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DIY battery spot welder: 12 V battery, MOSFET control board, 8 AWG probes and nickel strip

Bench build · Everyday gadgets · Intermediate · about 40 minutes

Welds nickel strip onto 18650, 21700 and 32700 cells to build battery packs. A spot welder is a huge, short pulse: a 12 V battery pushes several hundred amps for about 12 ms through the probes, and the strip melts in two small spots under them before heat can reach the cell. The control board is a bank of MOSFETs and a timer; in auto mode it fires 0.4 s after both probes touch the strip. Everything in the loop counts: the battery’s internal resistance, the 2 AWG battery leads, the 8 AWG probe cables and the MOSFETs, against a strip that is only about a milliohm. Too short a pulse and the strip peels off; too long and it blows a hole. Start short on scrap and work up.

DIY battery spot welder: 12 V battery, MOSFET control board, 8 AWG probes and nickel strip: the finished build running on the simulator's bench

Build it in the labGuided build, test sheet and fault finding, free in your browser.

zencircuitlab.com/l/diy-battery-spot-welder

Parts list

Test procedure

Each test has an expected value and a pass range, like a factory test sheet:

  1. 12 ms pulse: peak current
  2. 12 ms on 0.15 mm nickel: a good weld
  3. 3 ms: too short, the strip peels off
  4. 40 ms on 0.1 mm: blown through

How it works

What it does: Welds nickel strip onto 18650, 21700 and 32700 cells to build battery packs.

Build it (guided or ready-made), then run its test procedure below. Every test has an expected value and a pass range, just like a factory test sheet.

Background

Kits like this made DIY battery packs common: a car battery and a ₱1,000 board replace a ₱15,000 capacitor-discharge welder.

More builds: everyday gadgets