You need a spot-welder, large soldering iron, small soldering iron, solder, multi-meter, cells, nickel strips, insulating washers, insulation sheet, hot glue gun and glue sticks, 13g silicone wire for the power leads, thinner wire for the charge socket, heat-shrink sleeve in various sizes, hot air gun to shrink it, switch, BMS, charge socket and charger.
It isn't just a case of welding the strips onto the cells. You have to figure out settings and how to weld through more than one layer. Slotted nickel is best, but if you can't get that in the right size or shape, you need a Dremmel and carbide disc to slot the nickel yourself.
When you've done all that, you'll need a case to put it in. A proprietary case is relatively expensive. A home made one can look terrible if you don't have the skills to produce something nice. Plywood will need gluing, sanding, sealing and painting. GRP will require wooden patterns and all the tools to make them, release agents and the GRP stuff.
While you're making the battery, you cut your hands on the nickel and you scare yourself to death every time you drop something metal, like a piece of nickel strip, on the half-built battery and sparks start flying and the strip goes red hot so you can't get it off, then you have to figure out how to unweld all the damaged strips, then you need a single cell charger to get the drained cells back to the same voltage as the other ones before you can weld them up again. Or when you've nearly finished the battery, an edge of one of the nickel strips nicks through the heat-shrink cell covering and shorts it out. The whole thing starts smoking while you try to figure out what to do with it.
This is what I mean by "learning experience". It's not so bad when you've made a few; however, I guarantee that you'll learn some lessons along the way.
Compare all that with a professionally finished battery that comes by post with the charger, and all you have to do is screw it to the frame.
