DIY Battery Spot Welder: Build Your Own Power Pack Assembler

Ever wanted to build custom battery packs for electric bikes, solar power banks, or RC projects but found commercial spot welders too expensive? I've got the perfect solution! In this Instructable, I'll show you how to build a powerful yet affordable battery spot welder from scratch. This isn't just another simple project—it's a professional-grade tool that uses the classic NE555 timer and parallel MOSFETs to deliver precise, controlled welding pulses.

What makes this design special is its intelligent balance between simplicity and functionality. While commercial welders can cost hundreds of dollars, this DIY version achieves similar results at a fraction of the price. The secret lies in the carefully designed circuit that transforms a standard 12V battery into a controlled high-current welding machine. Whether you're assembling a power wall for home energy storage or building custom packs for your electric skateboard, this spot welder will become an indispensable tool in your workshop.

I've personally used this welder to assemble multiple battery packs, and I can vouch for its reliability and effectiveness. The design focuses on safety first, with controlled pulse timing that prevents battery damage and ensures consistent welds every time. Let's dive in and build something amazing!

Supplies

You'll need electronic components, basic tools, and some raw materials for this project. For the main circuit, gather an NE555 timer IC, six IRLB4132 MOSFETs, several resistors (10kΩ and 220Ω values), capacitors (100nF and 100μF), a 5mm LED, a momentary push button, and a piece of prototype PCB or a custom-designed board. You'll also need a 12V battery—a 7Ah sealed lead-acid battery works well to start, though you can upgrade to higher-capacity batteries later.

For the welding probes, source two pieces of 4mm solid copper rod, thick insulated copper cables (at least 12 AWG), heat shrink tubing in various sizes, and a small plastic enclosure or handle material. Basic tools include a soldering iron with solder, wire cutters, a multimeter, a drill with bits, and a screwdriver set. Optional but helpful are a PCB holder, helping hands, and safety equipment like gloves and safety glasses.

Schematic and PCB Design

Begin by creating the circuit schematic. The heart of our spot welder is the NE555 timer configured in monostable mode. This generates a precise 10ms pulse when triggered. Connect the NE555 following standard monostable configuration: pin 1 to ground, pin 8 to VCC, with a 10kΩ resistor between pins 7 and 8, and a 100nF capacitor from pin 6 to ground. Connect pin 2 (trigger) through a 10kΩ pull-up resistor to VCC and to one side of your push button the other button side goes to ground.

The output from pin 3 drives the gate of our MOSFET array. Since a single MOSFET can't handle the hundreds of amps needed for welding, we'll use six IRLB4132 MOSFETs in parallel. Connect all gates together, all sources together, and all drains together. Add a 220Ω resistor in series with an LED from pin 3 to ground as a pulse indicator. Finally, connect the MOSFET sources to ground and the drains to the positive welding probe terminal.

For the PCB, design wide traces at least 3mm for all high-current paths. Remember that thinner traces will overheat during welding pulses. Arrange components logically with the NE555 and support components on one side and the parallel MOSFETs with ample spacing for heat dissipation on the other. Include clearly labeled terminals for battery input, welding probes, and trigger switch. Once satisfied, export your design as Gerber files for manufacturing. I used JLCPCB, where all solder mask colors cost the same, so pick your favorite!

Circuit Assembly

With your PCB in hand, start assembling components from smallest to largest. First, solder the resistors and capacitors. Use an IC socket for the NE555—this protects the chip from soldering heat and allows easy replacement if needed. Next, install the LED, ensuring the longer positive lead goes toward the current-limiting resistor.

Now for the critical part: the MOSFETs. Apply a small amount of thermal paste to the back of each MOSFET before mounting them to the PCB. This helps with heat dissipation during operation. Solder each MOSFET carefully, ensuring no solder bridges between pins. The parallel configuration only works if all connections are clean and secure.

Install the terminal blocks for battery input, positive probe, and negative probe. These connections will carry high current, so use generous amounts of solder to create solid joints. Finally, solder header pins for the trigger switch connection.

Double-check all solder joints with a magnifying glass, looking for cold joints or bridges. Use your multimeter in continuity mode to verify there are no shorts between power and ground before proceeding.

Probes Assembly

The welding probes are what make contact with your nickel strips, so they need to be robust and well-insulated. Start with two 4mm copper rods, each about 15cm long. File one end of each rod to a slightly rounded point this concentrates the welding current for better results. The other ends will connect to your cables.

For the positive probe, solder a thick insulated cable (12 AWG or thicker) directly to the copper rod. Use plenty of solder to create a low-resistance connection. Cover this joint with multiple layers of heat shrink tubing for safety and durability. The positive probe will remain "hot" during operation, so ensure no exposed metal remains.

For the negative probe, we'll integrate the trigger switch for safe two-hand operation. Solder another thick cable to the second copper rod. About 10cm from the rod, connect your momentary push button in series with the cable. Mount the button to a comfortable handle, I used a piece of PVC pipe ensuring the button falls naturally under your thumb. Cover all solder joints with heat shrink tubing, creating a professional-looking insulated handle. This design ensures you must use both hands to weld, keeping fingers away from the welding area.

Final Test and Results

Before connecting any battery, perform a final safety check. Verify all components are correctly oriented, no solder bridges exist, and insulation on probes is complete. Connect your multimeter in resistance mode across the battery terminals you should see high resistance (MOSFETs off) until you press the trigger button, at which point the resistance should drop dramatically.

Now connect your 12V battery positive to the battery terminal, negative to ground. When you press the trigger button, the LED should flash briefly. This indicates the NE555 is generating a pulse and the MOSFETs are switching. Without touching the probes together, this is a safe no-load test.

For the real test, prepare a scrap piece of nickel strip on an old battery or metal surface. Press the probe tips firmly against the nickel strip and press the trigger. You should see and hear a quick spark, and the nickel should fuse to the surface. Adjust pressure and timing as needed different nickel thicknesses require slight technique adjustments.

Your spot welder is now ready for serious work! I've used mine to assemble multiple 18650 packs with excellent results. The 10ms pulse creates perfect welds without damaging battery cells. Remember to let your battery recover between welds, and consider upgrading to a higher-capacity battery or capacitor bank if you plan extensive welding sessions. Share your builds in the comments below I'd love to see what battery packs you create with your new DIY spot welder!