Amps From A Car Battery

[col3]

I am trying to explain what happens if a person touches the negative terminal or metal on the vehicle with a wrench
while touching the positive terminal on a car battery when both are connected.

I know it can heat up to the point of melting the wrench or metal on you, giving you serious burns,
Am I correct in saying this is because the current is so much higher, with practically no resistance?

Can someone tell me approximately how many Amps would be going through that wrench,
say compared to how many Amps travel through the load in the vehicle when it's running,
And what is the formula to figure that out?

Thanks,
Col3

 

[Resqueline]

You are perfectly correct in warning about that risk. The important rule to keep in mind is to disconnect the negative first, and to connect it last.
A car battery has a 12V EMF, and max 0.01 ohm internal series resistance.
Shorting it flat out (with no resistance) results in a current of 12/0.01=1200A. This heats the battery internally with 12x1200=14400W. It won't take long before "she blows"..
Now, I don't know how much resistance there is in a wrench (I can find out) but lets say it's also 0.01 ohm. The shorting current will then be 12/0.02=600A. The power developed in the wrench is 600x600x0.01=3600W (4.9hp). Try to stop the crankshaft of a fully throttled 5hp motorcycle engine with your bare hands (what hands?)..
A car alternator can deliver somewhere between 60 and 120A max (800-1600W=1-2hp). A car cig. lighter draws 4A (50W). A car starter draws somewhere between 150 and 600A depending on engine size & type.

 

[col3]

Thanks for the great reply mod!

I would love to know how much resistance a wrench has as well!

Col3

 

[Resqueline]

Ok, just measured a Belzer Streamline Vanadium Extra No 1953 1/2" + 9/16" and it turned out to be around 0.85 milli-ohms (or 0.00085 ohms).
That's far less than I'd have guessed - as steel is a poor conductor, but as the wrench grows hotter the resistance increases - which in turn increases the power generated in it.
The battery will be fully shorted & will heat real fast with a high risk of blowing up.
Initial wrench power: 1200A x 0.00085 x 1200A = 1200W = 1.6hp which is quite enough for a quick burn.
Red-hot power: 960A x 0.00255 x 960A = 2300W = 3.2hp which means it's soon to be white-hot.

 

[(*steve*)]

Just out of pure curiosity, how did you measure that?

 

[Resqueline]

Apply a constant current of your choice (0.1A, 1A, or even 10A if you have) to the ends of the wrench (using crocodile clips). Set your DMM to the lowest mV range and measure end-to-end on the wrench itself (not too near croc's). Divide the reading with the current.
This is called a Kelvin (or four-point) measurement and completely negates any contact resistance - allowing for micro-ohm accuracy & resolution.

 

[col3]

Excellent help Resqueline!

Now I would like to tell my readers just how hot that wrench will be if it has 1200 watts of electricity running through it, I'm sure its in thousands of degrees, but 2000, 4000 or more?

Do you have the formula to calculate that?

Also would it be an appropriate comparison to say touching a 25 watt light bulb (120 volts) gets too hot to touch in just a few seconds, how much hotter metal at 1200 watts?

Now am I correct in your calculations, the first would be with 1.02 volts, the second with 2.4 volts?
And if so how does that compare to 25 watts at 120 volts?

thanks for bearing with me!

col3

 

[(*steve*)]

If you look at the data Resqueline gave previously, it indicates the wrench is well on its way to white heat.

Almost certainly it will end up melting. Look up the melting point of steel.

The question is, will the wrench melt before the battery explodes, and what will melted steel do to the battery? Will the battery posts melt before the wrench?

I'm pretty sure that these are the sort of questions you'll only get a theoretical answer for since a) there are many variables, and b) it's the sort of thing that doing it in practice may get you a Darwin award.

 

[col3]

great points Steve!

Thanks

col3

 

[Resqueline]

Many wrenchers have escaped unharmed with just a little sparking (oops..) but it's a close call every time.
If it welds stuck it will surely melt & start a fire, the question is if it heats up too fast for you to react & get your hands off it before your skin is fried stuck to it.
Oh, and welder currents are often in the 40-90A range.
I forgot to measure the weight of the wrench, but it's "easy" math to find out how fast it heats, just look up the specific heat enth(whatever) for steel and plug in the numbers in an appropriate formula..

It's a little hard to compare this with lamps, but dare the non-believers to wrap their hands around a linear 500W lamp and turn it on.
Anyone who has ever tried to brake the shaft of even a 50W electric motor with their bare hands will know just how quickly it burns you.
The only thing that matters here is the wattage and the mass involved. Try to put a 22-gage wire across a car battery.. It will literally explode, instantly.

 

[col3]

Well, I did a Google search on how quickly steel heats up and it led me to a welding forum!

Before I join and ask there, what keywords should I use form the "specific heat enth" of steel to find a chart that would tell me that?

I also take it, a much lower gauge wire would probably not explode putting it across battery terminals.

This is fantastic information!


Thanks

col3

 

[Resqueline]

I looked up the term now and it's simply "Specific Heat Capacity" or just "Specific Heat" for short. Steel has a value of 0.45J/K*g
(J=Joules=Power[W]*time[seconds], K=temperature difference in kelvin (which has the same steps as degrees C), g=mass[grams]
I won't be able to measure the weight of that wrench until tomorrow, but let's assume it's 100g.
Applying 1200W for 1 second = 1200J. Divide that with 0.45 and 100g and you get 27 degrees C (=48'F) of temperature rise for each second.
Now, that was "disappointingly" low, not quite as much as I had expected, but most of the heating takes place in the handle where it's thin & has lower mass so I guess the number will be a little higher in practice. Still it seems you do get just enough time to react & let go before you burn you fingers.
Please do check up on my math yourself, I have been known to make a mistake on (rare) occations..

The wire I once "tested" was 10" of 24 awg. I'm not quite sure how thin you'll have to go before it becomes less spectacular (depends on length).

 

[col3]

When I used a Fahrenheit/Celsius converter 27C was 80.6F (a little more than 48F)

I weighed my Craftsman socket wrench (what I use to loosen battery terminal connectors), with my food scale and it was about 220g
using your calculations it would heat up at 55F a second.
However, my guess is that the handle is about 1/3rd the weight of the wrench or 73g, and more than likely that would be the part of the wrench that would touch metal and heat up first.

This would cause the handle to heat up at 97F a second, giving a person less than a second to release his grip on it to avoid serious burns, considering it only takes two seconds to receive 3rd degree burns at 150F, ( I assume the wrench would already be 50-70F)

Does this sound correct or are there other factors needing consideration?

*How large would the wrench have to be to deplete the battery before burning your hand?


Thanks
Col3

 

[Resqueline]

My reference to the temp's were relative (difference/delta), not absolute, so my numbers still stand.
Otherwise it sounds correct. I'll measure a real big wrench. It's bigger but longer, might only take longer to heat up.

 

[col3]

I believe the calculations would be the same, but would you know, would there be any difference in the effect if you touched a metal part of the car as opposed to touching the negative and positive terminals at the same time?


Thanks
col3

 

[Resqueline]

How can you claim the calculations to be the same when 1ºC = 33.8ºF and 1ºF = -17.2ºC whereas 1ºC difference = 1.8ºF difference, according to the formulas ºC=(ºF-32)*5/9 and ºF=ºC*9/5+32.

Yes, there would be a difference due to the resistance of the wiring and the thickness of the metal being touched. How much depends on what car it is etc., but the wires would likely melt & burn, and thin sheet metal could turn red hot, possibly before the wrench got hot. Either way great damage and a fire is the likely result.
Countless alternators have also been fried due to head covers etc. touching the positive post on the back of it, producing sparks & transients - killing it.
So sound thinking can only result in the decision to undertake the quick job of disconnecting the negative battery terminal before working close to heavy gage wiring, but I guess macho minds doesn't relate to that kind of thinking..

Oh, and a 1-1/4" + 1-1/16" Belzer Streamline Vanadium Extra No. 1953 has a resistance of only 0.4 milli-Ohms, is 34 centimetres long and weighs 650 grams. The smaller wrench I mentioned earlier weighs only 76 grams and is 16 centimetres long.
It seems to me that increasing size wrenches quickly becomes less hazardous with regards to hand burns.

 

[col3]

How can you claim the calculations to be the same when 1ºC = 33.8ºF and 1ºF = -17.2ºC whereas 1ºC difference = 1.8ºF difference, according to the formulas ºC=(ºF-32)*5/9 and ºF=ºC*9/5+32.

I was referring to the calculations of the temperature of the wrench touching metal and positive terminal compared to the wrench touching the negative and positive terminal.

So I guess you would have a more severe burn quicker touching terminal to terminal than pos. terminal to metal because of less resistance of the battery terminal.

I'm curious if anyone manufactures a wrench with rubber over the handle to prevent this type of accident and injury.

Thanks Res!

 

[Resqueline]

You do get rubberized wrenches, but that's for better grip & comfort, not for protection against stupidity..

 

[allabakash]

instylelife

hi i need more information about this
---------------------------------------------------------

 

[pleasedontfront]

Could always try using some heat shrink insulation in the wrench?