I just wanted to talk a bit about a design for a capacitor discharge battery pack spot welder that I’m working on. This project is motivated by the large pile of defunct Dewalt 18 volt battery packs that dad’s construction business generates. You can have these packs “professionally” rebuilt for around $35, and they last about half as long as the factory packs. This is not too good considering you can buy new packs for about $50. Also, the cost of the individual cells is low, somewhere around $0.75 each if you buy in quantity. Since the packs are a group of 12 cells welded together in series, one can rebuild a pack for about $9! That’s not too bad of a deal.

There is a problem with rebuilding these battery packs however. Joining the cells with a thin nickel strip is impossible without a spot welder. You might say “What about soldering the pack together?” Well, heat from the soldering process would significantly decrease the life of the cells, if it didn’t kill them outright. “So just buy a spot welder!” Not going to happen. I can justify spending a few hundred dollars on components to build a spot welder, but not the $1200 or so it would take to buy one. “So break out a soldering iron and an oscilloscope and build one!” OK, I think that is doable but its going to be a complex process. This will be the most complex electronics project I have undertaken to date.

The design that I will describe here is derived mainly from these three sources:

• http://www.pittnerovi.com/jiri/hobby/electronics/welder/
• http://www.pocketmagic.net/2012/07/capacitor-discharge-microspot-welder-cutter/#120208
• http://www.turtlesarehere.com/html/cd_welder.html

The heart of the design is a bank of four 330,000 µF 16 V electrolytic capacitors connected in parallel. This should give an equivalent capacitance of 1.32 F and should hopefully lower the capacitors ESR to a usable level for this application. The capacitors shown here were sourced from eBay and are made by Nippon Chemi-Con. The data sheet for these capacitors does not list an ESR but lists a ripple current. Ripple current really is not useful to us, and I do not have an ESR meter, so we will have to evaluate the capacitors in the testing phase.

All of that stored energy has to be delivered to the weld joints with some sort of flexible cable, for which I will cut up an old welding lead I have lying around. I am currently leaning on TIG welder tungsten electrodes for the actual contact tips, but that is currently up in the air. Copper buss bars will connect the capacitors together and will connect to the cable and switching silicone.

Switching this much power is quite tricky and has led to failures in other people’s projects. Simple spot welder designs use a SCR to switch the load, which is certainly durable enough to handle the abuse, but latches in the conducting state until the voltage drops low. A design like this relies on adjusting the capacitor bank voltage to tune the weld. This is OK but does not provide much fine control. More advanced designs use MOSFETs to switch the current on and off in short pulses. This allows for tuning the welder more accurately and allows a “pre-heat” pulse to clean the weld site before the main welding pulse. This is the design I will attempt to use.

The MOSFETs I chose for this design are International Rectifier IRFB7437. This is a fairly beefy device with a low on resistance and a fairly low gate capacitance. I plan on using six of these devices in parallel to provide some safety factor.

I will drive these with a dedicated low side driver IC under microcontroller control. The MOSFETs can be seen in the picture to the left in the tube. I purchased extra in case I destroy parts in testing. Also in the photo are the protection diodes and TVSs I plan on using to protect the capacitor bank, and a 4×20 LCD display to provide a user interface for the microcontroller. I plan on driving this project with a 40 pin PIC micro.

To provide power for the project I plan on using a 1u server power supply. This simplifies things since the PSU provides almost all of the voltages this project requires. It will also allow me to incorporate a  standby feature that may be useful.

I know this is not much in the way of a formal design, but I am back-of-the-napkin designing this project. I did run a few simulations to reassure myself that the expected currents would probably not destroy the MOSFETs. Now all I need to do is build up and test the circuitry and write the control code. I’ll keep you posted.