Battery question - car related

[srah1953]

When jump starting a car, why do they say to attach the negative to the dead car body rather than directly to the battery? As they are interconnected, I would have thought it made no difference.
Thanks
PS I'm sure there are car forums more appropriate for the question but I don't know any

 

[sailorman]

When jump starting a car, why do they say to attach the negative to the dead car body rather than directly to the battery? As they are interconnected, I would have thought it made no difference.
Thanks
PS I'm sure there are car forums more appropriate for the question but I don't know any

A lounge question
gets to power the starter rather that to a dead battery

 

[GrahamHR]

Assuming the positive lead was connected to the battery first, I'd have thought it's nore related to avoiding a spark near the battery when the negative lead is attached.

 

[vyv_cox]

Assuming the positive lead was connected to the battery first, I'd have thought it's nore related to avoiding a spark near the battery when the negative lead is attached.

I agree. There is surely no electrical reason. I have jump started loads of cars and always connected the leads directly to the batteries. There can be some issues with modern engine management systems and I have seen warnings about jump starting, but I don't recall anything about placement of the negative lead.

 

[Avocet]

I agree. I've never done it - I've always stuck it on the negative battery terminal or convenient earth strap. The last thing I want is damaged paintwork as it arcs to earth! It'll try and charge the dead battery anyway, unless you disconnect the negative terminal of the dead battery.

 

[pvb]

When jump starting a car, why do they say to attach the negative to the dead car body rather than directly to the battery?

It's a safety measure, to avoid the possibility of sparks near the battery.

 

[srah1953]

Thank you all.
It's been in my mind for a long time, ever since I read up on instructions how to jump start battery but having had occasion today to read manual going with my skoda, where it says this, I thought I'd ask the premier source for all knowledge.

 

[VicS]

Thank you all.
It's been in my mind for a long time, ever since I read up on instructions how to jump start battery but having had occasion today to read manual going with my skoda, where it says this, I thought I'd ask the premier source for all knowledge.

As said above it is to avoid the posibility of a spark igniting hydrogen

The important part of the sequence is that the last connection made, and the first broken, must be away from the battery.

The chances of of causing an explosion may be small but it can happen and does happen. The consequences of a battery exploding in your face would be very unpleasant

 

[pvb]

The consequences of a battery exploding in your face would be very unpleasant

Where's the "understatement" smiley when you really need it?

 

[wotayottie]

When jump starting a car, why do they say to attach the negative to the dead car body rather than directly to the battery?

As far as I am aware they dont say that, and there is no technical reason why they should say that anyway. To the contrary, unless you take care with the place on the body that you connected a jump lead to, you could end up with a high resistance path due to dirt, paint etc and more difficulty than you should have in jump starting.

If the car is dead, and you would not be jumping it if it werent, the possibility of any significant amount of hydrogen being given off is negligible. Most modern batteries are not vented anyway. The hydrogen risk is modern OTT H&S in my opinion - have you ever seen a small quantity of hydrogen go pop in the open air? Non event

 

[MrCramp]

As said above it is to avoid the posibility of a spark igniting hydrogen

The important part of the sequence is that the last connection made, and the first broken, must be away from the battery.

The chances of of causing an explosion may be small but it can happen and does happen. The consequences of a battery exploding in your face would be very unpleasant

It is better to be safe than sorry so I have allways done as VicS and pvb state. I have seen the aftermath of a battery that a youth tried to jump start by connecting the last lead to a battery, the battery exploded and youth got badly burnt with battery acid.

 

[Daydream believer]

As said above it is to avoid the posibility of a spark igniting hydrogen

The important part of the sequence is that the last connection made, and the first broken, must be away from the battery.

The chances of of causing an explosion may be small but it can happen and does happen. The consequences of a battery exploding in your face would be very unpleasant

When i was a teenager a battery exploded about 20 inches from my face.
Only quick reaction from my mother who shoved me face down in a bowl of washing up water saved long term injury to my sight
I still had bandages over my eyes for several days though plus bruise from where she caught my head on the damned taps

 

[VicS]

As far as I am aware they dont say that, and there is no technical reason why they should say that anyway. To the contrary, unless you take care with the place on the body that you connected a jump lead to, you could end up with a high resistance path due to dirt, paint etc and more difficulty than you should have in jump starting.

If the car is dead, and you would not be jumping it if it werent, the possibility of any significant amount of hydrogen being given off is negligible. Most modern batteries are not vented anyway. The hydrogen risk is modern OTT H&S in my opinion - have you ever seen a small quantity of hydrogen go pop in the open air? Non event

As I said the chances of an explosion are small The injuries caused if a battery does explode 2 feet in front of your face may well be very serious indeed and it is so so simple to avoid the risk altogether.

Its not modern H&S. Its a risk I have been aware of for decades. I remember an in-house warning being circulated long before the rise of the current H&S.

Yes Ive seen hydrogen go pop in open air many times...... a variety of interesting demos are performed in school science labs.
A mixture of hydrogen and oxygen, as would be the case with a battery, in an enclosed space is going to be something else.

If thats your considered opinion I suggest you keep it to yourself

 

[NickRobinson]

When I was in the trade I had a truck battery on a trolley as a starter pack. I managed to blow that up. It had exposed lead straps on top of tar sealed cells.

BTW, I always clamp my jumper start lead earth directly to the engine block, shortest path and usually something large like a manifold to give the clamp a good contact.

N

 

[GrahamHR]

Yes, I always do the same; after connecting the positive jump start cable I connect the negative lead to the engine somewhere. It eliminates any potential effect from a poor engine to car bodywork connection