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I built a spot welder that is powered by a car battery. The battery is 12V and 100amps. But unfortunately, it is a little bit too powerful, because it burns through the nickel strips and also through the battery really quickly most of the time.

So I would like to know what is the easiest way to reduce the power/voltage of the battery just a little bit? Will a voltage divider work in that instance? I added the picture of the spot welder.

It is really simple, no buttons nor switches:enter image description here

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    \$\begingroup\$ Get a weaker battery? Thicker metal strips? \$\endgroup\$ – Passerby Aug 16 '17 at 1:28
  • \$\begingroup\$ What are you trying to spot weld? Are you sure 100 A is all you need? Most spot welds for thin metals ( .1 -.5 mm) like battery straps require in the range of 1 - 1.5 kW for 400 mS or more. The safest way to do this is to charge some large capacitors and dump those into the spot weld. Trying to get small pulse sizes from a battery directly (even with a ballast resistor) is unlikely to give reliable results. \$\endgroup\$ – Jack Creasey Aug 16 '17 at 2:18
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    \$\begingroup\$ Use a 6V battery ("motorcycle") instead of a 12V battery? Or use fewer of the 2V cells instead of all 6 (since what you're doing is already inherently very very dangerous, why not open up the box of flammable/explosive hydrogen gas and caustic sulfuric acid -- what could go wrong?) Seriously, you will already need to take standard welding precautions like protective eyewear, gloves, well-ventilated outdoor area with shower / first aid / help available if things go south. \$\endgroup\$ – MarkU Aug 16 '17 at 2:19
  • \$\begingroup\$ @MarkU Dangerous, yes! \$\endgroup\$ – Mike Waters Aug 16 '17 at 2:49
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    \$\begingroup\$ That picture looks like it should accompany a Darwin Awards application. \$\endgroup\$ – Matt Young Aug 16 '17 at 2:49
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12 volts is WAY TOO HIGH!

I once worked for a firm (RADCO in Toledo, Ohio) that built and refurbished resistance welding equipment. The voltage at the tips was less than 2 volts, but the current was very high. I recall one of the transformer's secondary rating was about 1.6 volts and 100,000 amps!

Forget about using a battery, friend.

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    \$\begingroup\$ sounds like the Zirc-steel tube fusion welder's used where I worked in '70's... 4V, 100kA \$\endgroup\$ – Sunnyskyguy EE75 Aug 16 '17 at 2:21
  • \$\begingroup\$ That seems an extra ordinary power level.... 1.6 MW. \$\endgroup\$ – Jack Creasey Aug 16 '17 at 2:22
  • \$\begingroup\$ water cooled fireworks . transformer was taller than me. for nuclear reactor tubing, but in this case current is limited by spot size \$\endgroup\$ – Sunnyskyguy EE75 Aug 16 '17 at 2:24
  • \$\begingroup\$ @JackCreasey That voltage was applied to the water-cooled welding electrode for a fraction of a second. The supply was 480v 3-phase 800A. The lights in the entire industial parked dimmed! \$\endgroup\$ – Mike Waters Aug 16 '17 at 2:25
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    \$\begingroup\$ Stunning......, I used to design motor driven continuous feed welders in a long passed life, and I thought 1000 Amps was impressive. I will be forever humbled by both you and Tony. \$\endgroup\$ – Jack Creasey Aug 16 '17 at 2:29
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For the record, No, a resistor divider is not suitable for this kind of high-current application. When 100A flows through any resistor you can buy, it will instantly vaporize into stardust. The power (Watts) = resistance multiplied by current squared; and the temperature rise is closely related to power dissipation. Even with a 0.01Ohm shunt resistor, 100A requires dumping 100W of power out of the resistor.

A better strategy is to use either a transformer or a lower voltage source.

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  • \$\begingroup\$ he only needs to dump 1200W or so of energy, 1200W resistors are fairly common as heaters,,,, (not at 12V though) for 12V you could use a length of packing strap or steel wire. \$\endgroup\$ – Jasen Aug 16 '17 at 5:32
  • \$\begingroup\$ @Jasen So after the resistor heats up, how much power do you think will be left for welding? Not enough. \$\endgroup\$ – Mike Waters Aug 16 '17 at 14:37
  • \$\begingroup\$ @MarkU, you suggested using a transformer. As I know even less about transformers then maybe You can explain which tranformer is suitable for me. As transformers use AC then I cannot use my battery. If I would like to use tranformer then what power source could I use and which transformer? \$\endgroup\$ – Mark Aug 17 '17 at 1:31
  • \$\begingroup\$ As transformers use AC then I cannot use my battery. If I would like to use tranformer then what power source could I use and which transformer? \$\endgroup\$ – Mark Aug 18 '17 at 13:09
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Edit, just noticed how old OP is, sorry.

Using a surplus car audio "stiffening" capacitor of 1 farad or even 0.5 F (500,000 uF) and using a known value resistor to slowly charge the capacitor over a few seconds (maybe 10 ohm, 20 W?) You can use a voltmeter on the capacitor to monitor charge on the capacitor, when the charge reaches say 6 V, stop charging and try a weld on a piece of sample nickel first.

Adjust your charging time to vary the charge in the capacitor and thus how many joules are out into the weld.

In auto body collision when we weld new panels onto a car we sometimes use pieces of scrap metal from the old damaged pieces to do a destructive test on new welds to make sure the spot welder is set properly.

Many other ways to weld batteries, good luck.

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  • \$\begingroup\$ Perfect solution for diyers.. your method of energy metering is clever. \$\endgroup\$ – soosai steven Oct 31 '18 at 18:22
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This is going to be pretty much a link-only answer, and behind a paywall at that, but Digital Machinist magazine had a 2-part article last year on building a spot welder using a car battery as the source.

enter image description here

It uses a bunch of parallel semiconductors (MOSFETs or IGBTs, don't remember which) to control the power. Looks like a series inductor to control the current ramp as well.

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