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

[sevensages]

TL;DR 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?

 

[berkeman]

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.

 

[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.

That explains why one would want to disconnect the GROUND cable on a car battery before disconnecting the ground cable. But that does not really answer my other questions on the thread.

The mechanic acted like if I disconnected the positive cable first, that disconnecting of the positive cable first would AUTOMATICALLY cause damage to the battery and sparks to fly. You seem to be saying that if the positive cable was disconnected first, there would have to be an accidental connection from the exposed positive terminal to any part of the engine compartment to cause a short circuit. What if I disconnected the positive cable and left the positive cable laying somewhere where the positive cable would not touch any part of the engine compartment?

Edit: I initially made a typo in the first sentence of post #3. berkeman's post explains why would want to disconnect the ground cable first.

 

[sevensages]

If there was a short circuit, why would a short circuit cause sparks to fly?

I'm not an engineer or a scientist like many of you. Please be gentle.

 

[berkeman]

If there was a short circuit, why would a short circuit cause sparks to fly?

In the making or breaking of the short circuit of the battery, you will generally get some sparks. If you ignore the sparks and still continue with the short circuit connection, you can have the memorable experience of the battery exploding and covering your face in battery acid. Not a good experiment to conduct.

I think the main issue is more that often access to the battery is tight (especially in motorcycles), and if you leave the negative cable connected to the battery there is just too much risk of you accidentally contacting the positive terminal of the battery with one of your metal wrenches as you work in the engine compartment.

 

[berkeman]

The mechanic acted like if I disconnected the positive cable first, that disconnecting of the positive cable first would AUTOMATICALLY cause damage to the battery and sparks to fly.

No not automatic sparking. Just the risk of an accidental wrench contact with the positive terminal and some other metal part of the car/motorcycle engine compartment.

 

[berkeman]

BTW, the most dangerous time for sparks and shorting out the battery is when you are using your box wrench or socket wrench to disconnect the positive terminal first. If the wrench contacts any metal part of the engine compartment while doing that, you can short out the battery. Not so if you disconnect the negative terminal first (who cares if you short the negative terminal to the metal in the engine compartment -- they are already connected...

 

[hutchphd]

Once you understand it, its a no-brainer. Disconnect the chassis ground first. There is no downside.

 

[AZFIREBALL]

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 think the problem with removing the positive terminal at the battery terminal first is as follows.

While removing the positive terminal the metal wrench being used might come in contact with metal of the car. If the battery contained a charge, this might cause a lot of sparks as high current (several 100 amps) travels through the wrench to the car’s metal frame which is being used as a ground. This could cause the voltage in the system to momentarily drop to zero or cause a great fluctuation in the voltage. This might cause damage to the computer or its settings. I have heard, some cars require a small 12 volt battery to be attached between the two battery cables before changing the battery to provide a continuous small current supply to the computer. I do not know if this is true or not.
Removing the negative terminal first will prevent this from happening.

 

[DaveC426913]

Thanks all. Y'all may have saved the life of Future Dave, the next time he has to fiddle with his boat or car batteries.

 

[anorlunda]

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.

 

[berkeman]

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

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).

 

[Bystander]

the ring touches any part of the car body.

Nearly picked up my Darwin that way in high school greasing the steering on the family wagon...still shows the marks on my class ring.

 

[dlgoff]

@berkeman

I was thinking that the sparks mentioned were from the positive cable to the positive post. If that was the case, then the sparks is because there are items turned on in the auto. My two cents worth.

 

[Bystander]

are items turned on in the auto.

Clocks in the fifties, today? What isn't on?

 

[berkeman]

@berkeman

I was thinking that the sparks mentioned were from the positive cable to the positive post. If that was the case, then the sparks is because there are items turned on in the auto. My two cents worth.

True enough; when connecting the final battery cable you will generally see a small spark, but that's okay since it's just inrush into the normal load. The big fireworks when we accidentally brush the metal material around the battery when (incorrectly) loosening the postitive battery terminal first, not so good...

 

[dlgoff]

you will generally see a small spark,

I guess he didn't say how big the sparks were. Maybe @DaveC426913 can clarify.

 

[DaveC426913]

Maybe @DaveC426913 can clarify.

He can't. He wasn't there.

 

[Bystander]

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

..., or, "self-taught" ... experience/hard knocks.

 

[dlgoff]

He can't. He wasn't there.

Oh sorry. I meant @sevensages. So @sevensages, can you say how big the sparks were? See post #17 above.

 

[Baluncore]

True enough; when connecting the final battery cable you will generally see a small spark, but that's okay since it's just inrush into the normal load.

That inductive spark you see when making the final connection is high voltage on the B+ wire. That wire leads to where the voltage sensitive semiconductors are connected. Make sure everything in the car is turned off before making or breaking the battery connection.

 

[Borek]

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

There is more to it than just the electricity - any injury typically means swelling, which makes removing jewelry impossible (or at least very difficult).

Which is also a reason to not wear any jewelry during sport activities.

 

[dlgoff]

I just remembered that my '07 Jeep has a positive terminal insulated cover meant to protect it from touching the Jeep's body parts.

 

[berkeman]

I just remembered that my '07 Jeep has a positive terminal insulated cover meant to protect it from touching the Jeep's body parts.

Yeah, that helps to guard against dropped tools near the battery (do not ask me how I know this), but not for when you are actually wanting to disconnect the battery with a wrench.

 

[sevensages]

Oh sorry. I meant @sevensages. So @sevensages, can you say how big the sparks were? See post #17 above.

There were no sparks. The mechanic stopped me from removing the positive cable first and instructed me to remove the ground cable first. So I removed the ground cable first, and there were no sparks.

 

[dlgoff]

There were no sparks. The mechanic stopped me from removing the positive cable first and instructed me to remove the ground cable first. So I removed the ground cable first, and there were no sparks.

Never mind then. I thought you were asking why you go sparks when removing the positive cable.

 

[sevensages]

In the making or breaking of the short circuit of the battery, you will generally get some sparks. If you ignore the sparks and still continue with the short circuit connection, you can have the memorable experience of the battery exploding and covering your face in battery acid. Not a good experiment to conduct.

I think the main issue is more that often access to the battery is tight (especially in motorcycles), and if you leave the negative cable connected to the battery there is just too much risk of you accidentally contacting the positive terminal of the battery with one of your metal wrenches as you work in the engine compartment.

Although some people have given me some information that answers some of the (many) questions of the OP, I still don't feel like I fully understand this. The main question of the OP is: Why would disconnecting the positive cable first cause sparks to fly?"

I suppose it is arguable that berkeman answered that main question of the OP by answering something like the following: Disconnecting the positive cable first can cause sparks to fly if the positive cable touches any part of the engine compartment or frame because it would make a short circuit.

But I still don't fully understand this because I don't understand why a short circuit would cause sparks to fly. I asked "if there was a short circuit, why would a short circuit cause sparks to fly?" Berkeman responded with the quoted text above (in post #5). Your post does not tell me why a short circuit would cause sparks to fly. You just said "In the making or breaking of the short circuit, you will generally get some sparks."

In other words, I asked "Why would a short circuit cause sparks to fly?"

You basically responded, "In making a short circuit, there will be sparks." Yes, but that does not tell me why a short circuit would cause sparks.

Since the question "If there was a short circuit, why would a short circuit cause sparks to fly?" is just a continuation of "Why would disconnecting the postive cable first cause sparks", I did not feel like it is appropriate for me to create a new thread about this, but maybe I should have.

Should I create a new thread if I want to discuss this question? I know that creating a new thread about a tangent is preferable to hijacking another thread to discuss a tangent, but I thought that my subsequent question is such a continuation of the main question of the OP that it is not really a tangent. Please correct me if I am wrong.

 

[sevensages]

Never mind then. I thought you were asking why you go sparks when removing the positive cable.

I am asking "Why would there have been sparks if I had went ahead and removed the positive cable first?"

 

[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 am not even 100% sure precisely what short circuit means.

When line 2 on a circuit is connected to line 1 without a load in between, is that a short circuit?

 

[Baluncore]

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.

 

[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