Automotive Jumpstarting a dead car battery

[lucas_]

When using two cars to jumpstart a dead battery. Why can't you connect the negative jumper cable to the negative terminal of the dead battery (which is what I commonly do) but instead need to connect the negative jumper cable to grounded metal component?https://www.wikihow.com/Charge-a-Dead-Car-Battery
"Connect one end of the negative jumper cable to the negative terminal on the functional, charged battery. In most cases, the negative jumper cable is black.

Attach the other end of the negative jumper cable to a grounded metal component of the car that contains the dead battery. This will ground the car that contains the dead battery upon jump-starting. You can attach the ground cable to the frame, chassis, or another component that is reasonably clean and free of paint or oxidation. "

 

[anorlunda]

It is because there might be sparks at the connection point just before making the connection. Since lead acid batteries make hydrogen, we worry about explosion hazard. If there may be sparks, we prefer the sparks to be as far away from the battery as possible. At least, that is the way I learned it.

 

[AZFIREBALL]

There only two reasons I can think of: 1. To get you away from the battery when making the last connection in case the dead battery explodes. 2. To prevent a spark at the battery during the connection process, which might possibly ignite any hydrogen gas being expelled from the dead battery as it starts being charged by the good battery.

In the ’old days’ we always connected the two battery terminals directly to one another; negative to negative and positive to positive. However, I always tested for a very high current draw by ‘striking’ the last terminal to be connected to its counterpart. If there was a huge spark, while doing this, it would tell us there was a lot of current flow for some reason, and allowed time to look for the cause. There will be some spark because the good battery will start charging the dead battery when the connection is made but, it should not look like a welder.
Regardless of which method you use, it is always helpful to ‘strike’ the terminal to its intended location before making a permanent connection, just to get some idea how much current will flow. But, recognize you are taking a risk if you do this at the battery terminals! There might be hydrogen gas in the battery which could ignite.

 

[lucas_]

There only two reasons I can think of: 1. To get you away from the battery when making the last connection in case the dead battery explodes. 2. To prevent a spark at the battery during the connection process, which might possibly ignite any hydrogen gas being expelled from the dead battery as it starts being charged by the good battery.

In the ’old days’ we always connected the two battery terminals directly to one another; negative to negative and positive to positive. However, I always tested for a very high current draw by ‘striking’ the last terminal to be connected to its counterpart. If there was a huge spark, while doing this, it would tell us there was a lot of current flow for some reason, and allowed time to look for the cause. There will be some spark because the good battery will start charging the dead battery when the connection is made but, it should not look like a welder.
Regardless of which method you use, it is always helpful to ‘strike’ the terminal to its intended location before making a permanent connection, just to get some idea how much current will flow. But, recognize you are taking a risk if you do this at the battery terminals! There might be hydrogen gas in the battery which could ignite.

But the car metal is not connected to the dead battery negative terminal at all, how can it complete the circuit?

 

[AZFIREBALL]

The dead battery must be installed in the non-functional car as normally required, with the negative lead attached to the car's metal surface to provide a ground connection for all the operating systems within the car. If the negative lead is not connected between the battery and the car...the car will never start! It is always better to attach the negative lead of the 'jump-start' cables to the engine block somewhere, in case the engine-to-chassis ground strap is defective or missing.

 

[DaveE]

And you make that connection last. First connect positive to positive. This way if you slip and touch the car chassis there won't be a closed circuit yet. The last connection from negative to chassis is the one that will make a spark.
However, I wonder if this is a real concern. It's hard to collect hydrogen gas, especially since you've probably opened the engine compartment up to do all of this. Are there many real examples of hydrogen explosions, or is this a myth? It seems to me the real danger is in shorting the battery and "exploding" it that way.

 

[profbuxton]

As others have mentioned, the risk is igniting any hydrogen gas being given off by batteries. If you don't think it can happen there are probably enough examples on the net to convince you.
Lucas, I don't know what sort of car you drive, but let me assure you that the metal of the engine and car chassis is most definately connected to the negative of the battery. Unless of course you happen to have very old english car(model unknown) which did for a while have the positive to chassis(collectors will know which one).

 

[rbelli1]

It's hard to collect hydrogen gas, especially since you've probably opened the engine compartment up to do all of this.

Hydrogen that has accumulated in and around the engine compartment is not much concern. It will be a negligible amount and in the unlikely event you manage to ignite it it will burn off almost imperceptibly. As DaveE noticed it will float away rapidly on opening the hood and likely long before you open the hood.

Hydrogen trapped in and escaping directly from the battery is very much a concern. Igniting that can cause the battery to explode and spray sulfuric acid and shrapnel around. In general it might be a rare occurrence but there is essentially zero effort to mitigate the problem so why not?

BoB