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The Poor Man's Battery Tab Welder

Update 05 May 2009 21:23 BST:

I've fixed (another) mistake on the schematic. Seems I got the normally-open and normally-closed contacts on the relay mixed up, and forgot a small decoupling capacitor.

The missing capacitor probably won't do much harm, but if you've built one of these welders and it didn't work... try swapping the leads to the NO and NC relay contacts.

Click here for the latest version of the schematic

Lastly, people keep emailing me to ask where the relay coil is. "RLA1" (the relay) is split up into two parts -- the contacts and the coil. The contacts are at the top of the page near the capacitor array, and the coil is the rectangular box with a diagonal line through it near the diode and the power switching transistors.

As for the SCR, the 50RIA20 is getting hard to find. Any decently-powerful SCR with a surge current rating of 1000 amps or greater should be suitable. Something like that should cost around US $30 or so, brand new.

Car-audio capacitors are hit and miss as to quality -- if you can, use the computer-grade parts. I don't know where to get these cheaply, my original supplier had a warehouse fire a few years ago and all their stock was destroyed. I do NOT have any capacitors or other parts for sale.

If you're going to try a car-audio capacitor, measure it on a capacitance meter before using it (ideally before buying it). I've heard horror stories of some of these capacitors being made up of low-capacitance parts inside a cheap metal shell, or even being an empty shell with a few screw terminals on top...

A few other people asked me about the voltage regulators. IC1 is an STMicroelectronics (aka SGS-Thomson) L200C. It's available in two versions:

  • The L200CH sits flat on a circuit board, i.e. horizontally. This version is quite hard to find (assuming it hasn't been discontinued outright).
  • The L200CV, which stands off the board vertically. This version is usually quite easy to find.

If you can, go with the L200CV. It's far easier to find, and far easier to solder. That's not saying much, but you can bend the leads quite easily with a screwdriver to make one of these devices fit on ordinary 0.1" matrix stripboard.

I used the L200C because it has a programmable current limiter. In my circuit, it's set to the full 1.25A. Basically, the power supply I used to run the welder had a 1.5A output fuse. After a few welds, I got sick of replacing fuses.

IC2 is a standard 7805 regulator. These are made by a ton of companies, but the Fairchild Semiconductor LM7805 is probably the easiest to find. Alternative names for this part include:

So it's not exactly hard to find... :)

Update 08Jul2007 13:10 BST:

I've just fixed a mistake on the schematic, and made it a little more concise. Q2 is supposed to be a BC547 (NPN bipolar transistor, TO-92 package) - Q1 (BC557, PNP bipolar transistor, TO-92 package) was correct. Oops.

Also, for clarification I've marked the relay's pins - NC is the normally closed contact, NO is the normally-open contact and COM is the common contact. A few people seem to be getting a little confused due to my use of UK style schematic symbols instead of the more widely known US-ANSI and DIN symbols. Apologies for any confusion...

Update 17Apr2007 16:12 BST:

I've heard from a few people who have managed to build welders based on the one-farad car-audio capacitors, after removing the blinking-lights display circuitry from the top of them. I guess the car-audio caps aren't as bad as I thought.

Seems you could probably build one of these welders for about �65 if you got a car-audio capacitor from eBay and didn't mind spending a couple of hours desoldering the blinkenlites.

Update 29Nov2006 23:24 GMT:

Here's the schematic for the current version of the welder. One SCR, a pair of transistors, two voltage regulators, a relay, a handful of resistors, a foot-switch and a handful of resistors. Nothing too expensive. Oh, excepting (of course) the capacitor bank.

Update 07Sep2005 18:04 BST:

For those who have been asking, no that photo isn't one of the welder in its current form. The current welder switches on the low side (SCR cathode wired to ground), and has the SCR directly mounted onto the middle of the bus bar.

The reason the current shot up when I moved the SCR to the low side is because the current in the SCR gate has to find a path to ground to trigger the SCR. In the original welder, that current had to go through a metre of welding cable, a piece of nickel and two electrodes. Now it only has to go through a lump of thick copper. A lower resistance path to ground equals a sharper turn-on pulse, which means more current goes into the weld. Thanks to Ben Weaver (bjw105) for pointing this out.

Over the past few years, I've built up a few battery packs for myself and for other people. Most of them worked fine - in fact, one of the first packs I built over five years ago is still in service, working fine in a torch in the bottom of my cupboard.

The big problem with soldering to batteries is that you tend to damage the plastic separator, and the cell seals. This - as you might guess - is not a Good Thing™. In some cases, solder can splatter over the cell's pressure relief vent. There's a reason the datasheets make a big fuss about the vent - in an overpressure situation, the vent is used to release the excess pressure in the cell. Needless to say, blocking the vent with solder is never a good plan, unless you're trying to get a Darwin Award, or you happen to enjoy watching your battery pack undergoing rapid, uncontrolled self-disassembly.

In industry, resistance welding is used instead of soldering. Not only are the welded joints smaller than solder blobs, but they cause less damage to the cell. The only problem is the cost of resistance welding equipment. A low-end resistance welding machine can cost upwards of GB �2,000.

The solution

That's all well and good for industry, but when you only want to build up a few battery packs for an R/C car or re-cell your laptop battery, �2,000 is a lot of money. Thankfully there's a type of resistance welder that can be built for under �100 (well under �100 in my case). It's called a capacitance-discharge resistance welder.

A capacitance discharge welder is a relatively simple device - it consists of nothing more than a 600,000µF capacitor, a 1-25V power supply, an SCR (also known as a "thyristor"), a 5V power supply (for the SCR) and a pair of electrodes. The PSU charges the capacitor up to the desired voltage, then the SCR is triggered. When the SCR fires, the charge in the capacitor is dumped into the electrodes. A low-voltage, high current pulse heats up the metal at the contact point, which forms a weld. Simple enough, right?

My welder was built from an International Rectifier 50RIA20 SCR, five 120,000µF computer-grade capacitors, a Farnell L30/BT dual channel power supply, a bit of copper sheet, a lot of wire and a piece of 5mm thick copper rod.

The capacitors are wired in parallel using two pieces of copper sheet as bus strips. The 5mm copper rod was cut in half and ground down to create a pair of electrodes, which are connected to the capacitor array and SCR with thick cable (I think it's actually high-current automotive-grade cable designed for wiring up car batteries). The rest of the wiring is built out of normal 2A hookup wire.

I've included a photo of the welder below. The wiring is very simple - the 25V power supply is wired directly to the capacitors, the SCR anode goes to the capacitor bank's positive terminal, the SCR cathode is wired to one electrode and the other electrode is wired to the ground terminal on the capacitor bank. Finally, both power supplies have their ground terminals connected together, and the SCR's gate is wired to the switched 5V supply. Charge the cap with the 25V supply, then fire the SCR with the switched 5V supply. If none of this makes sense, look at the photo. The capacitors are the huge blue things, the power supply is behind them, and the SCR is the little tin can next to the capacitors. The welding electrodes are the two coppery bits that are shrouded in heatshrink sleeving and connect to the red cables. The croc-clip is there both to short out the capacitors, and also in case I need to weld pieces of sheet metal together.

I did want to fit a footswitch to the welder to make it easier to use. The fact that the local Maplin was out of momentary footswitches and wasn't expecting any more for two weeks kinda put a damper on that plan though...

Using the welder

Charge the capacitor - experiment with the voltage, start low and work up; every welder is a little different. Remember that energy goes up by the square of the voltage. Put a piece of nickel (0.003" thick) on the battery terminal and use one of the electrodes to hold it down. Hold the second electrode on the nickel on the other side of the battery cap (about 1mm away from the plastic wrapping) and push firmly. Fire the welder, then lift the electrodes off the nickel. Recharge the capacitor, then repeat the process - this should leave you with four weld spots. A battery assembly jig may be useful, if you can find (or make) one.


The welder seems to work better with the SCR wired in a low-side configuration. That is to say, the cathode connected to the -VE line on the capacitor bank, one electrode wired to the SCR anode and the other wired to the +VE line on the capacitor bank. I have absolutely no idea why this is the case - can anyone shed some light on this?
EDIT: this question has been answered - see above...

Hints, tips and warnings

Before doing actual welding of, say, a battery, do a few tests. Start at a low voltage, then work up in 0.5V increments. Do a 'pull test' to see if a weld is satisfactory. This involves pulling the nickel strip off the battery quite roughly. If nuggets of nickel get left on the base metal, then the weld is good.

If the joint appears burned around the location of the electrodes, then the capacitors are charged to too high a voltage. If the metal you tried to weld gets blown apart, the voltage is WAY too high...

If at all possible, buy a pack of alkaline batteries and use those for the voltage tests before welding your uber-expensive nickel-hydride or lithium ion (shudder) packs. A lithium fire is not easy to deal with, especially when the other components of the lithium cell liberate oxygen when heated. Alkaline batteries are, by comparison, pretty inert.

To avoid overheating the cells, it's a good idea to flux and tin one end of the tab you intend to solder to before welding it onto the battery. This will reduce the amount of heat the cells are exposed to (less heat is required to solder to pre-tinned metal).

When you've finished building your battery pack (and you've tested it), get some shrink-wrap and shrink it over the battery pack. This is covered in more detail in the Hobby Spot Welders battery welding tutorial (see links below).

Photo of the prototype

The welder

Click the image to enlarge it.

Parts list and possible sources

5x 120,000uF 25V capacitors
Sourced from Mainline Surplus Sales (who appear to have ceased trading after a major fire at their warehouse), part number 01-0552. Mine were branded "Cornell Dubilier", not Mallory. You'll have to source some 10-32 UNF machine screws and anti-shake washers. The length of the bolt should be between 0.358 and 0.435 inches. You could replace these with a single 680,000uF (0.68F) capacitor - I used five smaller caps because they were cheaper than one brand-new large cap.
Copper sheet - K&S #259 0.025" x 4" x 10"
Available from basically anywhere that stocks K&S sheet metal. Model shops, hobby shops and so forth. Used for the metal bus bars. Copper is used because it has a very low electrical resistance.
International Rectifier 50RIA20 thyristor (SCR)
A nice, beefy SCR. Obtained from Farnell - part number 394-1784. Feel free to substitute with any decent-sized SCR with similar specifications. I picked this one because it was relatively cheap and could handle 1200A surges (10ms maximum length).
Welding electrodes and cable
Home made - one of my friends is on good terms with someone who owns a metal lathe. Of course, if you don't have access to a lathe, it is possible to make the electrodes with a Dremel and a grinding bit. They're 5mm diameter copper rod, formed into a 0.75mm tip. The cable is not actually welding cable - it's 8AWG car wiring cable. You'll also need some heatshrink to cover the back end of the electrodes and the solder joint - any decent electronics supply house should have it. The electrodes I used are soldered into copper pipe, then the wire is soldered to the inside of the pipe - it makes it easier to make a clean-looking join, but it needs a lot of heat. This is well into 50W soldering iron territory - a 25W won't heat the copper up enough to melt the solder.
Nickel sheet
The only item I couldn't get from a supplier on the UK mainland. Ordered this from McMaster-Carr in the USA - it's order code 9707K33 0.003" nickel sheet. Cost about £40. No, you can't have any of mine.


Hobby Spot Welders
The inspiration for this project (I'm loathe to pay $600 for something that's cased up in a RadioShack project box). Expensive, but neat.
HSW battery welding tutorial
How to weld batteries together - a quick tutorial.
HSW example welds page
This page gives some examples of what good welds should look like.
Cheap nickel shim stock - you want Ni200/Ni201 nickel, 0.003", McMaster-Carr part number 9707K33. Ideal for battery tags and straps, as long as you don't mind cutting it down (use a pair of scissors).
NOTE: I've been informed that McMaster-Carr are no longer processing export (i.e. non-US) orders - can anyone confirm this?