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I agree with the above, it doesn't sound fail safe and wouldn't really be ok in health and safety terms
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I don't know why I missed this at the time.
I have known a couple of batteries erupt and it aint pretty, fortunately no one was hurt in the incidents.
So you have a solenoid, what is it? What's it designed for? Have you contacted the manufacturer to see if it's suitable?
A typical car battery has an internal resistance of 0.0001 to 0.001Ω depending on size, weight, quality, design criteria etc.
Do the sums for a direct short circuit... 12V/0.0001Ω = 120,000A to 12V/0.001Ω = 12,000A
Suitability of the solenoid... Let's assume the wiring adds another 0.001Ω to the internal resistance making 0.002Ω
12V/0.002Ω = 6000A and your solenoid is rated at 500A
Overload ... 6000A/500A = 12 ie there will be 12 times the amount of current flowing that the solenoid is designed for or to put it another way overloaded by 1200%.
I'll be generous and predict you get 5 welds out of it before it disappears in a bright flash puff of black sh1t
E
EddieM
Would it help if I pointed out the solenoid has a resistance of about 0.3 ohms? Surely this means it will only be delivering the same amperage as when cranking the engine and for a very substantially shorter time.
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The current that flows when cranking a car engine is determined by the resistance of the starter motor winding and the opposing back EMF generated by the rotating motor. The arc at the welding site will not generate any back EMF
Back EMF Electro Motive Force ...... roughly speaking this is a voltage that is generated in the windings of the motor. It is the opposite polarity to the source battery,
E
EddieM
Windings on a starter motor are about 0.05 ohms, cranking amperage is typically 300-400 amps.
A single coil resistance of 0.05 Ohms and 12 Volts applied results in a current of 240 Amps
E
EddieM
If it was 12v, typically 14-14.5v but anyway, even in the cobbled together scenario I described, you're not gonna be pulling much more than the cranking pull on the battery and for a fraction of a second. In other words, nothing is going to happen to the battery.
Other people have said that, I know only one who had serious burns from scalding hot sulphuric acid
It doesn't.
E
EddieM
Why? How can it pull appreciably more than a starter motor?
0.3 ohm resistance at 12 volts is 40 amps.
E
EddieM
Ah, yes, I see, thanks for explaining, well maybe have to use a transformer approach. I suppose you will be thinking why not get a proper tab welder, the answer is, the non pro ones (very expensive) are apparently rubbish. Maybe I'll just have the pack made up.
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To be fair as you mention the proper ones are rubbish and expensive, therefore if it were that easy to make a cheap, safe and reliable one using commodity components, it would have been done by now, as the demand is there.
Thinking about what might be stopping people, it has to be the safety thing.
Thinking about what might be stopping people, it has to be the safety thing.
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