How to jump a car

My husband says that when you jump a car, you should clip the negative clip to the car block instead of the negative terminal of the battery, on the battery that is dead. Otherwise, there could be a spark causing an explosion. Is this true?

Yes, it has happened. A battery gives off an explosive gas. When a spark occurs, such as when the battery jumper cable is being connected, that gas ignites KA-BLOOM! The result is explosive forces, shrapnel (of pieces of the battery), and battery acid on anyone near the battery.

Hmmm… Well, I’ve seen both methods used and haven’t seen an explosion yet. What my father always taught me was ‘Positive to positive, negative to ground’ which, if I am reading you right, is what your husband says.

The reason this is done is, if the battery you’re trying to jump start is expelling hydrogen gas, and if spark is created when connecting the negative jumper cable clamp, there’s less chance if igniting this gas causing a battery explosion.

If you’ve ever seen a battery explode from getting a jump start, from that time forward you would remember to make the final cable connection far from the battery.


Yes, that is the general rule, as long as the ground is somewhere away from the battery to be effective. Batteries give off hydrogen gas in small amounts, so usually there is no explosion. But you can’t be too careful.

That is the correct (and safest) way to do it.

No, it is not true. If both cables are already hooked up to the good battery you will get a spark in the weak car no matter where or how you hook up the cables to it. Even then there should be no risk of explosion. If the hood has been raised for a minute or so, any hydrogen that has outgassed is long gone.

There is no need to make heavy weather out of a simple jump start. Attach both terminals to the weak battery (or car block, it doesn’t really matter) and then attach to the good battery, observing polarity. (See the pretty spark! Harmless.) Now start your car.

As a side note, there is a true risk of battery explosion in very cold weather. The dead battery may be frozen. Here lies the true risk of the explosion during a jump start.

Although their is a small risk of explosion that is not the major reason for using the engine or frame as a ground. The dampening action of the frame or engine block helps to protect your electrical system from a voltage surge. It is also recommended after starting that you turn on your low beam headlights, heater blower and rear window deffoger if equiped to absorb the voltage surge from the alternator of the dead vehicle when the cables are removed.

All the reasons given are valid to some extent. It is good practice even if most of the time there is no real difference.

I thought the rule was that a battery being charged would give off hydrogen gas above the fluid in battery, occasionally getting through the vent holes. How can the dead battery, whose engine by definition isn’t running; be the battery w/hydrogen gas? If this is correct, and if we all agree that the final hokup of the 4 is the one that makes a spark; then doesn’t it make sense to hookup the dead battery last,

making its neg hookup last on a good ground away from battery, for the reason stated by DARTMAN? SteveF is right about a frozen battery; and this isn’t some wive’s tale. It’s in all the automotive textbooks I’ve seen. You shouldn’t jump it or charge it in a shop even till it’s thawed out. By the way, I know of at least 2 times where a battery has exploded and it was complete luck no one got hurt. One time the

customer was standing right there- how embarrassing! Afterthought- I can’t believe it’s not a crucial thing to try to prevent suphuric acid from spraying into your face!

I had a battery explode on me when I was about 18. It occurred as soon as I unclipped the cable and a spark went off. The explosion was so loud, it felt like a cherry bomb exploded in my ear. The whole top of the battery was blown off. Acid sprayed every where, including all over me. I felt so lucky I wasn’t blinded.

Batteries, car parts, and clothes with acid holes can all be replaced. You only have one pair of eyes.

JoeMario- right you are. Your post made me realize something I never thought of before: Not only do you get a spark during hookup of jumper cables (the last of the 4 hookups); you also get a spark removing cables- the first alligator clip removed will make a spark; so make the last conection the neg connection on the dead car on a good ground away from battery; and this same one for

the first disconnection when removing jumper cables. The only other way I could see a battery explode (besides a frozen battery) would be in DARTMAN’S scenario where you have a completely dead battery and you don’t turn on some electrical accessories after dead car is jumped and running. This battery has so little voltage to “push back” against alternator that the tremendous flow of

charging current through battery could just plain heat up batterry to point of explosion. I’ve always read about this scenario but never understood it- thanks DARTMAN! I’ve been a mechanic for 20 years and you learn something new every day. If you don’t it’s a wasted day! Afterthought: don’t let the 2 cars touch each other.

I had a battery explode on me once as well. I recieved a call from a customer that their car would not start. The customer claimed they had a dead battery. I told them I would be right over we would jump start it and take it to the shop. 10 minutes later I arrived at the customers house. I opened the hood on my truck and the customers LTD. Being in a hurry I connected both cables to the battery in my running truck then proceded to hook up the cables to the LTD. As soon as the cables touched the battery I disappeared in a cloud of battery acid. After a quick trip to the emergency room to have my eyes flushed out I returned to the customers home. Turns out the customer had tried to start the car that morning. When all he got was a click click click he drug out the charger hooked it up and turned it on. Since it was sprinkling (typical in Washington) he set the hood down on the LTD to keep the charger dry. 3 hours later the car still wouldn’t start so he called me. 5 minutes before I got there he removed the battery charger and lowered the hood. I feel really lucky I only have a few scars on my upper eyelids. It took three days for the ringing in m ears to go away. Now I am always carefull when it comes to jump starting and usually use a jump box with an on/off switch. I make sure it is off until connected and make sure to turn it off before removal.

Karl Sieger wrote:

I’ve been a mechanic for 20 years and you learn something new every day.

I’m always learning from the wealth of knowledge of the contributors in this discussion group. Thanks everyone for the time you put into all your replies. There is a lot of valuable information being shared.

You must be talking about suppressing voltage spikes. Connecting to the frame or block won’t make a bit a difference as far as voltage spikes go. You need a capacitor to suppress voltage spikes.

Another possibility of producing sparks is usually overlooked; after the jumper cables have been connected, power (sometimes) doesn’t seem to be getting through; so, what do we do? We wriggle the jumper clamps on the batteries to get better connections, and the engine/body. That wriggle causes sparks! Utt Oh!

Evel Knievel used to jump a lot of cars, but no more.

Couldn’t resist, could you?