I have used braided copper in a ton of packs. It’s awesome for low profile situations. don’t use flux, it will betray you. Trick is to use a very hot iron to tin the copper braid on the very ends. You have to do it fast, almost just barely tapping the flux core solder onto the braid with the iron. Then you put a bead on the nickel, as wide as the copper. Then you lay the lightly tinned copper on top of the bead, hold it in place with a wide flathead screwdriver to sink some of the heat off the nickel and keep your hands from burning (and it also seems to slow down the wicking) and put the iron on the bead and let it melt down and as soon as the copper drops into the wet bead you pull the iron off of it. It’s all about time, you don’t give it time to wick.
For my braided connections, i put a bit of FR4 under the nickel and push down the tinned braid with a piece of aluminium flat bar before applying heat.
I use a solder pot, liquid flux and hemostats to tin the braid.
I always solder after welding. I put the FR4 between the nickel and the sides of the cell to stop the heat getting pushed down onto the sides of the cell.
