Need help rigging up a spot welder for some 18650s

[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 ?