What is the correct way to disconnect and connect a car battery

aston

The Torque Meister
Points
282
From
South Dorset
Car
VOLVO and VW
having seen a screwed car from someone changing a battery What is the correct order, Some say remove the negative first and replace last others say remove the positive first and last so does anyone here know the correct procedure, Thanks as always J
 

old-git

Moderator
Points
617
From
Essex
Car
Elan & Robin Hood
Ok, why is the order of cable removal important? Whichever cable you remove first will break the circuit. Hopefully I can learn something here. Never taken them off in a particular order and never had any problems.
 

HDi fun

TC ModFather
Points
637
From
Buckinghamshire UK
Car
Passat 2.0 TDi
Electrically an open circuit is an open circuit therefore I agree.

On the odd occasion I have had to jump start a car (thankfully not mine !) I connect +ve to +ve firstly. If you do -ve to -ve firstly then you will create a circuit if you inadvertently touch bodywork with the red lead.

But for this purpose I can't see the difference. A lot of workshop manuals cite disconnecting the -ve terminal before commencing work, presumably because the whole chassis is then isolated. But then again if the +ve is disconnected there is still no circuit.

Sometimes I think these things are prescribed simply for the avoidance of doubt rather than any technical benefit.
 
Last edited:

T9 man

TC Pro Founder
Moderator
Points
1,077
From
London, UK
Car
Saab 9-3SS T9
For OG,

Because if your negative is still attached to the chassis and you slip with a spanner while removing the positive with a spanner then you will create a short circuit with the path created. If the negative is removed first and you have this accident then a path will not be available for the short circuit to occur.
For this exact reason if you reconnect the positive first with your spanner then connect your negative afterwards and touch earth by mistake then nothing will happen and you or should I say the car electrics will be fine.
 

old-git

Moderator
Points
617
From
Essex
Car
Elan & Robin Hood
I have experienced that on occasion, T9 :) A good reason to remove negative first on a metal car, but not so important on mine :).

However, I was looking for a more scientific reason rather than just to prevent burning a hole in a spanner.
 

aston

The Torque Meister
Points
282
From
South Dorset
Car
VOLVO and VW
Well A friends (mechanic) 911 had a flat battery, His wife changed the battery for a good battery and it screwed all the clocks up, The whole binnacle had to be replaced, I did read than when you disconnect a battery that current still remains in the circuit (which makes sense) So it is a good idea to join the neg and pos leads together with a resister in between (12v lamp or something) to drain the circuit......
 

aston

The Torque Meister
Points
282
From
South Dorset
Car
VOLVO and VW
The battery does not supply the electrons in wire, they simply pump them, so disconnect the battery and you have simple disconnected the pump, the electrons remain in the wire just as fluid would remain in a pipe. Wires are not hollow pipes, they are pre-filled pipes.

A battery or generator is like your heart: it moves blood, but it does not create blood. When a generator stops, or when the metal circuit is opened, all the electrons stop where they are, and the wires remain filled with electric charges. But this isn't unexpected, because the wires were full of vast quantities of charge in the first place.
 

old-git

Moderator
Points
617
From
Essex
Car
Elan & Robin Hood
So, if you diconnected a 240v AC house wire from the mains and touched it, would you get a shock? No, you wouldn't, because there is no current flowing thorugh it. Same with DC.

I think you missunderstand how electrical current systems work. You can't store electrical energy (other than possibly static) in a wire like you can store fluid in a pipe. You need a battery or a capacitor.

Disconnect the battery then connect the leads to a volt meter - it will register 0v.

However, if there is a capacitor in the curcuit, then you can get a shock. Maybe this is what you are thinking about.
 

HDi fun

TC ModFather
Points
637
From
Buckinghamshire UK
Car
Passat 2.0 TDi
It's got to be a capacitor you're thining of. A battery is simply a high capacity polarised electrolytic capacitor.

Electrons form part of everything and a simple conductor cannot store electrical charge.

It's the movement of electrons detween the outer shells of atoms in the conductor that constitute electric current.

All the battery does is supply an electrical potential difference to cause the electrons to move in the wire.

Electrons carry a net negative charge so they actually move from negative to positive. But convention is that current flows from positive to negative.

Electrons are not actually physical particles, they are clouds of electrical charge. That said they can be proven to have mass. This is where the wave/particle duality issue arises, for some aspects of physics it's convenient to consider waves, for others it's convenient to consider particles. Prof. Stephen Hawking, amongst others, has devoted a lifetime of work into establishing a unified theory for such things.
 
Last edited:

SLEEPER

Pro Tuner
Points
542
The battery does not supply the electrons in wire, they simply pump them, so disconnect the battery and you have simple disconnected the pump, the electrons remain in the wire just as fluid would remain in a pipe. Wires are not hollow pipes, they are pre-filled pipes.

A battery or generator is like your heart: it moves blood, but it does not create blood. When a generator stops, or when the metal circuit is opened, all the electrons stop where they are, and the wires remain filled with electric charges. But this isn't unexpected, because the wires were full of vast quantities of charge in the first place.
A battery is not like a heart which is a mechanical pump . If it stops for more than a few seconds it cannott restart on its own. A battery can.

If you cut eirher of the two main battery wires there is no change loss so you can re connect it and the staus quo is restored.
If you cut a main artery .........

Back on topic with pedant hat firmly on :toung:

It doesnt matter which cable is disconnected first as far as the circuit is concerned .
But to lower the chance of possible short circuits one is best advised to disconnect the earth wire (some vehicles are positive earth )
 

old-git

Moderator
Points
617
From
Essex
Car
Elan & Robin Hood
What hasn't been mentioned is the issue of power dependant items. For example, is there still an issue of losing the radio security code and having to retype it in?

Are there any other power critical components in modern day cars that don't like the battery being disconnected? Not having one I wouldn't know :)
 

T9 man

TC Pro Founder
Moderator
Points
1,077
From
London, UK
Car
Saab 9-3SS T9
^^ There are actually gadgets on sale for just such a thing OG, apparently a small battery pack made up of AA batteries plug in to the cigarette lighter and back feed the radio and other essential memory settings. It will never start a car obviously but there is sufficient power to keep the memory going of some of the more volatile car devices.
 

HDi fun

TC ModFather
Points
637
From
Buckinghamshire UK
Car
Passat 2.0 TDi
You might not necessarily want your engine (and especially auto transmission) adaption parameters erased. In some cars these are held in volatile RAM by the ECU so it does make sense to provide an low current 12 volt 'service' via the 12v fag lighter socket. In most cars now these are hard wired and not switched by the ignition.

I had some fun with mine when the transmission was reset!! The part throttle kickdown became invasive and started dropping all the way to 1st at 20mph!! Very amusing :-D
 

OriginalVirgo

Wrench Pro
Points
23
From
Worthing
Car
Peugeot 206 S 1.1
I was given training in the week regarding this subject. A few of my colleagues argued about it to our trainer. We were all given safety goggles too. Very fetching.
We were advised, like many of you have stated, that red on first, red off last.
At the auctions we have a battery on a sacktruck, well 3 of them actually, to use day to day as many of the cars are flat as hell.
We were also advised to turn on the radio and lights before disconnecting the leads. Something to do with preventing a surge or spike ruining the ecu?
I love the fact you guys managed to slip a bit of physics into your discussion, where would we be without those leptons of love...
 
Top