Author Topic: kWeld - "Next level" DIY battery spot welder  (Read 53819 times)

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Offline JohnG

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #225 on: August 28, 2017, 01:00:55 AM »
Aha. I was wondering why you didn't use thyristors instead of MOSFETs, since they come in rather large packages that can handle several thousands of amps, while being relatively cheap.

Have you looked into forced commutation? Possibly a class D commutation setup might do it.
I had considered them, and there were several reasons why I decided against them:
- a 300A SCR would probably have done the job, these are ~30€ which is same cost as I have now
- I was concerned if I would be able to design a reliable forced communitation circuit - it it fails to turn off the SCR, then the fuse is the last hope to prevent something from catching fire
- the voltage drop of the SCR would probably be 2V @ 1000A, which is more than 10 times of what it is now
- they are much bigger and I wanted to integrate everything on a circuit board
- last but not least, my knowledge of MOSFETs is several decades higher than that of SCRs

At low voltage, it is very tough to beat a power MOSFET. Cost is way down from where it used to be, they are robust, and fully controllable. Any bipolar device (SCRs included) have a saturation voltage of a few tenths of a volt, at least. Since this is a pretty slow application, the large capacitance of multiple high-current MOSFETs should not be a problem.

I don't have a ton of experience with thyristors, either, but what little I have had made me think that they are a lot more finicky and less straightforward than they seem.

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #226 on: August 28, 2017, 02:32:56 AM »
I've designed one with a Microwave Oven Transformer and and another version on its way. There the switch on time is always controlled  with a dual SCR in a back to back configuration.

SCR or thyristors are basically used with AC voltages as they switch OFF at zero cross but when dealing with dc supply MOSFTETS do a better job.
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Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #227 on: August 28, 2017, 03:27:28 AM »
Does the wire need to be silicon?
I can find 10 and 16mm2 awg7 and awg5 i believe in flexible wire for in the power cabinet.
Haven't heard of silicon wires yet  8) Are they any good? :P That reminds me of a scene from Terminator 2: in the German translation, Arnie is referring to his high performance silicone CPU. The audience in that movie theater didn't catch why I had burst into laughter  :o

I chose silicone high flex because a) it eases electrode handling and b) it is 200°C rated and won't immediately burst into flames when something unwanted happens.

a 1500W 45A  digital VC and CC controlled power supply  8)
I dream of one of these  ^-^

If that deal is happening then I am seriously looking for some 5 or 6 supercapacitors, I am only not sure how many F's they should have ?  Any link besides Ebay or suggestion is welcome  :)
I have some Maxwell BCAP0310 P270 T10 here, but I haven't found the time to test them yet. Their DC ESR is 2.2 mOhm, I first want to see if that is enough or not, before I can recommend them. I'll keep you updated!

I also see some protection boards with loadbalancing and current limiting, what is that about? Since we do not want current limiting in this application is it safe to use supercapacitors this way or could they also explode as LiPos?
Supercapacitors don't like being overcharged, similar to Lipos. In a similar way, they need balancers when connected in series. For low current applications, that can be a simple resistor, but in our cases we need an active circuit. Typically that is a (e.g. 2.6V) comparator that switches a transistor which connects a resistor in parallel with the cell.

What they like even less is being operated at high temperatures. It is the same as for regular electrolytic capacitors: every 10°C more halves their lifetime.

I haven't heard yet that they can explode, and I have never made this experience. They rather tend to put other things on fire because of their extreme power density  ;)
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Offline Kjelt

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #228 on: August 28, 2017, 07:10:29 AM »
I chose silicone high flex because a) it eases electrode handling and b) it is 200°C rated and won't immediately burst into flames when something unwanted happens.
Ah yes and you solder that wire to the electrodes that will also need the insulation of the wire to withstand the higher temperature.
Well I found bigger silicone wire (16mm2 upto 120mm2) from HELU or LAPP and othere manufacturers at www.tme.eu , a bit pricy but available if needed to lower the resistance even further  8)

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I dream of one of these  ^-^
me too, I so hope that the deal goes through, a dream would come true.
He even has one with 120A  :o  but that one he wants to keep to jumpstart his wifes car, he says he puts it at 16V and can start the car without battery just on that one power supply.
A bit rediculous because the listprice of these supplies are >5k€ but he has gotten them cheap so yeah if he has a practical use for it who am I to judge but I know some people that read this will burst in tears.

Quote
I have some Maxwell BCAP0310 P270 T10 here,I'll keep you updated!
Great! I am looking forward of hearing the experience.

Quote
Supercapacitors don't like being overcharged, similar to Lipos. In a similar way, they need balancers when connected in series. For low current applications, that can be a simple resistor, but in our cases we need an active circuit. Typically that is a (e.g. 2.6V) comparator that switches a transistor which connects a resistor in parallel with the cell.
If I get this correctly if you have 4 supercaps in series and charge it with a controlled Voltage of 10V, still one of the caps might get more than 2.6V due to imbalances in the capacitors?
Than a balancer circuit is also on the shopping list  ;)
« Last Edit: August 28, 2017, 07:12:28 AM by Kjelt »
 

Offline ozwolf

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #229 on: August 28, 2017, 09:07:20 AM »
G'day,

I've been watching this for a while now.  I had a look at your shop and find "out of stock".

Any plans to assemble more kits?

Ozwolf
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Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #230 on: August 28, 2017, 03:10:57 PM »
G'day,

I've been watching this for a while now.  I had a look at your shop and find "out of stock".

Any plans to assemble more kits?

Ozwolf
I will definitely make new kits after having received feedback on this first batch, and eventually having included the one or other improvement or feature.
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Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #231 on: August 28, 2017, 06:11:30 PM »
Ah yes and you solder that wire to the electrodes that will also need the insulation of the wire to withstand the higher temperature.
The electrodes receive a fraction of the heat that is generated at the weld spot, I don't see that we can do anything against this. When welding at higher energies like 100J, they, the electrode holder, and the connecting cable will indeed get quite hot. I alreday have this on my improvement list, because I think that some form of cooling and/or a better heat insulation for your hands is advisable.

available if needed to lower the resistance even further
Be careful when further lowering the resistance. In this application you are basically short circuiting a battery, and there is nothing except battery ESR and the cabling that limits the current. (The power switch is negligible with <0.2mOhm) The welder will go into overcurrent as soon as 2kA is exceeded. And I haven't done robustness testing at this current level yet, only at 1.5kA. The stored inductive energy will almost double from that (E = L * I^2 / 2).

jumpstart his wifes car ... some people that read this will burst in tears.
|O :palm:

If I get this correctly if you have 4 supercaps in series and charge it with a controlled Voltage of 10V, still one of the caps might get more than 2.6V due to imbalances in the capacitors?
Exactly. There are two contributing factors to this. a) of course the capacity variation between them, b) variations in leakage current that causes them to self discharge.
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Offline Kjelt

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #232 on: August 28, 2017, 06:19:22 PM »
Thanks for the info I have to dig somewhat deeper in that balancing circuit because 4 or 5 caps are in series you can only charge them with the 40Amps when there is no or little resistance between them, if a balancing circuit uses a fet to switch a resistor across one capacitor the resistor should be very low ohms not to influence the charge current ?
If the resistor is too low it drains the charged capacitor, so not sure how this exactly works then. But that is offtopic so probably open another topic about that  :)
 

Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #233 on: August 28, 2017, 06:30:24 PM »
Thanks for the info I have to dig somewhat deeper in that balancing circuit because 4 or 5 caps are in series you can only charge them with the 40Amps when there is no or little resistance between them, if a balancing circuit uses a fet to switch a resistor across one capacitor the resistor should be very low ohms not to influence the charge current ?
If the resistor is too low it drains the charged capacitor, so not sure how this exactly works then. But that is offtopic so probably open another topic about that  :)
I don't understand that current limit? Is that maybe the current carrying capability of the balancer's PCB track that connects the cells? We need special treatment for them in this case anyway, because they will probably immediately melt in this application. A few strips of solder wick will solve that.

The FET / resistor combination is in parallel to the cell, so it does not influence regular charging. It's only purpose is to reduce charging current when the cell voltage gets too high. For example, if we want to charge the cells to 2.6V, then the balancer can kick in at just this voltage and make sure that the steady state voltage will not exceed this. It does not need to hurry here, and it can "work" with the remaining 0.1V.

Additionally, the balancer should have a beefy shottky diode in series with another resistor to remove any possible negative charge. This can always happen when deeply discharging them.
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Offline Kjelt

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #234 on: August 28, 2017, 07:04:35 PM »
Ah yes I understand now, I am a kind of person that likes to see it (in a schematic)  :)
Ofcourse the normal charge "chain" is not broken so there is still current flowing through the capacitors. Thanks  :-+
 

Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #235 on: August 28, 2017, 09:28:00 PM »
The other capacitor option that I want to investigate, has arrived: 30x 100F far east cheapo capacitors (http://www.ebay.de/itm/263097739655). I almost would have bet that they are fake or at least overrated, but I measured 113F capacitance and 13mOhm ESR per cell. Not too bad!

But for now I have to concentrate on the user manual...  ???
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Offline SvanGool

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #236 on: August 28, 2017, 09:33:23 PM »
Quote
13mOhm ESR per cell. Not too bad!

Don't forget that the ESR multiplies with the number of cells, so you are going to get app. max 200 Amps @ 13 mOhm.
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Offline Kjelt

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #237 on: August 28, 2017, 10:24:26 PM »
Quote
13mOhm ESR per cell. Not too bad!

Don't forget that the ESR multiplies with the number of cells, so you are going to get app. max 200 Amps @ 13 mOhm.
per string, so five strings would be 1000 Amps.  8)
 

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Re: kWeld - &quot;Next level&quot; DIY battery spot welder
« Reply #238 on: August 28, 2017, 10:26:10 PM »
Yea close to an MOT. Out of curiosity how much max amps does the exiting give out ?


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Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #239 on: August 28, 2017, 10:58:08 PM »
Quote
13mOhm ESR per cell. Not too bad!

Don't forget that the ESR multiplies with the number of cells, so you are going to get app. max 200 Amps @ 13 mOhm.
don't forget that the voltage also multiplies. So if one cell delivers 2.7/0.013 = 207A, then six in series will also do that. And I have five strings available that I can parallel.
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Offline tatus1969

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Re: kWeld - &quot;Next level&quot; DIY battery spot welder
« Reply #240 on: August 28, 2017, 11:05:20 PM »
Yea close to an MOT. Out of curiosity how much max amps does the exiting give out ?
You mean the welder plus the current Lipo? That is around 1500A. I'm not sure if an MOT delivers that?
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Offline SvanGool

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #241 on: August 28, 2017, 11:15:37 PM »
Quote
13mOhm ESR per cell. Not too bad!

Don't forget that the ESR multiplies with the number of cells, so you are going to get app. max 200 Amps @ 13 mOhm.
don't forget that the voltage also multiplies. So if one cell delivers 2.7/0.013 = 207A, then six in series will also do that. And I have five strings available that I can parallel.

That is what I said, but I didn't realize you were planning on using all the caps (5 strings) for one welder.  :)
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Offline tatus1969

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #242 on: August 28, 2017, 11:16:43 PM »
Quote
13mOhm ESR per cell. Not too bad!

Don't forget that the ESR multiplies with the number of cells, so you are going to get app. max 200 Amps @ 13 mOhm.
don't forget that the voltage also multiplies. So if one cell delivers 2.7/0.013 = 207A, then six in series will also do that. And I have five strings available that I can parallel.

That is what I said, but I didn't realize you were planning on using all the caps (5 strings) for one welder.  :)
ok, got it  :)
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Online anishkgt

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Re: kWeld - &quot;Next level&quot; DIY battery spot welder
« Reply #243 on: August 28, 2017, 11:51:32 PM »
Yea close to an MOT. Out of curiosity how much max amps does the exiting give out ?
You mean the welder plus the current Lipo? That is around 1500A. I'm not sure if an MOT delivers that?
Yea MOT (microwave oven transformer) delivers much more than that close 1800A. That is of course after rewinding the secondary with 2AWG copper wire.


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Offline SirJMD

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #244 on: August 29, 2017, 06:53:24 AM »
A rewired MOT could be a good power source. I don't know how AC will affect the weld though.

I've ordered a 5000mAh Lipo and some various stuff. Will see how this works and continue from there.
 

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Re: kWeld - &quot;Next level&quot; DIY battery spot welder
« Reply #245 on: August 29, 2017, 07:41:07 AM »
Using a MOT would not fit the purpose of the K-weld because can be very bulky and heavy. On the other hand it can deliver enormous amperes after rectification and proper rewiring.


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Offline Kjelt

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Re: kWeld - "Next level" DIY battery spot welder
« Reply #246 on: August 29, 2017, 07:44:10 AM »
Well the six supercaps and supply will also be beefy as well as a car battery.

BTW received the kWeld today looks very nice  :-+ and will start this weekend.
Also the psu deal is going through although the specs are different it is a 3kW psu 0-45V 0-70A  8)
But it has some small issues so I also need to do some TLC there.
« Last Edit: August 29, 2017, 07:47:20 AM by Kjelt »
 

Offline SirJMD

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Re: kWeld - &quot;Next level&quot; DIY battery spot welder
« Reply #247 on: August 29, 2017, 05:52:18 PM »
Using a MOT would not fit the purpose of the K-weld because can be very bulky and heavy. On the other hand it can deliver enormous amperes after rectification and proper rewiring.


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The purpose is exactly what the user want it to be. If I want a stationary battery welder, then I don't care about size or weight. If you want a small portable welder, that's your choice :)

I like the kWeld so far because of the build quality and functionality. Now I just need to find a good and realiable power source. I'm a bit worried that the continued hammering of the lipo packs might be asking for trouble.
 

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Re: kWeld - &quot;Next level&quot; DIY battery spot welder
« Reply #248 on: August 29, 2017, 06:06:10 PM »
Then the MOT would be a good choice here as a power source.


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Offline tatus1969

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Re: kWeld - &quot;Next level&quot; DIY battery spot welder
« Reply #249 on: August 29, 2017, 06:27:43 PM »
Then the MOT would be a good choice here as a power source.
We would also need a bridge rectifier that can handle these currents, as the kWeld can only switch DC currents.
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