Why would disconnecting the positive cable first on car battery cause sparks?

[AZFIREBALL]

Summary:: Why would disconnecting the positive cable on a car battery before disconnecting the ground cable on a car battery cause damage to the battery and sparks to fly?

When I was 18 years old, I worked at an automobile repair shop doing oil changes and other light mechanical work. The automobile repair shop hired me for this work to free up their real auto mechanics to do more advanced automobile repair work. One time I was assigned the task of replacing a defective car battery with a new car battery. I got a wrench, and I started loosening the bolt holding the positive cable on the defective car battery. I was going to first remove the positive cable on the car battery, and then I planned on removing the ground cable after I removed the positive cable. The mechanic supervising me stopped me before I could remove the positive cable. The mechanic said that if I removed the positive cable first, sparks would have flew everywhere, and all sorts of disastrous results would have ensued. I'm not 100% sure, but I believe the mechanic also said that if I removed the positive cable first, it would have damaged the car battery and maybe other equipment on the car.

Why would disconnecting the positive cable on a car battery before disconnecting the ground cable on the car battery cause damage to the battery and sparks to fly?

I suppose that if one disconnects the positive cable on a car battery first, there would be an electrical circuit between the car battery and through the car from the battery to wherever on the car's frame that the other end of the ground cable connects to. I don't definitely know that there would be an electrical circuit between the car battery and through the car from the battery to wherever on the car's frame that the other end of the ground cable connects to. It is just my supposition. Why is that my supposition? Otherwise, why would sparks fly without an electrical circuit?

Am I correct that if one disconnects the positive cable on a car battery first, there would be an electrical circuit between the car battery and through the car from the battery to wherever on the car's frame that the other end of the ground cable connects to? If so, why wouldn't that circuit exist BEFORE the positive cable was removed?

If there is an electrical circuit if one disconnects the positive cable on a car battery first, why does the existence of that electrical circuit cause sparks to fly? How would that electrical circuit damage the battery of the car?

Having an exposed positive terminal with the negative still attached is just asking for an event!
Like throwing a bullet in a campfire.
Most of us tend to lay tools all around the open hood area as we are working with a difficult replacement like a timing chain or water pump. Think what might occur if you happened to reach up and lay a long extension bar mistakenly on that hot battery terminal while you are head-long down in the wheel well removing the water pump on a front wheel drive vehicle.

Best to remove that negative cable from its terminal!

 

[sevensages]

Why is everyone so reluctant to tell me why a short circuit would cause sparks? Is that classified information around here?

 

[sevensages]

Having an exposed positive terminal with the negative still attached is just asking for an event!
Like throwing a bullet in a campfire.
Most of us tend to lay tools all around the open hood area as we are working with a difficult replacement like a timing chain or water pump. Think what might occur if you happened to reach up and lay a long extension bar mistakenly on that hot battery terminal while you are head-long down in the wheel well removing the water pump on a front wheel drive vehicle.

Best to remove that negative cable from its terminal!

Not germane

 

[Baluncore]

Why is everyone so reluctant to tell me why a short circuit would cause sparks? Is that classified information around here?

A short circuit is when you connect the terminals of the battery, with a low-resistance shunt, such as a wrench. The energy appears at the contact point where high temperatures cause chemical reactions between the air and contact material that produces the sparks.

 

[sevensages]

You answered the question baluncore. Thank you

 

[sevensages]

A short circuit is when you connect the terminals of the battery, with a low-resistance shunt, such as a wrench. The energy appears at the contact point where high temperatures cause chemical reactions between the air and contact material that produces the sparks.

if a person connected the positive cable to the ground cable, would that also be a short circuit?

 

[Baluncore]

if a person connected the positive cable to the ground cable, would that also be a short circuit?

Yes. Any low-resistance connection across the battery terminals is a "short circuit".

 

[sevensages]

Yes. Any low-resistance connection directly across the battery terminals is a "short circuit".

Thank you. Youre doing good

 

[Borek]

Note that "a person" has much higher resistance than a wrench, so while we can talk about a "short circuit" chances of sparks are much lower.

Sparks are just a side effect of lots of heat evolving where the high current flows.

Edit: at least as long as we are talking about car voltages, when it comes to multi kV range things are getting more complicated.

 

[Mark44]

Why would disconnecting the positive cable on a car battery before disconnecting the ground cable on the car battery cause damage to the battery and sparks to fly?

Not so much an issue these days, but many cars of the 50s and possibly later had the positive battery terminal connected to ground. In the past I owned 11 cars that were manufactured in the 40s or 50s.

 

[Averagesupernova]

Not so much an issue these days, but many cars of the 50s and possibly later had the positive battery terminal connected to ground. In the past I owned 11 cars that were manufactured in the 40s or 50s.

6 volt systems were always positive ground. Some 12 volt systems were. I'm not sure of any US made vehicles that were positive ground after they switched to 12 volts. More industrial and heavy equipment stayed on positive ground after the switch to 12. Not sure for how long.

 

[Baluncore]

Why are so many being confused by the irrelevant polarity?
Disconnect the link between the chassis and the battery first.
Problem solved.

 

[Averagesupernova]

Why are so many being confused by the irrelevant polarity?
Disconnect the link between the chassis and the battery first.
Problem solved.

If I could like the above post about thirty times, I would. I don't get it either @Baluncore . What the heck is so difficult to understand about the idea of one node of a system being able to touch anything and make sparks versus the other node being able to touch anything EXCEPT ONE OTHER NODE and make no sparks.

 

[hutchphd]

6 volt systems were always positive ground.

I had a 66 VW beetle which was 6 Volt and, as I recall, negative ground.

 

[Baluncore]

I had a 66 VW beetle which was 6 Volt and, as I recall, negative ground.

With Kettering ignition, negative chassis was later preferred because the opposite polarity spark would more quickly erode the centre electrode of the plugs.

Early 6 volt VWs were congenitally night blind due to insufficient voltage to clean dirty connectors and switches.

Early VWs could reverse their polarity if the headlights were left on, then you push started the car. The generator and starter motor were universal.

 

[Averagesupernova]

I had a 66 VW beetle which was 6 Volt and, as I recall, negative ground.

To see that would be a first for me. Interesting you bring up the Volkswagen. I was at an auction yesterday that sold a handful of old bugs. Had these posts occurred prior to yesterday I would have looked. Lots of VW parts. A handful of dune buggies built from VW donors. I do know one of the buggies had a six volt battery.

 

[hutchphd]

I know that '66 was the last beetle that was 6V. It was my first car and taught me much. Eternal thanks to John Muir for a timeless book of philosophy and auto repair.

 

[sevensages]

I work with high voltages routinely at my EE job, and never wear conducting rings or watches. Luckily my wife is understanding about the silicone wedding ring (thankfully she has LEO in her background).

Does LEO stand for Law Enforcement Officer? If not, what does LEO stand for? How does LEO (whatever that means) make her understanding about having a silicone wedding ring?

 

[sevensages]

Another risk not negligible is that the positive post touches a ring on your finger and then the ring touches any part of the car body. You can lose your finger that way and the pain would be horrible.

Many mechanics are taught not to wear jewelry when working on engines.

It's my understanding that the reason that a person could lose a finger if the positive post touches a ring and then if the ring simultaneously touches any part of the car body, the electrical current would go from one terminal of the car battery to the negative cable and then through the ring and through the person's finger since it is touching the ring and then to the positive cable and back to the battery?

Is my understanding correct?

Here's one thing I don't fully understand: Assuming the person's finger is dry, wouldn't the person's finger have a lot more resistance than the resistance of the ring? If so, why would any significant amount of current go through the person's finger? I thought that the current goes through each conductor in proportion to the resistance of the conductor.

 

[sevensages]

The problem is that the negative terminal of the car battery is connected to all of the metal in the car and engine compartment. So an accidental connection from the exposed positive terminal to any part of the engine compartment can cause a short circuit. But if you disconnect the negative terminal, there is no short hazard from the positive terminal to any part of the car.

I just realized that if any person was reading this thread, and if the person was not knowledgeable about cars, there is probably a piece of information that they would not have that would prevent them from fully understanding this.

I never became a full-fledged mechanic. So I don't definitely know that to someone not knowledgeable about cars, there is a piece of information that someone reading this thread (before I made this post) would be missing that would prevent total comprehension and understanding of this. But I think there would be a piece of information missing.

Berkeman said that the negative terminal of the car battery is connected to all of the metal in the car and engine compartment. It's my understanding that the electrical return line on a car is the metal frame of the car. I think (don't definitely know) that that is why the negative terminal of the car battery is conneccted to all of the metal in the car and engine compartment. Am I correct that the electrical return line on a car is the metal frame of the car?

 

[hutchphd]

Is my understanding correct?

No. Your finger is a bystander. The battery can supply a hundred amps through the ring and it will get very hot very fast. Not good.

 

[sevensages]

No. Your finger is a bystander. The battery can supply a hundred amps through the ring and it will get very hot very fast. Not good.

Oh. So in the scenario in which a person removing a car battery was wearing a ring and the positive post touches the ring and then the ring simultaneously touched the car body, would the electrical circuit created cause a person to lose the ring finger by burning off the finger (as opposed to electrically shocking off the finger per se)?

 

[hutchphd]

I believe the burns cause the damage. High voltages (power lines) can burn you by the current conducted through flesh but not a 12V battery.

 

[anorlunda]

If so, why would any significant amount of current go through the person's finger?

No. The risk is that current goes through the ring, the ring gets hot, perhaps melts, while still on your finger.

 

[sevensages]

I believe the burns cause the damage. High voltages (power lines) can burn you by the current conducted through flesh but not a 12V battery.

I was about to say that your post quoted above is contradictory. But anorlunda's post #54 explained how your post ain't contradictory.