Why Does Jump Starting the WRONG way work?


#1

I’ve read the expert advice – when jump starting a car the RIGHT way to do it is to hook the cables from positive to positive, and then from the negative battery post on the good battery to a ground on the dead car – not to the negative battery post on the dead car.

But I’ve tried this twice on two different dead cars and it didn’t work for me. No signs of life – dead car didn’t crank at all.

Over my howls of protest someone moved the jumper cable from the ground on the dead car to the negative battery post on the dead car, and it started right up, ruining my rant on the proper way to do this.

What am I doing wrong? I’m does the ground I attach the negative cable too have to be somewhere special on the dead car, or will any big chunk of metal do? Help!

Thanks,

Glen


#2

The reason it works is you are making a complete circuit. The only reason to do it the “proper” way is for safety–the negative battery terminal is grounded to the car’s chassis, and by making the last connection to the chassis, you’re avoiding a spark at the battery which can ignite flammable hydrogen gas that lead-acid car batteries produce naturally. In extreme cases, a spark can ignite the gas and cause a car battery to explode, causing injury or at least destroying the battery and showering everything nearby with acid.

The reason it didn’t work when you did it the right way is likely because you didn’t have a good connection where you attached the final cable to the car’s chassis. Where you connected the cable may have been painted, corroded, or it may have looked good but actually been isolated from the chassis. Or the car itself may have had a bad grounding strap from the engine to the frame or battery to the frame. Modern cars have so much plastic under the hood that it’s hard to find a good ground. Look for a solid piece of metal coming from the engine block, such as the bracket that holds the alternator or other accessories, or a brace that runs across the engine compartment. Wiggle the jaws of the clamp around to make sure you have a good “bite” on it and have scraped through any grease or paint on your final connection point. Also make sure that all connections are solid and will not pop loose. (don’t wiggle the connection to the battery around after everything is all hooked up, make sure that one is tight first) Also, don’t immediately try to start the disabled vehicle–give the discharged battery 5 minutes or more to build up some charge first before trying. Turn off electrical accessories on the donor car to maximize the available power. Leave the key off, door shut, etc. on the disabled vehicle until you’re ready to start so you’re not wasting the energy you’re trying to get the low battery to take. You’ll have a better chance of success and it will be less strain on the donor car’s electrical system.

I’ll admit often connecting the last connection to the battery too if I can’t find a good ground on the car’s body. I always stand far from the battery, stretch out my arm, and turn my face away when doing so. Which is generally a good idea even if you’re doing it the proper way. I also have a pair of safety glasses I wear when working on or around a car battery.

Car batteries are an old, safe technology, but mishaps do occur sometimes. Car battery acid is dilute enough on a discharged battery that it will not generally harm skin if you wash it off immediately, but eyes are a very different story.


#3

Fantastic response (and in record time!) Thanks!


#4

I think @Oblivion said it all. Instructions with cables don’t say WHY you have to do it the way they say. They don’t want to scare anyone I guess.


#5

Some cars have a designated negative post located away from the battery for jumping.
It will be mentioned in the Owners Manual, but who ever looks at that?


#6

I admit I often hook the cables to the terminals of both batteries, but I’d never do that if I was connecting to an old (possibly leaky) battery or in an enclosed space with limited ventilation. I go through the motions of looking for a ground away from the battery, looking for that telltale spark, but oblivion is right that finding a ground on a modern car often isn’t easy.


#7

When I was in the USAF stationed in Illinois, it was well below 0 degrees. Someone jumped a battery directly and the spark caused the battery to explode. The acid scarred their face and blinded them. I’ll never forget that. I taught my 3 kids and my wife the correct way to do it and told them why.


#8

My wife’s BMWs have had terminals under the hood for jump-starting because the battery is located in the back of the car. I wish all cars had this feature, even those with the battery under the hood. In fact, it would be even better if these terminals had some kind of standardized connector that couldn’t be connected backwards. I know this would add a bit of extra weight, but I think jump-starts are still common enough to justify the weight.


#9

The wrong way works better if the battery doesn’t explode. If you install the jumper cables the right way you may have to wait a while to allow the dead battery to charge up a bit.

Way back in the gloomy past there was a hoist fitting on the engine block and I would attach the ground jumper there. This would enable the current to go through the big cable from the negative terminal which was connected directly to the engine block instead of the little cable from the negative terminal to the body/frame. More power to you!

I used to play Russian Roulette by doing it wrong. Ignorance was great when the battery didn’t pop.


#10

Did you hook up the negative jumper cable to a ground on the chassis or on the engine block. The starter ground is on the engine block and the big battery cable from the battery negative post goes to the block for this reason. The chassis to engine ground is a small cable so if you hooked the jumper cable to the chassis or body, you don’t get a good circuit.


#11

My car also has the battery in the trunk. It has terminals under the hood for jumping, but I’ve never had to jump it or jump anything from it. It’s a nice arrangement–the battery is protected from temperature extremes in the trunk and lasts longer. It also helps the car’s weight distribution. So far I’m still on the 8+ y.o. factory battery.


#12

Back when I was 19, I connected the cables the “wrong way”. The battery exploded. Acid sprayed all over me and my face. I am so lucky it didn’t blind me.