Spot welding is very much a manufacturing process, and in Britain we seem to have largely given u making things. However, try a local car body repair specialist Phil, they may have the spot welding gear.
You can do it yourself easily if you have a usable car battery and a pair of goggles to protect your eyes just in case of any splatter.
Connect a couple of heavy cables to the charged car battery, old jump leads will do. Then clip the tag onto the end of the negative cable or pinch the tag against the cable end with pliers using one hand to make a good connection.
Now hold the positive end against the edge of the end plate of the cell you want to connect to and touch the negative connected tag accurately into place. It will instantly flash and weld into place so just as instantly lift away your positive connection to stop the current flow.
Since the connections are both on the same end plate no current will flow through the cell. Make sure you don't cross connect the battery cables together while welding of course.
You can practice this first on the end of an old tin can of course to perfect your technique and see if you are confident enough to do it on the cells.
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