Field battery boxes, long-range packs and goggle batteries for FPV quads are all 21700 NMC inside; if you can build your own you save money for other parts. Tools: 21700 cells, a cell holder, nickel strip, copper strip, heat-shrink and a spot welder — a soldering iron can hardly get nickel strip to hold, and poor temperature control raises the internal resistance. The creator uses an AWithZ machine that welds nickel strip or 0.15mm copper strip with ease, max gear 99; the nickel + copper combination of about 0.3mm is usually around gear 25. First stick fish paper on the cell to insulate the outer rim; when welding, one pen presses the strip down while the other welds; on the other side, weld + and − separately — never weld them together; solder the balance lead and power lead to + and − (red positive, black negative); wrap in fibre tape, add flame-retardant foam, slide on heat-shrink, and a goggle battery is done. Reminder: lithium batteries are flammable and explosive, so take extra care while building.