Dual pulse battery spot welder

I recently bought 20 Sanyo NCR18650GA Lithium batteries. 12 of them will be used for developing a BMS for a upcoming E-bike project. From the other cells I will put together 2 battery packs for my bicycle lights, 4 cells each in 2s2p configuration.

This type of cells are usually spot welded together using thin nickel strips. I’ve read that many have been successful soldering the cells together but how fun is that when you can build a DIY battery spot welder.

Most designs that I have found is based on a SSR (Solid State Relay) controlled MOT (Microwave Oven Transformer) with the secondary replaced by few turns of heavy wire which converts mains voltage to high welding current. This doesn’t seem like a good solution since the current comes in 100 Hz pulses which makes welding energy control very difficult. I know some SSR can only break the current at the zero-crossing-point of the mains voltage resulting in a pulse time resolution of 10 ms. I don’t think this is good enough to get a consistent result.

There are also variants which discharge a large capacitor bank through a MOSFET which seems like it would give a much higher degree of control. I also found a similar design using a car starter battery instead of capacitors which seemed even more interesting.

I don’t have any spare car batteries, instead I’ll use some high power LiPo batteries. I have a pack of 4 Turnigy 6S 20C 5Ah batteris that I could connect in parallell. This will result in a 6s4p 20C 20Ah pack capable of delivering  ~20 V 400 A continuous. It will not have any problem delivering enough current for battery tab welding in short ~10 ms pulses.

The main focus of the build was to use as much parts from my junk-bin as possible.

Electrodes

The electrodes are built om 10 mm copper rods sharpened in one end and threaded with an M10 thread in the other. On the threaded end a 25 mm² welding cable are connected. To set off the welding pulse I have placed a small button on the top of one electrode.

Electrode
Left hand electrode, a sharpened 10 mm copper rod
Trigger
Right hand electrode with trigger placed for thumb activation

Electronics

To switch the current I found six FDP8440 MOSFETs rated at 40 V with a very low RDSon of 2.2 mΩ. If the welding current reach 1200 A they will handle 200 A each resulting in ~90W losses. This will easily be handled for a few hundredths of a second every 10 s or so. Especially since they are mounted on a thick copper busbar.

The control circuit semi-temporarily built on a Veroboard
The control circuit semi-temporarily built on a Veroboard

The mosfets will be controlled by a Microchip TC4421 from a 8-bit PIC microcontroller. Haven’t used a 8-bit PIC in ages, nowdays i prefer ARM Cortex-M processors, mainly the STM32 and LPC series. Since I’m building this on a veroboard I needed a DIP-casing so I decided to use an ancient PIC16F648A from the junkbin. This processor was perfect for a quick job like this, extremely simple keeping the datasheet reeding to a minimum

Spot welder schematic
Spot welder schematic

It has an internal 4 MHz 1% oscillator which will work fine for this application since there are no asynchronous communication, and 1% precision of the pulse timing is more than enough, no need for an external crystal oscillator. The reset-pin can be turned off with the config bits and all I/O-pins on Port B have internal pull-ups saving me a few resistors.

To program the processor I used Microchip MPLAB X IDE, XC8 compiler, and a PICkit 2 programmer. I’ve never used MPLAB X before, but it worked rather well. This is the application firmware, perhaps a little bit overdone, but why not?

The pulse length is currently hard coded in the application, it’s easy enough to update. If needed i will later add a switch with a few presets. The microcontroller has no A/D-converter, otherwise a potentiometer for setting this would have been nice.

A 6 S LiPo used as a power source discharging in a 3,3 ohm power resistor.
A 6 S LiPo used as a power source discharging in a 3,3 ohm power resistor.

There is a lot to read about pulse welding online, essentially the first pulse is to break any oxide layers and the second performs most of the welding. I have set pulse 2 to 10 ms, and pulse 1 to 1,25 ms which works fine for 8 x 0,18 mm Nickel strips.

I haven’t received the nickel strips I ordered yet, so my first test subject was some pieces of box cutter blades. It worked perfectly, but perhaps I need to shorten the pulses a little with the much thinner nickel strips.

Top side of welded knife blades
Top side of welded knife blades
Bottom side of welded knife blades
Bottom side of welded knife blades

Future plans

I will use this to build a few batteries, if the design works well, I’ll probably make a new PCB with a display, rotary encoder and a more modern processor.

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