MIG welding batteries?

I was on the verge of building a capacitive-discharge spot welder to join a mess of NiCad batteries for cordless tools.
Then I got to wondering. And wondering. Whether a 90-amp MIG welder would substitute? Seems like 0.030" wire is pretty close to a spot weld...
Anyone ever try this or something similar?
I guess I could experiment, but preliminary cautions and advice would be appreciated.
Thanks.
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On 3/20/2013 6:35 AM, HeyBub wrote:

Capacitive-discharge gives you a limited and controlled energy to the weld. How are you going to do that with MIG? Maybe put a 5A fuse in the welding lead to control the time?
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bud-- wrote:

I guess I'll stop the weld when it seems done.
I don't think a 5A fuse would hold up long against a 90A current...
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Oren wrote:

No need. There must be 100 plans on the net.
The simplest seems to be the capacitive-discharge model. You charge up one or more capacitors whose combined capacity is in the neighborhood of 1 Farad (with a 12-24V battery charger) and discharge the pack through the substance to be welded.
There's more to it, of course. The switch is a Thyistron that can handle 2400Amps, and it, in turn is driven by a 12v wall-wart and a doorbell switch. The electrodes are copper rods, and so on. Parts, however, should be less than $100.
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I have a friend who welds with 2 car batteries in series. Affordable, works great, and requires no real equiptement other than 2 or 3 car batteries, some connecting wires and clamps and welding rods
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On 3/20/2013 10:38 AM, HeyBub wrote:

Lots of luck doing that welding on a NiCd.

That is the point. The 5A fuse can pass a much larger current but will open fast to limit the energy at the weld. It is a kludge to make a spot welder.
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bud-- wrote:

??? What's the problem. The terminal on the battery is, I think, nickle. Why would there be any unusual difficulty?

Ah! I see. How about a 5A circuit breaker? Surely something dramatic would happen when it sees 2,000A.
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On 3/21/2013 1:09 PM, HeyBub wrote:

Send a picture of your weld with a 90A MIG welder on a NiCd. Or better, a video.

2,000A from a 90A welder? And when using fuses (and circuit breakers) their "fault current" rating should be appropriate to the circuit they are used on.
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bud-- wrote:

Good idea. I will.

90A x 120V = 10,800 (momentary) Watts
10,800W / 5V = 2,160 (momentary) Amps
It could happen.
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On 3/22/2013 4:21 PM, HeyBub wrote:

90A is the secondary side amps.
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