The biggest problem with assembling custom battery packs has always been that the stainless steel terminals and cases of cells are very difficult to solder; there are special solders and fluxes available, but they typically also require higher heat, which has greater potential to damage the cells; there has been a lot of work devoted to developing inexpensive methods to spot weld the batteries together, but I think I've come across a reasonably easy procedure that allows you to use regular old rosin-flux solder to attach wires to a stainless battery quickly and with minimal risk of heat damage. I don't recall reading about anyone else using this technique, so I thought I'd share it..
What's the trick? Very simple, I copper plate the terminals first.. What you'll need is a bottle of copper plating solution from the craft/hobby store (basically just a solution of copper sulfate) and some fine sandpaper for metal. First pour out a bit of the solution and get a cotton swab ready, then gently sand each of the battery terminals, and quickly apply the plating solution to the freshly abraded stainless steel.. Some battery bodies may have some additional plating or some other surface treatment applied, so if it doesn't work right away, sand down some more and try again: once you get past the surface treatment and apply the solution, the steel surface will instantly turn a copper color.. Do this to every battery and then heat up your soldering iron and once it is good and hot, you can quickly tin each terminal- you can go very quickly and the solder will wet out the copper plated surface instantly so there should be no issues with heat damage (you can quickly quench each cell with a damp rag if you're worried, but all the cells I did were just barely warm to the touch just seconds after tinning). Once this is done, mount the batteries in whatever jig you want and cut out wires or braid or whatever you choose to use to connect the batteries together, then tin this stuff, and lay it down on the battery, and quickly touch your iron to it- it takes less than a second per joint and it is well wetted out and not prone to failure like most attempts to solder onto bare stainless steel..
I haven't applied this method to e-bike batteries yet, but I recently rebuilt a couple of dead cordless drill battery packs with new cells and the most tedious part of the job was all the sanding of the terminals, but once the battery ends were copper plated, the soldering was incredibly fast and easy, and the reassembled battery packs were better than new (higher capacity cells than the originals)..
What's the trick? Very simple, I copper plate the terminals first.. What you'll need is a bottle of copper plating solution from the craft/hobby store (basically just a solution of copper sulfate) and some fine sandpaper for metal. First pour out a bit of the solution and get a cotton swab ready, then gently sand each of the battery terminals, and quickly apply the plating solution to the freshly abraded stainless steel.. Some battery bodies may have some additional plating or some other surface treatment applied, so if it doesn't work right away, sand down some more and try again: once you get past the surface treatment and apply the solution, the steel surface will instantly turn a copper color.. Do this to every battery and then heat up your soldering iron and once it is good and hot, you can quickly tin each terminal- you can go very quickly and the solder will wet out the copper plated surface instantly so there should be no issues with heat damage (you can quickly quench each cell with a damp rag if you're worried, but all the cells I did were just barely warm to the touch just seconds after tinning). Once this is done, mount the batteries in whatever jig you want and cut out wires or braid or whatever you choose to use to connect the batteries together, then tin this stuff, and lay it down on the battery, and quickly touch your iron to it- it takes less than a second per joint and it is well wetted out and not prone to failure like most attempts to solder onto bare stainless steel..
I haven't applied this method to e-bike batteries yet, but I recently rebuilt a couple of dead cordless drill battery packs with new cells and the most tedious part of the job was all the sanding of the terminals, but once the battery ends were copper plated, the soldering was incredibly fast and easy, and the reassembled battery packs were better than new (higher capacity cells than the originals)..
I plead guilty to killing another A123 M1 cell on Pack A with a 100 watt soldering iron. It's my first time killing a cell with heat. 
