Need help rigging up a spot welder for some 18650s

[VoltJesus]

Hello everyone. First post here so take it easy on me, k? 
I do not have the money to buy a tab welder, or buy parts to build one. Trying to use some of the stuff I have to get me by for now.

Right to the point, I'll list the specifics...
I am needing to rig up a spot welder for an 18650 pack(s) I am building. Tab material is pure nickel strip 0.2mm x 8mm wide. I'm expecting ~2000 welds total between the 4 identical packs I am building. All welds do not have to be done at the same time. I can wait if things need to cool down every so many welds.

To keep from making a wall-o-text, I made a quick graphic to explain what materials I am working with. (please see attachments)


In that mockup, the torodial on the left is 120v to 12v. Primary wire is 12awg, secondary is either 8awg or 10awg. Big enough hopefully?
The rectifiers are rated for 50amp cont., 400amp peak. I've got about 10 of them. Should be more than enough if run in parallel.
The momentary switch on the bottom left is quite beefy and pretty fast. I think a quick actuation will be in the 5ms to 20ms range.


What I would like to know is if this will work at all. There has to be something I am overlooking. Will there be an inductance spike within the wiring on the toroidial that will destroy the windings? Will it go up in a ball of flames and glory?
I also have that capacitor seen in the other photo attached. It has a bunch of switches that let you change the value. Could add that in the circuit to give it more punch.


If all else fails, I've got that bank of 4 heavy duty 12v solenoids that could be used with a car battery. However, I know it is incredibly unreliable and hard to control.

[Ian.M]

Doesn't look very practical.  You probably need a capacitive discharge system (using a bank of pulse discharge rated capacitors) so you can accurately control the energy per spot.  Remember, if you get *ANYTHING* wrong, the cell is likely to detonate.

[VoltJesus]

Understood. I know its not practical, but I'm trying to make do with what I got.
For better understanding, I have a background in HVAC. Needless to say, I've got a ton of old contactors and capacitors I can put together to get the BANG! needed.
I am most worried about the load the transformers will undergo. Will there be an inductive current feedback in the windings, causing them to contract violently and possibly short against the core?

There will be many many test welds before I actually attempt to use them on the cells. Figure welding 2 pieces of the battery tab material together a few hundred times will prove if it will be reliable. Just wanted to know if it could work at all before I destroy my gear. 
Also, I expect the rectifiers to take the brunt of the kickback, since they are the last component before the welding leads. If they burn out or blow up, not a big deal. Super cheap and I got plenty.

[Ian.M]

When operating, the rectifier and transformer are effectively operating into a dead short.  Typical welding currents are in the hundreds of amps range and 400A is probably on the low side.  Rectifier peak ratings are typically only for one half cycle at line frequency, and without a pulse length control you'll exceed that, the rectifier will fail shorted first, and may take out the transformer by fusing the primary.  Also its going to be a PITA running back to the mains supply panel to reset the breaker after every weld.

Why not build an AC spot welder based on a Microwave oven transformer?  However instead of having the usual arrangement of two separate electrodes with the workpiece clamped between them, for tabbing, you'd want a split electrode with the two halves insulated by a mica sheet, and firmly bolted together with insulating sleeves on each bolt and an insulating washer under each bolt head.

You'll also want precision timer control of the AC switching.  The best option is to use a heavy duty SSR rated for inductive loads with zero crossing switching, controlled by a 555 timer for simple single pulse welding, or by an Arduno if you want a more complex dual or multiple pulse.

Due to the risks of battery tabbing, you'll want a low flammability clamping system for the cell or pack, and a clear polycarbonate blast shield.   I wouldn't care to manually handle the pack during the welding operation without heavy leather and kevlar jacket and gauntlets, and a helmet with a full face impact resistant visor.

[jpanhalt]

I built both modified MOT welder and CD welder.   The MOT (microwave oven transformer, 1000 to 1200 W) was controlled with a timer and zero-crossing switch so I could regulate the actual number of cycles at 60 Hz.  Secondary was 6-1/2 wraps of #4 welding cable.  OCV was 6V at 120VAC input.  Input voltage was controlled with a Variac.  I used various weld times and settled on 170 ms per pulse for testing.

The CD was simply a bank of caps controlled by a PIC MCU.  It was a modified version of this: http://www.philpem.me.uk/elec/welder/

Pictures of the finished unit as well as the prototype that was used for evaluation are attached.  The prototype was very close to the philpem design.

After testing both welders, I decided to use the CD welder for battery tabs and built the finished unit shown.   The main reason to reject the MOT was that it produced a much larger heat affected zone.  I could not get a good weld without being able to see the metal turn red.

An example of some test CD spot welds is also attached.   Those welds could not be separated without tearing the bottom sheet.   Note that the heat affected zone is very small.

[VoltJesus]

When operating, the rectifier and transformer are effectively operating into a dead short.  Typical welding currents are in the hundreds of amps range and 400A is probably on the low side.

Are you referring to the rating of a single rectifier? Could I not hook up 3 of them in parallel to get 1200A peak capability? I've got about 10 of them.

Why not build an AC spot welder based on a Microwave oven transformer?

I've been hunting for one to use. The microwave I have now has a puny transformer that I do not think would work well. My torodial is nearly twice the size.
If I can ever find an older microwave to take apart, that is what I had planned on using. Time is a factor however, and I've been looking for one for almost a year now.

Oh also... I have not seen anyone on the internet talk about using a portable 12v battery charger. Surely the transformer in these are bigger than come in a microwave.
 I have 2 of them, but I really do not want to tear them apart and rewind them just for this spot welder. The units I have are 250A start capability, are older, and very heavy.

You'll also want precision timer control of the AC switching.  The best option is to use a heavy duty SSR rated for inductive loads with zero crossing switching, controlled by a 555 timer for simple single pulse welding, or by an Arduno if you want a more complex dual or multiple pulse.

Thank you for mentioning this. I actually have some questions regarding a setup like what you mention.
Why is Ardrino the go to for these sorts of things? To be honest, I am a complete noob with programming. Can a Rpi Zero not be programmed to do the same operation that the Ardrino could? I  bought a Rpi Zero W+ to poke around with just last week.
For the 555 timer, I am sure I have one somewhere in all the scrap boards I've collected that I could salvage.
I was hoping that I could use my momentary switch to manually provide the pulses. Probably a bad idea... LoL

[VoltJesus]

The MOT (microwave oven transformer, 1000 to 1200 W) was controlled with a timer and zero-crossing switch so I could regulate the actual number of cycles at 60 Hz.  Secondary was 6-1/2 wraps of #4 welding cable.  OCV was 6V at 120VAC input.  Input voltage was controlled with a Variac.  I used various weld times and settled on 170 ms per pulse for testing.

After testing both welders, I decided to use the CD welder for battery tabs and built the finished unit shown.   The main reason to reject the MOT was that it produced a much larger heat affected zone.  I could not get a good weld without being able to see the metal turn red.

This is why I was going to use the first transformer to step the voltage up. Amps produce heat, so I figured a little higher voltage would produce less heat and stress on the components I was using. That may well be a flawed logic on my part though.

An example of some test CD spot welds is also attached.   Those welds could not be separated without tearing the bottom sheet.   Note that the heat affected zone is very small.

What thickness are those strips? And was this the same thickness you were using to test the MOT welder?

[Ian.M]

Paralleling rectifiers doesn't guarantee even current sharing: whichever one is hottest will have a lower Vf than the rest and on the next pulse will hog more of the current, getting even hotter till it blows.  You'd have to add ballast resistors in series with each bridge to keep that from happening, which would limit the energy you could put into the workpiece, and as you don't actually need DC for spot welding its all rather pointless.

If you've got a heavy duty toroidal  transformer (~ 1KVA) with an open center, just wind a few turns of welding cable through it and insulate and ignore its usual secondary.  You can stack lower VA torodial transformers for this - primaries in parallel and a series secondary wound through all of them with fewer turns in proportion to the number of transformers.  Add the primary connections one at a time and check the secondary open circuit voltage increases with each to make sure they are properly phased.

Raspberry PI sucks for hard-realtime stuff.  The LINUX OS keeps on getting in the way, running off to do its own thing like serving interrupts and task-switching.   Arduino is a bare-metal MCU, no OS, just AVR-GCC and some libraries to make low level stuff accessible to non-nerds + a horrible IDE that attempts to hide C++'s need for functions to be declared before you use them, and in the process mangles your code to the point that some C++ constructs wont work if put in the sketch's .ino file but will if put in a separate .cpp file in the sketch folder.

Also Arduinos have analog inputs, so you can easily make a fancy timer/sequencer with pots to adjust pulse duration, and spacing, and a switch for pulse count, and don't have to mess around adding a display or writing a UI.

The battery chargers would be good for a fast recharge capacitive discharge welder - take a monster cap or a bank of caps, put a stainless steel strip ballast resistor between the cap bank and the charger to limit the charging current to the charger's max output, and pulse the discharge with a >1000A MOSFET or IGBT module (as it needs to be able to turn off at the end of the pulse so you cant use a SCR).   As it has to turn off under load, a fast free-wheeling diode that can handle the peak current across the load leads at the supply end would be essential to suppress the kick-back from their self-inductance so it doesn't kill the MOSFET or IGBT module.

[jpanhalt]

An example of some test CD spot welds is also attached.   Those welds could not be separated without tearing the bottom sheet.   Note that the heat affected zone is very small.

What thickness are those strips? And was this the same thickness you were using to test the MOT welder?

Same thickness of strips was used to evaluate both designs.  I got various widths of pure nickle strips in 0.003" and 0.005" thicknesses.   For battery tabs, I use 1/8" wide strips that are usually 0.003".   Sometimes I use the 0.005 thickness.  With CD there is no effective current control per se.   I control the welding with voltage and generally charge to 17 to 18 V.   Too much voltage can cause burn through.   Good contact and reasonable pressure help alleviate burn through.

I also use the welder to fixture thin-wall, SS tubing prior to brazing.

John

[Richard Crowley]

This video shows an ultra low-cost spot welder design that will perform much better than an DC circuit kludged from a battery charger, etc.  It uses a microwave oven transformer (MOT) which should be available free for the taking if you are clever.  The only bits that can't be found in a good junk-box are the 1m length of welding cable and the timer (which is very nice with easy knob adjustment and digital readout, etc. for $5. You can't beat that.  The nice thing about AC is that you can precisely turn it on/off with small/cheap components. 

https://youtu.be/tPuKu5J-_64



If you are really cheap, you can make your own "welding cable" from scrap stranded mains cable as shown in this video.

https://youtu.be/neR_RFTblqg

[VoltJesus]

Paralleling rectifiers doesn't guarantee even current sharing: whichever one is hottest will have a lower Vf than the rest and on the next pulse will hog more of the current, getting even hotter till it blows.

THIS ^^^   This is exactly what I was overlooking, and would be the mode of failure.
And like you mentioned, there could be a chance for the rectifier to fuse the transformer secondary when it goes.
I have a 1000A breaker laying around somewhere. Maybe I'll throw that in too for good measure? 

Same thickness of strips was used to evaluate both designs.  I got various widths of pure nickle strips in 0.003" and 0.005" thicknesses.   For battery tabs, I use 1/8" wide strips that are usually 0.003".   Sometimes I use the 0.005 thickness.  With CD there is no effective current control per se.   I control the welding with voltage and generally charge to 17 to 18 V.   Too much voltage can cause burn through.   Good contact and reasonable pressure help alleviate burn through.

Yeah, that's why I was asking. The strips in the photo you posted looked a lot thinner that what I've got. 0.2mm x 8mm translates to about .008" x 3/8"
Think I am going to need a bit more juice to stick these strips.

[sokoloff]

Ali has the welder I have for <$135 delivered. You're probably going to be $50+ and many, many, many hours into anything you build. I don't want to dissuade you from a DIY path, but you're likely to spend as much money and way more time down that path.

If you're near Cambridge, MA and want to drop by sometime to play with the one I have (don't have time to do 2000 welds with you, but could run some test welds and then let you borrow it), PM me. Or post your location in the US and see if someone else has one you can test or rent/borrow with a deposit.

I honestly don't know if mine will weld strips that thick, but there's an easy way to find out...

[jpanhalt]

Be sure to watch the temperature of your 18650 cells.   If they start getting warm after welding, you may have ruined them.   Be sure they are in a safe place. 

I have never had a problem with NiCd or NiMH of any size.   With lithium cells, RCR123A's have been welded with no problems either.   With 18650's, I have had problems.   Maybe it was the lot of cheap cells I got, maybe not.  Of about 10 I have welded, I had one or two go into autonomous over-heating.  For others, early failure rate was quite high.  I rarely use 18650's anymore, unless it is just a single cell in a holder.

In any event, be careful of how much extra "juice" you use.

John

[VoltJesus]

Thanks for all the valuable input guys. Curiosity has got the best of me, so I'm afraid I NEED to test this wild rig of a contraption I devised. For uh... science! (plus I love me a good light show every now and then).
It's not going to work well, if at all before it self-destructs. Just can't help but give it a go. Need to see how she bangs. Plus I got a few ideas to implement now.

@sokoloff - I am not even remotely close to your location. But thank you kindly for the offer. Much appreciated.

@jpanhalt - Noted. I will keep an eye on the temps if I even get to that stage. I'll be welding 2 pieces of the .2mm strips together to test before I even attempt to ruin my investment in these cells.

[Ian.M]

If you leave out the bridge rectifier, and limit the input voltage, your wild contraption will probably survive if you are careful to only blip the button.

*NEVER* try to go more than a few percent over a transformer's nominal input voltage, or it will saturate and either fuse something or trip the breaker as soon as you switch it on, even if the secondary side is unloaded.

When you do get some sort of DIY welder set up, if initial tests go well, I suggest practising welding tabs onto small steel washers.  If they ever get too hot to comfortably hold, you *really* wouldn't want to be welding onto a 18650 cell!

[VoltJesus]

About the over saturating on the transformers... Is that true for torodial as well? I know about it being an issue with laminated core and shell styles.

I've got a few "china special" FartFire 18650s that I will test on before moving on to my good cells. This way I can cut the base off and see if there is any penetration or hotspots inside.

[Gyro]

The best option is to use a heavy duty SSR rated for inductive loads with zero crossing switching, controlled by a 555 timer for simple single pulse welding, or by an Arduno if you want a more complex dual or multiple pulse.

Actually, skip the zero crossing switching. Zero cross is ideal for resistive loads but is bad for inductive loads (transformers in particular), it's a common misconception. Transformer core sizing normally relies on there being some residual reverse core magnetisation (from the previous half cycle) as you go into the next cycle. If you switch on from a zero cross then the core will typically saturate on the peak of the first half cycle.

Transformers will handle this with normal random switch-on points but switching at the zero crossing every time is bad. I was once required to do an analysis on high failure rate of transformers in traffic signal heads. Initially it was put down to poor manufacturing quality, but it turned out that the controller was using zero crossing switch timing, this resulted in failures at the ends of the primary windings. Switching to mains peak switching completely cured the problem, it also made the signal head enclosures go from sounding like bongo drums (flashing) to blissful silence.

Peak switching does result in higher dV/dt in the switching device but this is more than offset by the reduction in dI/dt at switch on.

[Ian.M]

I agree it will be *really* hard on the transformer, but if you randomly trigger a SSR without the zero crossing relationship to the mains phase, the energy for short weld pulses of the order of a few mains cycles will be unpredictable. 

As you point out, peak switch on and zero current switch off would be nice, ideally switching on on the opposite half cycle to the one before the previous switchoff, but I fear the complexity of the control circuit will be more than the O.P. can cope with.   Also, unlike traffic signals, it isn't going to be running thousands of cycles a day 365 days a year, unless the O.P. goes into the battery pack rebuilding business.

[MagicSmoker]

Commercial battery tab welders deliver two 1ms to 10ms long pulses in rapid succession. The first pulse cleans the weld area while the second does the actual welding. Needless to say, you aren't going to be able to reliably perform double-pulse welding with mains frequency AC directly from a transformer secondary. That said, inexpensive commercial spot welders for ferrous metals do use a big single turn secondary transformer, so it probably works well enough, most of the time (I feel like a Ron Burgundy quote is apropros here...  )

On a related note, I've been kicking around the idea of making a CD welder in which the energy is stored in the capacitor at high voltage and then dumped into a forward mode step-down transformer to get to the necessary peak current level while (hopefully) saving in overall cost (less expensive capacitor and switches but adding a transformer and very high current rectifiers).

[texaspyro]

Soooo.... you're gonna weld 2000 lithium cells with a crap-ass welder built by somebody with no experience in the dark arts of tab welding...  or even electronics design.  I'm sorry to say this, but you're gonna die! Or burn.  Or flame out.  Or...

I'm well experienced the art.  I have built the mother of all CD welders and used it to make many, many thousands of welds.  It is NOT a task for the amateur.  A single bad weld can lead to disaster.  This kind of task requires a very good welder with precise pulse control and also precise weld held pressure control and lots of experience.  It is the ONLY way to get reliable welds.  And with 4000+ welds to do, you WILL mess up... even with a decent welder.  Been there... done that.

I STRONGLY suggest you don't pursue this project.  I have seen the results of similar projects and they ain't pretty.  There is no way to build a safe pack  with what you are proposing.

[Gyro]

I agree it will be *really* hard on the transformer, but if you randomly trigger a SSR without the zero crossing relationship to the mains phase, the energy for short weld pulses of the order of a few mains cycles will be unpredictable. 

As you point out, peak switch on and zero current switch off would be nice, ideally switching on on the opposite half cycle to the one before the previous switchoff, but I fear the complexity of the control circuit will be more than the O.P. can cope with.   Also, unlike traffic signals, it isn't going to be running thousands of cycles a day 365 days a year, unless the O.P. goes into the battery pack rebuilding business.

Yes, fair enough. You could maybe achieve an (Edit: inexpensive) improvement though by doing a discrete zero cross detection plus a fixed delay to get somewhere near peak and a non zero crossing SSR.

[karoru]

Main reason why people use microwave oven transformers for these kind of projects is that these are stray-field transformers, like ones used in welders - they have built-in current limiting and can survive near short-circuit conditions for quite a long time. I wouldn't try to use typical toroidal transformer for such application, as it's "too good" magnetically (stray-field transformers are made to "be bad", ie have high leakage inductance by design).

[Richard Crowley]

Commodity SSR (Solid-State Relays) essentially all have zero-crossing switching built-in. 
Ask anyone who has tried to build a lamp dimmer how hard it is to find a "random-trigger" SSR in modern times.

[Ian.M]

Main reason why people use microwave oven transformers for these kind of projects is that these are stray-field transformers, like ones used in welders - they have built-in current limiting and can survive near short-circuit conditions for quite a long time. I wouldn't try to use typical toroidal transformer for such application, as it's "too good" magnetically (stray-field transformers are made to "be bad", ie have high leakage inductance by design).

Good point, though I *think* that could be worked around with a high power ballast resistor (e.g. a kettle element) in the primary circuit.

[james_s]

I built my battery tab welder a number of years ago. Mine is a capacitive discharge design, I used a bank of 22,000uF 25V capacitors adding up to a bit over 1F and a very simple power supply with a LM317 to charge them up. For triggering I used a big old stud mount SCR that probably came from some kind of motor drive. I've used it to weld a lot of batteries and it has continued to work well.

[VoltJesus]

Did not have time to rig it up today, but will sometime this week.
I did however pop the side cover off both my Schumacher and Sears battery charger/jumpers to have a look.
The transformer is identical in both, except for the secondary winding. The Sears unit has separated coils (4 terminals) on the secondary vs. what you can see on the Schumacher (photo attached).
Transformer is a little smaller than I was expecting, but looks bigger than any MOT I've ever worked on. That includes the Amana and Panasonic commercial units.

By stroke of luck, someone replied to my ad looking for an older MOT today. I will be going to pick that up tomorrow.

[Housedad]

This thread scares the crap outa me.  I've done welding for all of my adult life as a hobby.  GMAW, SMAW, Oy/Acet,  spot, Plasma. I have all the common types at my disposal.  But the current levels that you guys are talking about to weld on a nickle tab and some of the design ideas are, well,  not too kosher in my book.  They look dangerous as all hell.

[james_s]

There's really nothing unusual about them. A typical capacitive discharge spot welder will develop peak currents in the 10kA range but the voltage is relatively low and the pulse is very short. The idea is to heat the joint very quickly so it can cool before any significant heating occurs in the battery being welded. It's not particularly dangerous, the average power is low. A small CD spot welder is a heck of a lot safer than a MIG, TIG or stick welder, I use mine on the kitchen table.

[Housedad]

I guess I am definitely not used to this form of welding.

[james_s]

It's a similar concept to the nitrogen laser. Peak optical power can be in the kilowatts but due to the extremely short pulse width the average power is similar to a 99 cent laser pointer. I measured the output of mine years ago by scoping the drop across a piece of #4 copper wire and IIRC the pulse peaked at just under 10kA but was only about 5nS long.

Actually xenon flashes are another similar concept, the tiny little tube in a disposable camera can have a peak power of hundreds of watts, somewhat larger camera flashes can have peak currents of hundreds of Amps, but again it's a very short pulse so the average power is low.

[edpalmer42]

I have a 5V power supply that's rated for 250A @ 115VAC input or 360A @ 220 VAC.  It has remote sense and enable/disable capability.  Would it work as a spot welder?  Set up a timer to pulse the enable lead as required.  Current control would only be via selecting the appropriate lead size, but maybe?

Ed

[james_s]

If it's a linear supply it might work for light duty spot welding, but if it's a switchmode type it will almost certainly detect a short circuit and shut down. Spot welding is normally in the thousands of Amps at very low voltage. A friend of mine built a conventional transformer type spot welder and he used a microwave oven transformer core with only a couple turns of very heavy secondary. Only about 1V but lots of current.

[edpalmer42]

It's a switchmode power supply, but don't forget that full load for 5V @ 250A is only 20 milliohms.  5V @ 360A is about 15 milliohms.  So what kind of contact resistance do you have with a spot welder?  Don't forget that the only way to heat the spot is if there's resistance there.  I just used my millohmmeter (20 milliohm full scale, 0.01 milliohm resolution) and got 1-5 milliohms with two big copper electrodes pressed down onto a few typical battery strips.  So with appropriate lead selection I'll be able to limit the current to a 'safe' value.

A long time ago, I took about 20 cm of 20 ga wire and connected it across the power supply and plugged it in.  It just turned red and melted.  No drama at all!     Obviously, that isn't comparable to spot welding, but it was interesting to do.

I looked at the battery spot welders on ebay and saw values from 50 to 800 amps so 250 or 360 seems feasible.

I'll probably test this one of these days, but I have too many other toys to play with for now! 

Ed

[james_s]

Try it and see what happens, I'd love to hear the results. My friend used copper ground rod to form the electrodes for his spot welder.

[DBecker]

Also need a low-usage spot welder for cells.

The lowest cost approach is using a spare 12V car battery, jumper cables, and 8 AWG sold copper wires taped to a wooden handle.  I've seen it done successfully on YouTube, but it seems to require just the right battery age and lots of practice to get the contact timing right.

The next step up is using a relay and pushbutton with the car battery.  The timing is easier and you can align everything rather than having to jab.  But the timing is still not repeatable.

My thought is to move the backyard scrap approach into the modern age.  Instead of a precisely-worn car battery, use a lithium jump start pack.  And instead of using an unpredictable  relay, use a MOSFET.   Lithium cells have significant intrinsic capacitance due to their structure, and the cells used in jump start packs are optimized for brief high discharge rates.

[VoltJesus]

Wow, so many questions! LoL     I'll try to get most of them in this one post. Look for your user name.

As has been mentioned, battery welding is a finicky process that needs EXCELLENT control to do it properly. And there is quite the risk if things go wrong. Exploding batteries is one.
I would say that as long as you understand the risks involved, protect yourself and nearby things accordingly, then you might be ok. Not meaning to downplay safety at all.
My example in the original post is NEITHER SAFE nor FINE CONTROL. So although I posted asking for advice/insight, I would otherwise not recommend what I am attempting to anyone.
That said, you are welcome to do whatever you wish. Just know what you are up against and the ramifications if you mess up (your fault or not).

@Housedad - Honestly there is danger everywhere. Driving a car on the road has very high risk, yet most of us do it every day. The best we can do is understand the risks and try our hardest to control the outcome.
I have a background in HVAC, so I am more aware of safety concerns than the average joe. I've be electrocuted, cut, fingers smashed, and had things go boom right in my face. Very few of those were my fault. My comment above should explain my position on this welder setup. Safe? No. Workable? Possibly.

@edpalmer42 - Just remember that welding is actually shorting the power supply. I do not know the design of your power supply, but there is always the chance something inside could burn up or go pop. Welding is getting the material hot enough to melt together as one, so when you melted that 20ga wire that's the same principal. Will it do battery spot welding? You need to understand that there are a multitude of battery tab strip thicknesses, and they all require different power levels to weld properly. Then there are different tab materials to consider. I am specifically working with (99% pure Nickel) what is considered to be quite thick strips at 0.2mm. Most common thicknesses are in the 0.1mm to 0.15mm range. A 0.2mm strip will need at least 2x the power to weld as the 0.1mm.
For 0.15mm strips, the avg. power needed to weld those is in the ~1100w range. However many Amps and Volts it takes to get there is depending on the application. Too high a voltage will cause too much spark, and will either blow holes in the material or stick your welding leads. Too low a voltage is not good either. Excessive heat at the weld could internally damage the battery, which just might make it explode. DO NOT kid yourself. This CAN and WILL happen if you do not have your settings right. Know your enemy.

@evb149 - See my reply to edpalmer42
About welding with a car lead-acid battery... The problem is that it is way too hard to control. Every time you put a load on that battery, you could get wildly different amounts of amps. This makes it unreliable for tab welding. See my original post. That was my last resort option with the bank of 4 solenoids. Not advisable and wont do a good job.
bout the battery tabs... I've yet to find a good source. Ordered mine on Ebay. You want Pure Nickel 99.6% strips. Many many listings on Ebay are flat out scams where they claim pure Ni, but are actually Ni coated steel. You do not want that because of higher resistance in steel, and it will rust. Nickel strip is the best for battery packs.
Different materials have different characteristics. Copper is the best conductively, but is hard to weld. Brass has much higher resistance. Sheets of Ni are fine, the thickness is key.
For chemical joining, I can promise you if a viable low-cost method existed, all the big name tool manufactures would be using it. So I assume no such beast exists.

@Dbecker - See my reply to evb149
Using lithium cells to weld can and has been done. The guys I bought my 18650s from are using old 123 cells and a control circuit to do spot welding on the packs they build and sell. The 123 cells they're using are 3.3v, but are capable of several 100 amps a piece and much safer that hobby grade LiPo packs.
I would not advise you use LiPo cells from things such as those jump-starter packs. The quality of LiPos in those things are horrible. Welding is a dead short, and not something you want to do to a low quality LiPo. If they managed to survive without blowing up, you would most likely damage them severely with repeated shorts.

@texaspyro - Not 2000 cells, 2000 welds. Packs I'm making will have strips across both series and parallel, needing at least 2 welds per strip. 4 welds per side of the 18650 for a total of 8 welds/cell. I'm trying to go for 3 welds per strip, making 12 welds per cell. If I do 12welds/cell that would add up to ~2000 welds.
Is my setup safe? No, not really. That is ok. Not afraid of a little risk. Guess all the things I've seen though my years of work has kind of dulled my senses a bit.

[jpanhalt]

Possibly off topic but maybe not.

Anyway I'm not seriously suggesting it for this particular application but it seems like some kind of "glue for metals" process shoud be possible for some combination of metals that leads to something about of the quality of a spot weld or soldering joint but is basically electro-chemical / chemical in origin.

1) There are many , low melting alloys that can  be used for "gluing" metals (think hot melts glues).  Wetting the surfaces can be a problem but some do wet well.  Many contain bismuth and/or indium.

2) A few years ago researchers announced a modified Tollen's reagent that deposits pure silver.  Original Tollen's was a very thin coating.  The new reagent sets at relatively low temperature and presumably can even be soldered.   Here's a link to the original: http://pubs.acs.org/doi/abs/10.1021/ja209267c  Unfortunately, the ACS (publisher of JACS) removed details from the abstract.  They are readily available on the internet.   Basically, the authors replaced glucose in the original Tollen's process with a reducing agent that gave gaseous products.  All of the reaction products in the revised procedure, except for the silver,  are gaseous.

3) If you do not need high currents, there are  variety of elastomers and paints that are conductive (zebra strips and window defroster repair paint).

4) Use a battery holder

The main problem with most of the alternatives just mentioned is mechanical strength.   Welds are very strong and relatively resistant to vibration.

John

[james_s]

I got the nickel strip I use from Sunstone Engineering, it seems to be quality stuff.

http://sunstonewelders.com/


I did blow a hole in a battery once, it happened because I bumped the foot pedal before I had pressed the electrodes down firmly. Didn't explode or anything but it did ruin the battery.

[DBecker]

@Dbecker - See my reply to evb149
Using lithium cells to weld can and has been done. The guys I bought my 18650s from are using old 123 cells and a control circuit to do spot welding on the packs they build and sell. The 123 cells they're using are 3.3v, but are capable of several 100 amps a piece and much safer that hobby grade LiPo packs.
I would not advise you use LiPo cells from things such as those jump-starter packs. The quality of LiPos in those things are horrible. Welding is a dead short, and not something you want to do to a low quality LiPo. If they managed to survive without blowing up, you would most likely damage them severely with repeated shorts.

The jump start boxes are using pouch cells.  I didn't believe the advertising at first, but tests show that they put out an amazing amount of power for a few seconds.  A 600 amp one might only deliver 300A, but that's still really impressive.  And a few years on the market shows that some models are reasonably long-lived.

The appeal for someone that only occasionally does a few dozen welds is that it can be "off-budget".  You can keep it in a car for its intended purpose, and hook it up only when needed.

Edit: fixed quote

[jpanhalt]

@Dbecker - See my reply to evb149
Using lithium cells to weld can and has been done. The guys I bought my 18650s from are using old 123 cells and a control circuit to do spot welding on the packs they build and sell. The 123 cells they're using are 3.3v, but are capable of several 100 amps a piece and much safer that hobby grade LiPo packs.
I would not advise you use LiPo cells from things such as those jump-starter packs. The quality of LiPos in those things are horrible. Welding is a dead short, and not something you want to do to a low quality LiPo. If they managed to survive without blowing up, you would most likely damage them severely with repeated shorts.

The jump start boxes are using pouch cells.  I didn't believe the advertising at first, but tests show that they put out an amazing amount of power for a few seconds.  A 600 amp one might only deliver 300A, but that's still really impressive.  And a few years on the market shows that some models are reasonably long-lived.

The appeal for someone that only occasionally does a few dozen welds is that it can be "off-budget".  You can keep it in a car for its intended purpose, and hook it up only when needed.

Your quote is screwed up.

John

[james_s]

I use LiPo packs in my RC aircraft and I can say from experience that if you abuse them they will puff and the internal resistance rises. Sure you may get huge current from them, a few times, then they'll quickly degrade.

[VoltJesus]

The jump start boxes are using pouch cells.  I didn't believe the advertising at first, but tests show that they put out an amazing amount of power for a few seconds.  A 600 amp one might only deliver 300A, but that's still really impressive.  And a few years on the market shows that some models are reasonably long-lived.

The appeal for someone that only occasionally does a few dozen welds is that it can be "off-budget".  You can keep it in a car for its intended purpose, and hook it up only when needed.

Yes, those pouch cells are LiPo packs. Essentially RC car/plane battery packs. Most of the companies making these jump-packs are buying the lowest grade "no name" LiPos they can get their hands on and stuffing them into plastic cases.
There are many brands and qualities of hobby LiPos. Have a look at HobbyKing and see just how many different variants of the same 3s1p LiPo they sell.

Also, very very few of these jump-packs offer any real protection. When you flip the little flap up to attach your jumper cables, you are looking right at the battery connector. There is no circuitry between that connector and the LiPo cells. Only the box on the jumper cable attachment has any kind of protection. And most are very poor, to say the least.
The only jump-pack I have seen that offers built in protection in the actual pack is the Noco Genius Boost.

Bottom line, using those to weld would be more dangerous than my retarded setup. 18650s are much safer than pouch cells (the good quality 18650s at least).
By design an 18650 is a steel shell, with gaskets, and a vent under the positive terminal to release excess pressure should something go wrong internally. LiPo pouch cells just keep swelling until they go BOOM.
I see james_s also has experience with these. So there are 2 guys that can tell you first hand not to play with pouch cells. I don't mind toeing the line on safety when it only involves myself, and others are not in harms way. That said, I do not play around with pouch cells. Too high a risk.

[Ian.M]

Even the humble Lead Acid battery can be unsafe for welding  or other applications that draw an extremely high current.  All it takes is a cracked or loosened internal strap between the cells and if its recently been fully charged, you risk internal sparking resulting in a hydrogen explosion.  Its also possible for them to short internally if the links are corroded or otherwise weakened and they are grossly overloaded.  Chunks of lead and boiling sulphuric acid being thrown everywhere is not something you want to be complacent about.   It doesn't happen often, but when it does it ruins equipment and if anyone is in the way, even lives.

The hazards of a mistreated LiPO pouch cell mAH for mAH are probably less than a mistreated 18650.  The stored energy is the same but without rigid confinement all it can do is burst and burn.  An 18650 with blocked vents can explode like a grenade or launch its casing like a rocket, and do comparable damage similar to being shot with a shotgun slug.  A 2600mAH 18650 cell has about 31KJ of stored energy.  For comparison that's about double the maximum muzzle energy of a .50 BMG round, and if even 5% of it is converted into kinetic energy of the casing or fragments, that's equivalent to a .22LR round.

[mc172]

Just buy one. You will spend more money messing about building your own or buying and bodging up some other contraption not intended to do welding.

http://www.ebay.com/itm/US-Shipping-18650-Battery-Charger-800-A-0-1-0-2-mm-36V-60A-788H-Spot-Welder-/171866427579?hash=item28040990bb:g:ZEsAAOSwPCVX9kTw

$138 shipped.

[m.k]

I built my MOT one as a simplified version of this: http://www.avdweb.nl/arduino/hardware-interfacing/spot-welder-controller.html
Quite a nice circuit, you get very repeatable results with it. One thing i learned is you need current, as much as possible, i don't think mine does have more than 3 or 4 volts open circuit voltage on the secondary. So if you use the transformer above you would have to replace the secondary. But it seems big enough. I definitely needed a thick cable from a welder for the secondary, a 10 mm² car jumper cable did not do the trick.
And if you use a MOT: remove the magnetic shunts. I skipped this step in the first place but the welding times were too high.
Costs were about 10€ for the components and 25 for the copper Pins

[VoltJesus]

So if you use the transformer above you would have to replace the secondary. But it seems big enough. I definitely needed a thick cable from a welder for the secondary, a 10 mm² car jumper cable did not do the trick.

Cool. I am working on something similar. Picked up that microwave yesterday, and got it torn down today. Mine is a Sharp 1100w MOT from 2000.
Removed the secondary and replaced it with a piece of 2awg I had laying around. Hope that is big enough? I see most people end up using 4awg and can just barely get that to fit. I had quite a time getting the #2 in there. Nothing bigger will fit without some major modifications.
I had to use my framing hammer, if that puts it in perspective.     I'll post pics up later.

The hazards of a mistreated LiPO pouch cell mAH for mAH are probably less than a mistreated 18650.

Good info Ian. Scientifically speaking, an 18650 would be more dangerous than a pouch cell.
I was mostly addressing using the LiPo as your power source for welding. Dead shorting a LiPo accidentally is scary enough. The reason I would consider them more dangerous is because they are notorious for being tempermental. That and the astonishing lack of quality control with all the various brands running amok.
You are correct in regards to the info you posted, however.

[james_s]

I have personally seen LiPo pouch batteries erupt into surprisingly large fireballs. It happens occasionally when an electric RC plane or helicopter suffers a severe crash.

18650 cells are relatively safe, they can blow the end out and flame in a really severe case, stored energy is dangerous, no way around that but I've never seen one make a fireball.

[Conrad Hoffman]

I've seen schematics for thermocouple welders, which are the same or similar to what's needed here. They start with a small isolation transformer (I think) and then a Variac. The output of that is rectified and charges a large cap through a big limit resistor to whatever voltage is set by the Variac, 0-400 VDC for the one I looked at. The limit resistor is such that you can discharge (essentially short) the cap without having to disconnect the charge circuit.

A mercury relay is used to discharge the cap through a transformer having one or two turns as the secondary. That gives you a low voltage pulse capable of hundreds of amps, depending on the Variac setting.

Pretty simple device and easily built, save for the mercury relay. You'd probably want something like a big contactor relay with a snubber, even if it had to be cleaned now and then. I don't know if the newer units have replaced the mercury relay with something solid state but it would be surprising if they still used the mercury devices.

[james_s]

That's certainly a valid approach. When I built mine I decided the stored energy was already dangerous enough so I didn't want high voltage too. I went with a lot of capacitance, around 1F charged by a variable voltage of up to about 22V.

[VoltJesus]

Which Arduino is suitable for this task? Uno, Leonardo, Mini Pro, Mega... ??
I was at the store just a few days ago and was planning to pick one up, however I could not find an answer online while in the store. Didn't get one.
Got an open box Rpi 3 model B for $24, but it has WiFi issues. The Rpi Zero W I got a few weeks back wont stay connected to bluetooth. I'm just going to return them both.

I will still setup the first design in the OP just for kicks, using the microswitch to manually actuate the pulse, to see if it would work.
But I want a more precise setup with the MOT I am building, and an Ardrino circuit seems like the easiest/cheapest option.

One more thing I am curious about. What makes a SSR "zero cross switching" ?  I have worked with a few different types, but have never come across this terminology. At least not "zero cross switch" printed on the SSRs I've messed with. I might even have one of these laying around in all my stuff.
There is this particular type of multi-pin relay used in Hobart equipment that we refer to as "Ice Cube" relays. I am wondering if these are zero cross switching.

[Richard Crowley]

Which Arduino is suitable for this task? Uno, Leonardo, Mini Pro, Mega... ??

ANY microcontroller would work. None will directly drive a mechanical relay (like your "ice cube" gadgets). You would need at least a transistor to handle the higher current of the relay coil.  None of the Arduinos (or Raspberry Pi or other microcontrollers) will work directly, but ALL of them will work when properly buffered.

I will still setup the first design in the OP just for kicks, using the microswitch to manually actuate the pulse, to see if it would work.
But I want a more precise setup with the MOT I am building, and an Ardrino circuit seems like the easiest/cheapest option.

Switching a large, inductive load like a MOT directly from a microswitch is pushing the envelope somewhat farther than I would be comfortable with.  I wouldn't do it.

I do not agree that using a microcontroller (Arduino, RasPi, or ANY microcontroller) is either "easiest" or "cheapest" and certainly not both.  Look at that video I suggested. He uses a timer board which is far easier to use than any microcontroller.  Strongly recommended.

One more thing I am curious about. What makes a SSR "zero cross switching" ?

It means that the SSR will not turn ON until the mains AC passes through the "zero crossing".
Virtually all commercially available SSR have built-in zero cross switching.  That is likely why they don't mention it specifically.
https://en.wikipedia.org/wiki/Zero_cross_circuit

There is this particular type of multi-pin relay used in Hobart equipment that we refer to as "Ice Cube" relays. I am wondering if these are zero cross switching.

No. There are NO mechanical relays that have zero-cross switching. It can only be done with a solid state relay (SSR).

[VoltJesus]

It means that the SSR will not turn ON until the mains AC passes through the "zero crossing".
Virtually all commercially available SSR have built-in zero cross switching.  That is likely why they don't mention it specifically.
https://en.wikipedia.org/wiki/Zero_cross_circuit

No. There are NO mechanical relays that have zero-cross switching. It can only be done with a solid state relay (SSR).

Thanks for the reply. Guess I worded my question poorly. I understand the idea of zero cross switching, though I am trying to understand how it is achieved. As in what components in a SSR make this action happen.

Switching a large, inductive load like a MOT directly from a microswitch is pushing the envelope somewhat farther than I would be comfortable with.  I wouldn't do it.

I do not agree that using a microcontroller (Arduino, RasPi, or ANY microcontroller) is either "easiest" or "cheapest" and certainly not both.  Look at that video I suggested. He uses a timer board which is far easier to use than any microcontroller.

I'm not planning on welding on the 18650s with that setup unless it proves effective. I just want to try it out and see if it will work and possibly be useful for other projects.
That switch I posted in the OP is actually a very heavy duty switch for its design. These are used in commercial convection ovens to break mains power to the motor when the door(s) is open. Most of the time 120v, but can be 208v as well. So high temp, high amp, long use conditions. The switch actuation is very stiff and sharp. You wont accidentally push that button unless you intentionally do so.

Maybe not the absolute cheapest, but very cheap. I've been wanting to learn how to build circuits and use the Ardrino platform for a while now. So even if there are cheaper complete circuits that will accomplish the goal for this welder, I could use the Ardrino for other projects down the road. Not planning on spot welding 18650s every day (hopefully).

Microcenter had some of the Mini Pro boards for $5-7 I think it was. Almost got one, but wasn't sure the I/O was suitable for the task.
Correction, $4 : http://www.microcenter.com/product/431996/Arduino_Pro_Mini_Board
And the plus is that this stuff is local. Also can pick it up or swap it out if it is faulty. I hate waiting days-weeks for shipments if I can help it.

[Ian.M]

There are several different ways of handling zero crossing switching.  Most SSRs use a TRIAC load switch.  By their nature TRIACs only switch off at close to zero current so once triggered will stay conducting for the rest of the half cycle (assuming a resistive load).  To reduce stress on the TRIAC (apart from when driving easily saturatable inductors, e.g. transformers) its desirable to switch when there is minimal voltage across it.   As the control circuit needs to be isolated anyway (to provide equivalent functionality to an electromechanical relay), and to minimise component count, its usual to use an optocoupler that includes a zero crossing detect circuit on the output side.  It will only switch on when the input commands it AND the voltage is near zero.  A typical device is the MOC3063.

If you are new to Arduinos and don't have experience with Atmel AVR MCUs or much experience with other MCUs, you should probably start with an Arduino Uno, as 99% of the introductory to intermediate level tutorials that are out there use the Uno. If you can already program in C, you'll learn a lot quicker.  Boards like the Mini Pro or Nano are a half-way house between the Uno and building the bare ATmega328P MCU directly into your project.    If you are breadboarding or building on protoboard, they are very handy, but as soon as you want to make a PCB you might as well put an AVR directly on your board. The clones are cheap enough not to worry about building them permanently into small projects.

[james_s]

Well of all the choices you listed, I would say the Raspberry Pi is definitely *not* what you want. It's somewhat challenging to achieve truly real-time control that you need for this sort of thing when you're running a multitasking operating system and surely you don't want to wait for your welder to boot up and launch the program that runs it. Honestly I don't think you need a microcontroller at all, you can do it all with a few jellybean ICs and no software at all. If you'd rather have the flexibility of software then any microcontroller you're comfortable developing with will work. If Arduino is your thing, one of those $2.50 nano clones from China would work just fine. A 20 year old AVR or PIC would be more than adequate, there is really nothing fancy needed here.

[VoltJesus]

Surely my idea for this build is not that far off the mark. Just found this last night: https://hackaday.io/project/9227-look-inside-and-repair-of-1970s-prof-spot-welder
A spot welder from the 70's that uses a giant relay (aka contactor) to apply power to the welding transformer. Certainly not fine control enough for battery welding, however does exactly what it was built to do with a very simple circuit. I found it an interesting read.

you should probably start with an Arduino Uno, as 99% of the introductory to intermediate level tutorials that are out there use the Uno. If you can already program in C, you'll learn a lot quicker.

Thanks for the explanation. Yeah I am a complete noob with programming, so I am starting from zero. I have done some poking around with android and ADB/Command-line stuff, but not much beyond that. No coding language experience, per se.

surely you don't want to wait for your welder to boot up and launch the program that runs it.

Could it be any worse than a Fluke 287?      Seriously though, I got the Rpi to tinker with. Not necessarily for this project. Guess I used this as an excuse to get one.
But yes, using the Ardrino platform will be flexible and easier for me right now (i think). Something I've wanted to learn anyhow.

[VoltJesus]

WARNING   Long post. Skip to the end and see the pretty pictures.


Well... I did it. Not sure what I did exactly, but I did it.
Just updating this as some may be curious if I went up in a ball of flames 

There was absolutely no question that this was a dangerous endeavor. Spot welding on 18650s is no joke, and you best be on your toes at all times. (more on this at the end)

As I stated earlier in this thread, I would not be attempting to use this setup unless it proved worthy/reliable. While it was not 100% reliable, after performing over 250 test welds on food can lids, I was fairly confident in my ability to make this work. Even so, I took extra precautions, just in case.

To me, the welds seemed pretty decent. As you can see in the photo, there is a slight bulge inside the base on the 18650 where the weld was a little too hot. After a lot of testing, I came to the conclusion that the "weld pressure" was more critical than the power of the weld alone. Pressing too hard while welding made the metal cave in, and made for a weak weld. Too little pressure also produced a weak weld, while also having the chance of "blow out".

Also, I found the the length of the wire (all of it.. primary, secondary, incoming power) had a dramatic affect on the welders output if changed at all. I am talking about inches of length difference, not feet. I must have been EXTREMELY lucky with the initial build of this welder, as it was nearly perfect.
What I ended up doing is going back to the original wiring and design, after attempting to improve it and failing every time.

I tried welding wire, was just a bit longer than my original wire, so I cut it to the same length. Even then, the fine strand welding wire did not perform as well as the large stranded wire I had used initially. Had to go back to my old wire to get enough power for a decent weld.
Then I built a foot switch, so I could use both hands, instead of having to hold both welding leads with one had while pressing the switch with the other.
Foot switch was nice, but I had a bad loss of power. And all of that loss through about 4ft of 10awg going to the MicroSwitch mounted on my foot switch, which was on the primary side of the welder. I was not expecting that big of a power loss from slightly longer high voltage primary wiring. If the foot switch was controlling power on the secondary side, sure I would have expected the losses.

During my testing of that foot switch (about 100 welds in) it decided to stick, and burned a hole in the can lid I was testing on. I was quick to remove the stuck welding lead to the material, but it had already made a large hole. All of this happened in under 1sec (something near 70-80ms I would guess). Check the last photo.
Dangerous? Very much YES. Imagine if that had been on one of the 18650s. Would have made for a very unpleasant evening.

Anyways, had to return to my original design to get decent welds again.
In the photo that shows the battery pack welded up, that is only the parallel ties. Each parallel string was also welded in series to the next string. That photo is about 20% completion on ONE pack. I made 3 packs in total.
During my welding, I only had one event where a weld got stuck and could have blown a hole in my pack. Still have no idea how that happened, but I was quick to break the lead loose when it stuck. Beyond that, no other issues came up besides some of the welds not being consistent, which I just went back over and redid after letting them cool a bit.

Overall, I accomplished my goal. Nearly 2200 welds, and each one manually controlled with a press of my finger. Sketchy is putting it lightly...
Packs are all finished up now and work just fine. No damage to any of the cells, and my recharge of each pack is actually yielding more capacity than what the spec sheet for these cells show. Which is amazing considering all I put the poor things through. 

[anishkgt]

here is one that i came across on ebay and it looks decent.https://www.ebay.com/itm/282827305599. Is the cost worth it ?