Starting on the battery packs now. I picked up some old laptop batteries from a local computer store for free and practiced soldering nickel strips to those. I’m very glad I practiced and I would highly recommend it if you haven’t done this before.
I’m soldering 3 pure nickel strips between batteries, each 8mm x .15mm (that’s about 5/16" x .006"). It’s pretty beefy and should handle high current nicely. My practice demonstrated that soldering one strip at a time is the way to do it. I wait a bit between iterations so the batteries don’t get too warm (i.e., I solder one each on all the connections, then cut more strips, etc., then do another iteration).
I’m aware of the warnings against soldering, but I have never seen any tests demonstrating any ill effects.
I sanded the terminals before starting and I used plenty of flux. I think I’m getting very good flow. Also, as you can see, I added insulators to the positive terminals. I experimented using hot glue between the batteries, but determined it didn’t really add much strength. I’ll just be gentle with the packs until I have the heat shrink in place.
I was going to use 10 gauge wire but decided that it would be difficult to make some of the connectors with wire that big, so I’m going to use 12 gauge. I think that’ll handle 20 amps continuous, so it should be fine. The batteries I’m using (Sony VTC6) will put out 30 amps up to 80° C (so temporarily) and 15 amps continuously. I doubt that I’ll ever have the guts to drive the board that hard.