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
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
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
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
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
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
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.
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
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
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
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,
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
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?