Does it matter order of battery disconnection in a car?

[Srah 1953]

I'm studying electrics and battery theory at the moment. The question relates to a battery in a car. I'm sure I read somewhere in my "investigations" that there was an order in which leads from a car battery should be disconnected to avoid, I thought, an electric shock. But when I think about it I can't see how it matters. If I disconnect the positive lead first and say touch it to the car metal, how can there be a shock as it's no longer connected to the battery? It seems to me the same as if I touched the negative terminal with my hand. As long as I don't touch the positive terminal at the same time I won't get a shock. Similarly, if I disconnected the negative terminal first and touched it to metal, there won't be any reaction as it is no longer connected to the battery. In either case, it seems to me, that once one lead (whichever) is disconnected from the battery, there is no longer a circuit, but I feel I'm missing something.
Am I wrong?
Thanks

[Wotname]

In a car, always disconnect the negative lead first (i.e. the one going to the metal chassis of the car).

The reason is that if you accidentally touch the chassis with the spanner while you are removing the positive lead and the negative lead is still connected, you will create a short circuit of the battery - from the positive terminal though the spanner to chassis to the negative terminal. With the negative terminal removed, this cannot occur.

EDIT: Of course if you have a very old car with a positive ground, the reverse is true.

[Srah 1953]

Thank you for this. I couldn't figure out where the risk came from. It's the spanner touching metal and positive terminal at same time.

[Wotname]

Correct
And it can cause serious damage; to the spanner, to the battery, to the chassis and especially to the spanner holder!

[GILow]

Originally Posted by Wotname

Correct
And it can cause serious damage; to the spanner, to the battery, to the chassis and especially to the spanner holder!

And to the wallet.

[thomm225]

Originally Posted by Wotname

Correct
And it can cause serious damage; to the spanner, to the battery, to the chassis and especially to the spanner holder!

But usually it just scares the crap out of you when it happens.

As a teenager changing batteries from my car to my boat this happened quite often as I was usually in a hurry. Plus many times I didn't have the proper wrench and the oversized ones easily would hit ground/metal and cause an arc. Also, I had to do this with the car running to charge up the dead battery coming off the boat...…..so dropping the wrench near the fan was even worse

All this because the alternator on my outboard didn't work.

[model 10]

Also; current flow in a battery is from - to +. You can get a spark from the + term to the + lead, so disconnect the - lead first. If there is a spark, it will be smaller because the load has used up any juice in the system.

[mikecambrai]

Always disconnect the negative terminal first. It has no voltage and is therefore harmless. If you then disconnect the positive it again is harmless because there is no negative across which it can arc.
The big reason for disconnecting the negative first, particularly on a boat, is that in a confined space you can get a build up of hydrogen across the terminals. One spark and the battery will blow, and you can not only be defended by the noise but can also get splashed by the acid.
It is good practice to blow across the top of the battery to remove any hydrogen before you disconnect anything from the battery.
When reconnecting the terminals, you should always connect the positive first and finally the negative.

[waterman46]

Originally Posted by Wotname

In a car, always disconnect the negative lead first (i.e. the one going to the metal chassis of the car).

The reason is that if you accidentally touch the chassis with the spanner while you are removing the positive lead and the negative lead is still connected, you will create a short circuit of the battery - from the positive terminal though the spanner to chassis to the negative terminal. With the negative terminal removed, this cannot occur.

EDIT: Of course if you have a very old car with a positive ground, the reverse is true.

Right! But if you have a battery disconnect switch on starting battery and another one for house battery, those can both be set to the disconnect position, which will make it safe to work on the electric system as well.

[Kalinowski]

Always black first. Black (b) comes before R (r) in the alphabet.

[waterman46]

Originally Posted by Ecos

Also; current flow in a battery is from - to +. You can get a spark from the + term to the + lead, so disconnect the - lead first. If there is a spark, it will be smaller because the load has used up any juice in the system.

Hmmm. I think you will get the same small spark when connecting/disconnecting either + or - leads at the battery if the other battery lead is still connected at the time. The spark is caused by very small loads which are always connected to the battery even when you think you have everything turned off. Modern electronics in cars draw a small amount of power all the time and some probably have capacitors and inductors as filters, internally on their power inputs. Capacitor charging during connect and inductor current interruption on disconnect will cause that small spark.

[wiekeith]

Originally Posted by waterman46

Hmmm. I think you will get the same small spark when connecting/disconnecting either + or - leads at the battery if the other battery lead is still connected at the time. The spark is caused by very small loads which are always connected to the battery even when you think you have everything turned off. Modern electronics in cars draw a small amount of power all the time and some probably have capacitors and inductors as filters, internally on their power inputs. Capacitor charging during connect and inductor current interruption on disconnect will cause that small spark.

+1

If there's enough current being drawn to cause a spark it matters not which lead you disconnect first - you'll get a spark.

However, I do agree that it's best to disconnect the -ve first as that prevents an accidental short when undoing the +ve, as has been said by others.

[Srah 1953]

Originally Posted by mikecambrai

Always disconnect the negative terminal first. It has no voltage and is therefore harmless. If you then disconnect the positive it again is harmless because there is no negative across which it can arc.
The big reason for disconnecting the negative first, particularly on a boat, is that in a confined space you can get a build up of hydrogen across the terminals. One spark and the battery will blow, and you can not only be defended by the noise but can also get splashed by the acid.
It is good practice to blow across the top of the battery to remove any hydrogen before you disconnect anything from the battery.
When reconnecting the terminals, you should always connect the positive first and finally the negative.

Good advice. Thanks

[AZ-Oldguy]

Originally Posted by Srah 1953

Thank you for this. I couldn't figure out where the risk came from. It's the spanner touching metal and positive terminal at same time.

I had a friend who touched the car frame with the spanner and the spanner to his wedding ring and the wedding ring to the battery post and the wedding ring turned instantly white-hot. Fortunately it was not too tight and he was able to jerk his finger out and only had permanent scarring, superficial.

[GILow]

Originally Posted by mikecambrai

Always disconnect the negative terminal first. It has no voltage and is therefore harmless. If you then disconnect the positive it again is harmless because there is no negative across which it can arc.

The big reason for disconnecting the negative first, particularly on a boat, is that in a confined space you can get a build up of hydrogen across the terminals. One spark and the battery will blow, and you can not only be defended by the noise but can also get splashed by the acid.

It is good practice to blow across the top of the battery to remove any hydrogen before you disconnect anything from the battery.

When reconnecting the terminals, you should always connect the positive first and finally the negative.

Sorry, but this is just weirdly wrong. The order of connection and disconnection simply does not matter. Either way can cause a spark. It is, after all, an electrical CIRCUIT.