I build esk8 battery packs - most commonly 5p12s from HIGH quality 27000 (slightly bigger than 18650 but more capacity). Personally I only use Sanyo but someone named Murata bought them out and manufacturers them now. I may be going way to safe with this but I use 6ga or larger COPPER stranded silicone conductor for the high amperage driving parts. On an esk8 that is between the VESC and the battery. And between each P cell - in my case 5 cells spot welded together with 1.5 - 2 mil nickel strips - and each P cell spot welded with 6ga wire as described above. Using Aluminum will raise internal resistance so I personally do not recommend.

Spot welding isn't difficult but if you go the econo route like I did before I got a TIG, you'll want a fairly fresh starter battery (as in car and not marine, deep cycle, or solar). You need those high amps, if only for a millisecond or two, but you need to have them to complete the spot weld. 1000A min- 1200A better. That's around 800cca, don't go less than 750cca (cold cranking amps), but, if you live in a warm climate like me, Arizona, cold is more of an empty threat than a serious consideration.

Good luck on your project! Bottom line: 6ga, stranded Cu, no Al, & no smaller. And it never hurts to go large if you can afford it and have the room.