jumping a car and the negative battery terminals

They always say when jumping a car hook up the positive to the positive but to hook the negative to something grounded (e.g. engine block) and not to the other negative terminal. That’s the way I learned how to do it, but I just watched a guy in the parking lot hook his negative to another negative terminal and the car started up just fine.

Isn’t there some safety reason for not hooking up a negative to another negative terminal? Couldn’t doing so damage one’s car?

Failing batteries sometimes outgas hydrogen. The last connection you make is the one that makes the spark, so making the last connection to the battery would be the one that makes the battery explode in your face.

Similarly, if you connect the cables backwards, or if you have a dead short in a failed battery, the heat generated can boil the electrolyte in the battery, causing it to explode.

In either case, the end result is the same. You make the last connection to the frame so that if the battery explodes, it doesn’t explode in your face. Getting sprayed in the face with battery acid and bits of plastic and metal moving at high velocities is generally something that most people regard as being rather unpleasant.

A lot of people connect both cables to the battery. Fortunately for them, battery explosions are fairly rare. They do happen on occasion though. Better safe than sorry.

Lead/acid auto batteries can give off hydrogen. The reason to make the last ground connection away from the battery is to avoid having a spark near a possible high concentration of hydrogen near the battery.

A guy I know had a new car, and on about the 3rd or 4th day, it didn’t start. It was completely dead. He lifted the hood, and wiggled one of the battery post. It was a defective battery, and the post was disconnected internally to the battery. Wiggling the post created a spark internal to the battery, ignited the hydrogen inside the case, and it exploded. The acid and other parts missed his eyes, but it did ruin his cloths. He got a new car from the dealer.

I was jumping a car just yesterday. I actually couldn’t find any frame metal near the battery to hook on to. Every reachable part was plastic, tucked under a plastic shield, or shaped such that you can’t hook a jumper cable to it.

It doesn’t need to be near the battery. Back in the old days, some folks would jump cars just by pushing them together so that the bumpers touched, but that was when bumpers were metal.

You can use any chunk of metal that you can find that is big enough to get a good connection on it, like a radiator bracket or the mounting bracket for the alternator.

ETA: Just make sure that whatever you use, you won’t end up getting the cable caught in anything that moves once the engine starts. In other words, if you use something like the alternator bracket, make sure that the jumper cable can’t get caught in the serpentine belt once the car starts.

The last car I had that I had to jumper routinely, I used one of the upper mounting nuts at the top of the front struts. Large, sturdy, and definitely grounded, as well as well away from the battery.

Because I surely couldn’t find an exposed ground in the vicinity of the block or accessory train that wouldn’t impinge on something after the engine started turning.

I can’t recommend enough one of those portable jumpstart kits. Saved me a few times when I was stuck at night with no one else around to help jump my car. They have them now small enough to fit in the glovebox.

Second. I was shocked when my BIL could not start the inboard engine in his boat after winter storage, but whipped out a portable jumpstart unit and then got it going in seconds. I bought a similar one that very afternoon. A friend of mine has a similar unit that charges from the accessory outlet in his car the entire time he’s driving.

And of course most of the ones with Lithium Ion battery’s can also charge your phone and tablet as well.

The Official “GaryM” of the Internet

Others have hinted at this, but just to be clear, what you want is for the LAST of the four connections to be away from the battery. If you were to do the negative connections first, using a grounded engine part, and then you made the last connection to the positive battery terminal, that would be just as dangerous as connecting them all to the battery directly.

Also, you only have to do that one one car. On the first car you connect on its negative side, that can be on the battery terminal. It’s just that the LAST of the four connections should be away from the battery.

I’ve always had bad luck trying to connect the last cable to the body. It just never seems to work.

I’ve always wondered why they don’t make a set of jumper cables with a switch in the middle so you could just connect all four ends to the two batteries and then complete the circuit by flipping the switch. This would also help
when you are trying to jump the battery by yourself – you wouldn’t have to worry about keeping the cables connectors from touching each other or the ground as you are stretching to get between cars.

I have searched through a lot of data from OSHA and other safety sites but I have found none that reports a Hydrogen Fire by connecting car battery terminals (negative terminal included).

It’s an extremely small risk that a modern sealed lead acid car battery will generate hydrogen in flammable quantities especially once discharged. And having worked with Hydrogen for a long time, I am sure that any Hydrogen evolved will diffuse away from the car - really really fast.

So, I challenge all that have posted the Hydrogen flame reasoning, to show a real documented case for this or do something like the myth busters do.

You probably have a million times more probability for dying from a car accident than lighting a fire due to Hydrogen from a battery during a jump start.

So - feel free to connect terminals. I do !

Here you go:

(it’s youtube so it might be faked, but it looks real enough to me)

I don’t know what the actual odds are, but they are pretty low. I personally have only ever seen one battery explode in my lifetime so far. Just about every car mechanic I have known has experienced a few of them though.

So maybe not a million times, but I will definitely concede that it’s a low risk. I did say they were rare in my first post.

By the way, if you want to go all Mythbusters and try this yourself, you’ll have a significantly easier time if you use a non-sealed battery type and remove some of the electrolyte so that there is more room inside the battery for the hydrogen and oxygen to build up. It will also help to use an unregulated battery charger, as the newer “smart” chargers will stop charging once the battery is full, leading to significantly less gas production. The more you mercilessly overcharge the battery, the more hydrogen and oxygen will be produced from electrolysis of the water in the electrolyte.

I have had 4 or 5 blow up in my face over the course of almost 50 years. I never witnessed a flame. The explosions always amounted to a face full of battery acid. I am not aware of being hit by any shrapnel.

Fully agree. If you see the YouTube video - he does all the things you mentioned - it is highly staged.

HoneyBadgerDC - do you remember if these were sealed batteries ? Also if they were fully charged or discharged (like a battery needing a jump start)?

In every case but one they were in a group of 4 truck batteries and were overcharged and over heated due to one defective battery in the group and yes they were sealed batteries but leaking. One case was in the 1960’s, not a sealed battery, I was a kid and don’t remember anything but hooking up the cables to jump start the car and it blew.

You’d expect the YouTube video to be staged-- Why else would he have had the camera running? The question isn’t whether it was staged; it’s whether the staging represents a realistic model of something that could happen in a non-staged situation.

Why can’t you make the last connection to the terminal on the good battery? Is that one also likely to have hydrogen buildup?