Another what size cable question

[EddieM]

I'm thinking of making a DIY spot welder, the power source being a 100 Ah car battery (12-14v).

The welder itself will pulse full current from the battery momentarily as it's effectively a short circuit.

What size cable should I be looking at, ideally I'd like it to be as small as possible as flexibility is a consideration.

TIA for any direction.

 

[flameport]

Think again.
Shorting out a car battery is dangerous.

Proper spot welders use a transformer, which by it's design will limit the current even under short circuit conditions.
They also use voltages far lower than 12V.

 

[EddieM]

Think again.
Shorting out a car battery is dangerous.

Proper spot welders use a transformer, which by it's design will limit the current even under short circuit conditions.
They also use voltages far lower than 12V.

Hmm ok, the pulse is a fraction of a second I.e. about 0.1

 

[flameport]

the pulse is a fraction of a second

Controlled by what?
What happens on the inevitable day when it welds itself closed?

 

[EddieM]

Controlled by what?
What happens on the inevitable day when it welds itself closed?

A 0.1 sec momentary contact switch operating a relay (solenoid)

 

[Gasguru]

You do realise you'll be trying to switch a 1000 Amps or more...

Spot welders normally use transformers with the timing done on the low current side.

 

[EddieM]

You do realise you'll be trying to switch a 1000 Amps or more...

Spot welders normally use transformers with the timing done on the low current side.

That's what the solenoid is for.

 

[Gasguru]

You need to understand the basics of current flow....

The current from a car battery is only limited by its own internal resistance and the resistance of the spot welding cables, the materials you're welding and any switch contacts.

This current will be several hundred amps and more.
You won't have any control over it.
Your solenoid switch will weld itself together.

That's why transformers are used and the low current primary side is switched (the secondary winding providing the high current for welding).

 

[EddieM]

You need to understand the basics of current flow....

The current from a car battery is only limited by its own internal resistance and the resistance of the spot welding cables, the materials you're welding and any switch contacts.

This current will be several hundred amps and more.
You won't have any control over it.
Your solenoid switch will weld itself together.

That's why transformers are used and the low current primary side is switched (the secondary winding providing the high current for welding).

Why? The load won't be going through the switch, otherwise, yes, I'd agree with you.

The solenoid is rated at 500 AMPS.

Just think of it as akin to starting a car, except the load isn't a starter motor, it's a 0.1 second pulse between two electrodes.

 

[Gasguru]

Draw a diagram, by the sound of it you are simply switching the output of a car battery directly through your spot welding fixture.
By load I assume you mean the spot welding materials...how is the current not going through the solenoid switch contacts?
Is this solenoid designed to switch so fast?

 

[bernardgreen]

Listen to the advice given, what you propose is extremely dangerous. The switching contacts controlling the welding current would need to be very large surface area pads AND forced separation ( an insulator is forced between the contacts to ensure they separate, this does not always work.

Think about it, the current is high enough to weld together two pieces of metal at the location of the spot weld, therefor the current is also high enough to weld together the two pieces of metal that are the switch contacts.

Boiling sulphuric acid erupting from a shorted car battery does a lot of damage and causes deep tissue burns if it reaches the skin.

 

[flameport]

The solenoid is rated at 500 AMPS.

Woefully inadequate, and unlikely that it will switch fast enough.

A starter motor isn't a short circuit.
Your spot welding deal is.

 

[EddieM]

Draw a diagram, by the sound of it you are simply switching the output of a car battery directly through your spot welding fixture.
By load I assume you mean the spot welding materials...how is the current not going through the solenoid switch contacts?
Is this solenoid designed to switch so fast?

OK a momentary switch is used to activate the solenoid (relay) this handles the load c 100 AMPS it's more than capable of that. Yes the solenoid will have a response time of 0.1 of a second. It only needs to spot weld 0.2mm thick nickel battery tabs, nothing major.

 

[Gasguru]

But what's limiting the current?

 

[EddieM]

But what's limiting the current?

Nothing apart from the battery.