Correct way to jump-start a car

[mointainwalker]

Hi

Have just watched a car magazine video which gave the following instructions for the correct method.

1) Connect red/live to battery of donor car and then same to "dead" car.

2) Connect black /earth to donor car battery and then same to an earth point in the engine-bay.

Then start engine of donor car.

Removal is reverse of above.

I think I have seen this sequence before touted as the "correct " method and don't understand the following.

1) Why not start with live of dead car and then to donor car. This ensures that you are not waving a live contact around that could potentially short on something ?

2) Why is it important to use an earth point rather than the battery earth which is handy ?

Why is this removal order important ? I would remove live from donor car first to cut power asap to the "jump " circuit and avoid shorting danger as in my preferred "connection " method.

 

[Doggit]

Now this is interesting, because I've never even managed to get the car to start by putting the negative lead on the engine, but the negative battery connector goes straight onto the engine or the car body, so putting it straight on to the engine near the starter, cuts down the distance for the charge to travel. I know the principle, but can never get it to work.

As to the order of fitting and removing the leads, I think fitting the live first is sensible, but I always remove the live first as well, and yes, I can see you're reasoning for fitting to the dead car first, and although I've never had an issue, I'll follow your suggestion in future.

Once the dead car has started, it's then producing electricity from it's alternator, so you want to remove any feedback issues as soon as possible. And you'd still be waving a live cable around whichever car you remove it from, as both are then live.

 

[Mottie]

When you make the final connection, there is likely to be a spark as you connect it. If the final connection is to the battery of the doner car, with that battery being charged and likely to still have gasses present near the battery, you could cause an explosion. A spark when making final connection to the earth point in the engine bay of the dead car will be much safer.

 

[mointainwalker]

You both make good points - thanks for the comments. I've just looked up what I think is a more informes site than the magazine I looked up earlier and it repeats the same procedure , but with explanations.

https://www.lifewire.com/dangers-of-exploding-car-battery-534782

 

[motorbiking]

We are worried about a bit of hydrogen in an engine bay full of oil and petrol ?

 

[Keithmac]

I saw a BMW battery explode so it's deffinitely feasible (been in the boot I assume all the hydrogen gas would have welled up rather than blowing away in the wind).

They go with a bang!.

 

[Burnerman]

Just to add.....when I have the two vehicles connected up, I run the donor vehicle at 2000 RPM or whatever for a couple of minutes before cranking the casualty over......if the engine isn't running, often such things like the radio code can be lost due to the high current draw.
John

 

[Mottie]

I saw a BMW battery explode so it's deffinitely feasible (been in the boot I assume all the hydrogen gas would have welled up rather than blowing away in the wind).

They go with a bang!.

Many years ago, I connected jump leads to a battery the wrong way. I had an old Austin J4 van with the battery behind the passengers seat. It occasionally overcharged so had a fair bit of gas around the battery area. As I made the final connection to the good battery......Bang! The gas exploded and all 6 caps blew off the battery and were ricocheting around the cab like champagne corks. How I didn’t get a face full of battery acid I’ll never know. It definately does happen.

 

[mointainwalker]

Motman , that is 40-50 year old technology. Any more recent examples ?

 

[mointainwalker]

Sorry, hadn't noticed the remark about wrong terminals.

 

[DaveHerns]

Same happened to me 40 years ago jumpstarting a Mini, not knowing the live cable had shorted to the exhaust underneath. Battery blew up in my face.

 

[bernardgreen]

I had a battery charger that was 12 or 24 volt The toggle switch was 12 -- OFF -- 24

Charging a 12 volt battery and it was gassing so I flicked the switch from 12 to OFF and took the croc clip off the battery termimal. BANG, Two cells exploded, the window of the shed was cracked and I was deaf. The switch was in the 24 volt position... My own fault for not checking the switch was in the OFF before removing the clip. Hearing came back a few hours later but one ear was noticably less sensitive.

 

[Mottie]

Motman , that is 40-50 year old technology. Any more recent examples ?

Nope. I learned from my mistake. What’s changed 'technology' wise? Still have lead acid batteries, still have alternators, still have jump leads and they still have warnings about naked flames on batteries because of the explosive gasses present. Are you saying we shouldn’t be careful with naked flames or sparks around batteries nowdays?

 

[DaveHerns]

I'm more concerned these days with the risk of spiking ECUs etc. I'm not sure I'd volunteer to use my car to start another. You read about turning on lights , HRW etc to lessen the risk but I'm not sure.

 

[Keithmac]

"Sealed" batteries still vent, don't think your safe because it's clased as a Sealed unit..