I would like to add several things to take into consideration. The entire bottom and sides of an 18650 cell are the negative electrode. Only the "button" tip of the positive end is the positive elctrode. there is a small area of insulation between them on the positive end. The sides and even a small portion of the negative electrode that is at the edge of the positive end, is typically covered in a plastic sleeve, upon which the name and type is usually labeled.
Due to vibration over time, a short can develop across the positive central "button" electrode, and the circumferential negative "shell" that is very close to it. For this reason, many pack builders add some type of additional insulation. In the pic below, the 4-cell Paralleled string has a tan-colored paper insulator. Individual silicone "washers" are also available, at very affordable prices.
In the pic below, the cell on the right has an additional "washer" style of insulator on the positive end.
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The next item is adding a space between the places where the "two-probe" spot welder will create the welds. Due the variability of the thickness of the nickel strips, and also the resistance of one strip to the next, some of the spot-welding current will travel
through the strip itself, rather than only through the places where the strip touches the exposed end of the cells. This is why production spot-welded packs have a short and thin "split" between one side and the other. This air-gap forces all of the spot-welding current to travel from one side, through the end of the cell, and then up through the other side. Doing it this way can create a more-consistent weld.
This slit can be added to plain nickel strip material with a dremel tool, using a thin abrasive disk.
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Individual cell-fusing:
If one cell in the middle of a pack begins to short out and become very hot, very rapidly...having the connection between the cell and the rest of the pack formed as individual fuses can (in some cases) stop a runaway short from affecting the rest of the cells. The pack might still be ruined, but having only one cell overheat,
rather than all of them....is a much more desirable situation.
The pic below is a design of a laser-cut nickel strip that connects one end of a collection of 12 cells, to form a 2S / 6P sub-group. The six tabs on the left side have the central split (described above), and those would be spot-welded onto the negative ends. The six tabs on the right side not only have a central split, they also have the connection from the center out to the rest of the cells formed with four thin "S" shaped strands. These are the fuses. All four strands would get very hot during a shorted cell, and...as soon as one of them melted completely through, the current flowing through fewer fuses would cause an immediate cascading failure of the remaining three strands. The fuse "strands" are to be located on the postive "button"
How about a discussion on what is the best way to connect the cells together in series and parallel? Most batteries I see that people make themselves have strips of Nickel welded to make the parallel connections and then more strips of Nickel to make the series connections. I remember dmun brought up a different way to make those connections that takes a lot less Nickel strips and despite my ignorance on the matter, it just makes sense to me. Make the series connections with small lengths of Nickel strips and then the parallel connections on the strips with a small gauge wire soldered on. Solder heavy gauge wire on the Nickel strips for the positive and negative poles of the battery. Below I included a couple drawings of a 4S14P battery, first the all Nickel strip battery I often see and second, the way dmun suggested many times. Sorry for the bad choice of colors representing some materials.
