium battery spot welder is that the voltage is the ~4v, as one would expect. This makes it unable to cope with much resistance, the larger spot welders these jaws are meant for operate at slightly higher voltages.
In the end the simplest solution was the best for what I needed; just use screw terminal and solid core housing wiring.
It is easy to take off and on, took a few minutes to make and works well, with the caveat that you need to wear gloves when using it since the copper will get warm. It is very easy to spot weld thin wire together like resistor leads. The wire is flexible enough to also act as the a spring, so nothing else is required. If I had to do a ton of welds I might want something neater. But this is perfectly serviceable, sometimes it is best not to overthink things.