Build your own CD battery tab welder for about $100.00+-

[RLT]

Been promising this for a couple of weeks. Finally got it written up and put on one of my own sites. Very extensive description and photos of a very crude, bare-bones , yet very functional Capacitive Discharge welder for welding tabs for homebrew battery packs.

You can build your own for very close to $100, even if you have to purchase everything for it. Most of us reading this are probably enough of a 'packrat' and/or 'scrounger' to be able to build it without having to spend much money at all.

It will be improved and refined over the next few weeks, and of course by spending a little more money we can make it more powerful, efficient, prettier and safer.

I'll try to excerpt the article suitable for posting here on the forum late tonight, and try to answer any questions that may come up. But I gotta get some sleep now.

So, I'll leave you with a teaser photo:


and a link to the page on my LEDHACKS.COM site with the initial writeup on it.
http://www.ledhacks.com/power/battery_tab_welder.htm

I haven't even fully proofread it yet, so there are bound to be errors and omissions, so if you see anything that doesn't seem right, let me know.

More to come!
(Thanks, Eric, for letting me hijack your Project #2 thread occasionally before. I'll try not to do that anymore )

[Doctorbass]

That's a good news!

Since i have 320 Kinion cells to link in 20s 16p.. That should be helpfull !

I already have all the stuff to built the spotwelder.. i will try that for sure!

Doc

[Ypedal]

Hey doc.. got extra capacitors handy ?

[RLT]

Thanks Doc.

Oh, one important thing I forgot to mention:

The way it is set up now, the SCR will keep draining power after the pulse, even if you release the foot switch. So you need to break contact of one of the electrodes very quickly after the pulse.

Not a real big problem, just something to be aware of. Might be kind of hard on your power supply if you maintain full contact for more than a few seconds.

Wouldn't be a problem if we were dealing with AC, but under DC, the SCR works like a latching relay.

Anyone got any ideas for a cheap simple circuit that will fix this problem? I have a couple of ideas for getting around it, but they either complicate the manual process or cost a fairly significant amount of money (compared to the cheapness of the components for rest of the basic project.)

[RLT]

Hey doc.. got extra capacitors handy ?

Hey, yeah, I'll bet a flux capacitor at 1.21 gigawatts would work great.

How 'bout sharing some, Doc.?

[fechter]

Anyone got any ideas for a cheap simple circuit that will fix this problem? I have a couple of ideas for getting around it, but they either complicate the manual process or cost a fairly significant amount of money (compared to the cheapness of the components for rest of the basic project.)

In the "old days" dc-ac inverters used SCRs. There was some kind of commutation circuit that used an inductor to give a short reverse current pulse to turn off the SCR. I'll have to dig to see how that one worked.

What about just charging a capacitor bank to the desired voltage, then disconnecting the charging supply. Fire the caps through a SCR and let the caps drain completely.

[Doctorbass]

Hey doc.. got extra capacitors handy ?

Hey, yeah, I'll bet a flux capacitor at 1.21 gigawatts would work great.

How 'bout sharing some, Doc.?

I tried to sale the last one i had on ebay last week, but the military guy knoked at my door and told me to destroy its because previously they detected some large EMP pulse around my city caused by my previous experiments with it that caused some crash in their...radar tracking system...


Sorry !

Doc

[RLT]

What about just charging a capacitor bank to the desired voltage, then disconnecting the charging supply. Fire the caps through a SCR and let the caps drain completely

Yeah, that's one of my less than elegant solutions. I thought of using two foot switches, one to power the charger... Or maybe rig some sort of two stage single pole double throw switch. in one foot control.

Gotta be a better (but still cheap and easy) way.

[TylerDurden]

I tried to sale the last one i had on ebay last week, but the military guy knoked at my door and told me to destroy its because previously they detected some large EMP pulse around my city caused by my previous experiments with it that caused some crash in their...radar tracking system...

I guess you better not show them your rail-gun.

[Doctorbass]

I tried to sale the last one i had on ebay last week, but the military guy knoked at my door and told me to destroy its because previously they detected some large EMP pulse around my city caused by my previous experiments with it that caused some crash in their...radar tracking system...

I guess you better not show them your rail-gun.

Doc

[RLT]

A few new things, not unexected, but confirmed after more experimenting;

The 8ga wire makes a noticable difference over 10ga. And the super flexible stuff makes manipulation of the electrodes so much easier and more pleasant.

The more amps your SCR can handle, the better. The lot of new SCRs I bought on eBay came in. There were four in there that the seller thought were probably 225A.... I'm guessing those huge things are at least 300-400A, but I can't find any info on the internet using the parts number to confirm. But they make a noticeable improvement in the welds over the original 130A SCR.

Also tried paralleling a couple of 110A SCRs. That works very well too; So, if you have a few lower amperage ones laying around, rather than searching for a reasonably priced big one, you can probably get by by paralleling them. (Although I think ...Guess...that it is probably important that they be all the same part number, or at least the same specifications in all respects.

Mounting / connecting the SCR on the capacitor rather than on the electrode end of the wire is better.

Drilling and tapping a 1/4in bolt hole in the end of the stud, if you are using a large stud mount SCR, makes it much easier, neater (and probably more electrically efficient) to make connections.

With all of the above improvements I was able to tack weld two pieces of 1/4 inch thick titanium together, with just the 1.5F cap with 14.2V charge. It wasn't a 'structural' weld by any means, but it was strong enough to survive modest handling and a slight amount of torque. Two 1/8 " thick steel plates tack welded together much better than I would have imagined.

Battery tab welds were just about perfect, (at least by MY criteria).

Along the lines of Fechter's comment, and one of my earlier 'crude' ideas to overcome to the 'latching' action of the SCR under DC:
I think the problem might possibly resolve itself, although I haven't actually tried it yet:
My commercially made foot switch has a SPDT switch, with the normal position being N.C. and the actuated position being N.O....
If I am figuring this right;... If I wire one side of the wiring between the of the PS and the capacitor (probably have to use a relay or solid state relay switch) to the NC pole of the foot switch, and (of course) wire the SCR gate actuator part to the NO side of the foot switch, the cap will charge normally until I press the foot switch, then cut off a few milliseconds before the gate is actuated, then resume charging the cap when I release the foot switch.

I guess it depends on whether or not the cap retains enough residual voltage after the pulse to keep the SCR latched.


I also learned that I can't quickly draw a simple circuit diagram that wouldn't be an embarrassment to make public. I'll have to put that off until I can spend some time on it.

[fechter]

You could have a relay that disconnects the charger from the capacitor bank at the same time it triggers the SCR. When you let off the footpedal, the relay reconnects the charger (through a resistor maybe).

Putting some inductance in series with the discharge circuit might help commutate the SCR. A few turns of your 8ga wire in a coil.

I have one of those big honking SCRs around somewhere. They are fairly common on eBay.

[RLT]

....
Putting some inductance in series with the discharge circuit might help commutate the SCR. A few turns of your 8ga wire in a coil.
...

Wouldn't that rob some of the welding power? maybe even a significant amount?

I've seen those reactance coils for welders made out of high frequency aircraft generators
(like this:

which I assume is sort of the same idea as you mention.

I dunno.... We are getting into stuff that is a step or two above my brain being able to grasp easily.

[fechter]

Wouldn't that rob some of the welding power? maybe even a significant amount?

Good point. From what I've read, you need to get the wiring resistance as low as possible.

Is there any data on what the optimum starting voltage for the caps is?

Seems like you could use a higher voltage and a smaller capacitance to get the same energy. Too far toward either extreme will run into problems. I just wonder what the best range is.

You could use one of those solid state relays on the input to the charger to kill the output when you discharge so the SCR can unlatch. With the right timing, you'd be able to zap a weld and recharge almost immediately depending on the current output of the charger.

[Beagle123]

Thanks RLT:

That's really excellent information. It would be great if someone made this setup and helped others with their batteries. I'd do it myself except I don't have batteries to weld now. However, I do have a good power supply.

[RLT]

...
Is there any data on what the optimum starting voltage for the caps is?

Who knows what the REAL data is on those car audio caps. I haven't been able to find anything that I'd trust. There is so much exaggeration in their marketing and in their manufacturing specifications, that you really can't trust anything. Since they are making products intended for consumers rather electronics engineers, marketing hype substitutes for precise specifications.

Seems like you could use a higher voltage and a smaller capacitance to get the same energy. Too far toward either extreme will run into problems. I just wonder what the best range is......

I've been trying to remember what voltage I used on the one I built 15 years ago... I think it was somewhere between 24 & 30V. but I'm a LOT happier with how this new setup works. (Although the main functional difference may just be due to using copper instead of tungsten electrodes).

I think that somewhere between 15 & 16 V is optimum for this kind of setup, but the way it is now at 14.2v (the adjustment limit on the power supply I'm using now) gives consistent, solid and neat welds that I am VERY happy with.

Years ago, I worked in an aerospace repair facility for a while, and the commercially made industrial one they had there that I got to play with when no one was looking, was, (if I remember correctly), adjustable between 30 & 40 V. I doubt that it was more than .3 or .5 Farad, (my estimate based on the size of the unit and the available caps at the time.) And I think that it was adjustable between about 100 watt/seconds and 225ws.

So, yeah, increasing voltage should allow you to use smaller caps; But I don't know what is optimum. There must be some kind of trade- off, and the type of materials you are trying to weld must be a significant factor in choosing voltage.

(you are making me work harder than I planned here and math is not my best friend)either

So, if the formula I just found ( ws=F/2 X V²) for converting volts & farads into watt seconds is right and my failing memory isn't too far off:
:
.3 to .5 farad works out right , if you factor in losses due to resistance, inductance etc:
.3/2 X 30² = .15 X 900 = 135ws
.3/2 X 40² = .15 X 1600 = 240ws

Which means that the setup I have now should be (if the farad specs are not vastly over rated) putting out about 140- 150ws.
1.5/2 X 14.2² = 151.23ws

Looking more closely at the Sunstone Engineering CD welder, it looks like they are using 16.5 V as a maximum in their units ...
http://sunstoneengineering.com/site/pages/spDatasheet

I wonder if they are using car audio caps too?

You might have led me to something important:
Since I haven't been able to spot weld to aluminum with my CD experiments yet, I'm wondering if it might be possible to make a cheap CD welder for doing the aluminum cans of A123 cells by using just a couple of 220,000 mfd higher voltage capacitors charged at around 40 volts. Have to try that if I can find the caps without sending a fortune, and rewire the secondary of a microwave transformer for the power supply.

You could use one of those solid state relays on the input to the charger to kill the output when you discharge so the SCR can unlatch. With the right timing, you'd be able to zap a weld and recharge almost immediately depending on the current output of the charger.

Well, I thought about that, but the residual voltage in the power supply caps would just add to the timing problems if you cut the input, rather than the output. Of course If I just use a transformer and bridge rectifier rather than a regulated power supply, cutting the input would simplify things.

[RLT]

.....It would be great if someone made this setup and helped others with their batteries. .....

My situation is too screwed up now to offer to do stuff for other people; But if some 'respected' member wants to help out other members here by providing this service 'gratis' or at least really cheap, I'd be willing to make and give them the basic wiring,the SCR / footswitch setup, the electrode/handpieces) ... pretty much everything needed other than the power supply/charger, the capacitor and the nickel strip.

Of course the whole point of this exercise was to come up with something that any of us could make and use ourselves, for not much more money than shipping batteries back and forth would cost

[Beagle123]

I'd be interested in building a battery rig, and helping them put-together their packs. I just need the capacitor and one of those really good scrs RLT has. I already have a 10+ amp power supply that handles 50 A.

Let me know.

[fechter]

I was looking at the Sunstone unit too. Doing the math, I think you're right about the voltage. The car audio caps look like they'd be in the right ballpark, but a bunch of surplus smaller ones in parallel might be cheaper. Their larger unit used something like 2.4 farads.

If the charging current isn't too astronomical, you could use a relay to disconnect the charging supply (dc) when discharging. I looked up some commutation circuits for SCRs, but they are all a bit complicated. One used a second SCR to shunt the first one through a smaller capacitor and coil.

[RLT]

I'd be interested in building a battery rig, and helping them put-together their packs. I just need the capacitor and one of those really good scrs RLT has. I already have a 10+ amp power supply that handles 50 A.

Let me know.

Sure, Eric, I'll send you an SCR and whatever else you want; (except the capacitor... I'm short of them myself).

PM me with your address.

[YodaKrawler]

so then the scr that i just got has a red and white wire coming out of the cable side i was wondering if one of those wires is the negative and one the positive for the activation switch? this is a pic of it. thank for any info on this.

<a href="http://www.putfile.com/pic.php?img=7124762" target="_blank"><img src="http://img2.putfile.com/thumb/11/32122060434.jpg" alt="Click to enlarge"></a>

[RLT]

I would need a closeup photo of where the small wires actually go into the SCR body to tell you for sure.

[YodaKrawler]

10-4 i will have to get you that thanks.

[fechter]

The one I have has a little diode symbol on the side indicating polarity.

The other way is to look up the number and find a datasheet.

SCRs come in both polarities, so you do need to look it up.

[YodaKrawler]

well just looking at the pic it looks like the red wire is attached to the big cable and the white is attached to the white collar. but when it gets here i will look and let you know. and i have tried to google and yahoo it and no instruction or data sheet unless you guys have a great place to look thanks