@ heating cells in video there, its possible.
a good weld on nickel has little discolouration but is still strong.(just from observation, im not much of an expert as say, nobuo or okashira to name a few...
with welding of copper, i just thought id share some insights/observations here.
the dynamics in the process of welding copper is quite different from that of metals with higher resistance like nickel.
because it conducts the weld current so well, and also conducts any heat away so fast, it requires much more current to get it to heat to the point where it melts and fuses to the cell.
this makes it very difficult to weld, as it requires a huge amount of current in a very short pulse, and any factors such as bad contact or dirty surface could blow the arse of a cell to bits!
my solution to this was is to use tungsten rod as the electrode tip,($3 from any good welding store (from tig electode) drilling a hole in the end of the copper dn10 electrode tips(as pictured earlier) and inserting some~15mm long piece of tungsten with a press fit into each electrode tip.
this has a very high melting point, quite high resistance and is very hard.
the result is that the tungsten remains hard but glows cherry red for an instant as the weld pulse travels through it, producing the heat that creates the weld.
because the copper is so good at conducting the heat away, the area where the weld is is barely warm to the touch afterwards. ( although the welding pens get warm after a while) im absolutely confident that it doesnt get hot enough to damage the cells, while creating a very high current connection.
i usually only do one weld in concession on the neg pole especially to avoid overheating a single cell to much.
as you may see from the pics above, i took great care to minimise any path for air or moisture to get into the finished pack, so as to avoid any potential corrosion.
the other consideration is that 0.1mm copper is not particularly mechanically strong, even though the welds are good, so the pack must be very soild build with spacers or hotglue, but i would not like a nickel pack with just the strips holding the pack together either.
also, i did not solder anything to the cells but soldered to the copper before welding it on, to avoid soaking heat into them.
iv built 2 of these 22s12p packs now,for 2 separate bikes, and can vouch that they can produce some savage power