Why do the instructions for removing a car battery tell one to remove the negative terminal first, and then replace it last?
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2 years ago
Removing wires from car battery terminals - Why -ve 1st
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2 years ago
Because that's the earth termimal. You are disconnecting the battery from the chassis.
If you disconnected the other terminal first, and dropped/touched it on the chassis, it could get very hot and sparky.
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- posted
2 years ago
Where would the power come from to make it hot? There would be nothing connected to the battery +ve terminal. Some of the electronics might not that happening if there are capacitors storing electricity.
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2 years ago
When 'spannering', if you touch the bodywork with the other end of the spanner while undoing the neg, no problem. Do this while undoing the positive first and you'll get a nasty surprise.
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2 years ago
There would be power from the charger or donor battery at the other end of the leads.
I've heard it said that the earth of the jump leads should be connected to an earthed part of each car that is some distance away from the battery (ie
*not* the earth terminal of the battery). This is because the rapid charging of the recipient battery as soon as power is supplied to it could cause hydrogen and oxygen to be given off by the battery. The hydrogen could burn explosively in the oxygen (or the oxygen in the air) if it was ignited by a spark. Hence connect the earth last and disconnect it first so the spark is at a point that is a reasonable distance from the battery.The hazard is not only the H-O explosion but the risk of the battery itself exploding - and battery acid is not something you want flying around. A teacher at my school had a big bald patch on his head. He explained that it was caused when a spanner dropped across the terminals of a car battery when he was doing some work on his car e few years earlier. The battery exploded showering him with acid. Fortunately he wore glasses so his eyes were shielded from the acid.
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2 years ago
Is the right answer. That said, for most cars these days the geometry is such that this is not usually a problem. Sometimes more of an issue in commercial vehicles.
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2 years ago
Exactly. the same would happen with anything Dc where a mass is normally connected to onne side. What I don't understand is why some modern cars do not have a way to store stuff like radio codes and settings if it takes more than a certain time to replace the battery. I overheard a couple of blokes a few gardens away have a whinge about that. Brian
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2 years ago
+1 and I always remove terminals in this order.
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2 years ago
Thanks for the replies, using a small spanner is likely to be safer.
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2 years ago
Sorry, yes. I meant the spanner while undoing.
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2 years ago
So that if your spanner makes contact with the car's body, there will no no voltage beween terminal, spanner or the body. Once the -ve is removed, there will be no voltage between +ve and the body.
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2 years ago
Why? You wouldn't do it with the engine running would you?
Bill
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2 years ago
Places that regularly replace car batteries usually have a special power supply to maintain the supply voltage during the battery change to overcome this problem. John
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2 years ago
Not if you remove the positive terminal from the battery and manage to let it touch the chassis. And few cars have the positive battery lead long enough to do that anyway.
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2 years ago
My cars (body) computer was confused after I reconnected that battery. fotunately it recovered after I had driven in about 50 yds.
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2 years ago
Try accidentally letting the spanner touch something metal on the vehicle whilst disconnecting the positive first, then you will know!
Mike
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2 years ago
Why not just do it in the sequence recommended?
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2 years ago
Especially if the spanner is in contact with a wedding ring that makes the contact to the chassis.
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2 years ago
I was not trying to suggest not doing thst. Some tools if you drop them might b able to cause a short circuit.
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2 years ago
A metallic watchband will cover a longer distance.
And the pattern it burns into your wrist is interesting. You can see the metallic links, an image of them, burned into the skin.
Someone at school managed to do that while working on his Mini.
Seeing an injury like that, works much better from an elfen safety perspective, than just reading of the possibility.
Paul