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Electrically, when I think of jumping my car, I visually see basically putting a stronger voltage source in parallel with a weaker voltage source (see https://i.stack.imgur.com/M0S72.jpg). However, on every car forum, they say to ground out the negative jumper on the engine-block of the dead car (see https://i.stack.imgur.com/LslIn.jpg)

All I hear is "there's gas that's formed when a battery dies, and there might be a spark that causes an explosion", that makes reasonable sense and all, but aren't we opening the circuit? Is the car's negative technically "ground"? (I reference this post: When a battery is your power source, what is ground?)

If we've defined (as described in the other post) the engine block as ground, is there actually a potential formed between the car and earth-ground?

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IDLacrosseplayer
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1 Answers1

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The battery's negative terminal is connected directly to the car's bodywork. The whole of the car becomes the equivalent of the negative terminal.

Connect the positive terminals together as per normal, then connect the negative of the good car to somewhere stable on the body of the dead car - somewhere where there is no paint - like the engine block.

The car body will link that through to the negative terminal of the battery, and if there is a spark it will be well away from the battery.

Another way is to take 2 sets of jump leads. Connect one set to one battery, and the other to the other battery, then link the jump leads together between the cars. Be careful not to let the + and - leads touch in the middle though.

Majenko
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  • Thanks! Can you also let me know why, if the whole body is part of the circuit, I don't shock myself when I connect my hand to the car, and my feet to earth-ground? Is there just not enough current for me to notice? – IDLacrosseplayer Oct 28 '11 at 19:51
  • There is no connection between + and ground, which there would need to be for the current to then pass through the ground, through you, through the car, and into the - terminal. Touching the + and the car body could be nasty though. – Majenko Oct 28 '11 at 20:03
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    @IDLacrosseplayer: there is insulation on your feet (shoes) and your car (rubber tires), so it's not dangerous. 12V isn't enough voltage to cause any issues anyway unless you really try hard to form a good conductive path (e.g. electrodes across your eyeballs or tongue, or with wet conductive pads right next to your heart -- please don't try any of these) – Jason S Oct 28 '11 at 20:11
  • @Majenko: Why would it be nasty to touch a 12 V battery? – endolith Oct 29 '11 at 01:07
  • Under normal circumstances, it is "fairly safe" to touch the + and - terminal of a car battery. The voltage is too low to overcome the skin resistance. In certain circumstances, however, it is possible to get a shock off a car battery. Even if this isn't enough to kill in itself, the actual act of getting a shock can be shocking enough to cause you to drop something heavy on your foot, for example, or worse, to cause a winch to drop something heavy on you - or for you to jerk backwards into something nasty. It is never safe to say that you cannot get a shock off something as it ... – Majenko Oct 29 '11 at 09:02
  • ... entirely depends upon circumstances. Cuts & grazes, protruding wires, sweat, humidity, etc., they all play a part. That is why if you stick a 9v battery on your tongue you can feel it. One of the worst ways that a car battery can injure you by shocking you is by causing a minor shock that causes you to drop the metal tool that you are holding, thus shorting the + to the - terminal. This can short circuit the battery causing a massive build-up of hydrogen and oxygen gasses at a rate that is far in excess of the ability of the venting system to get rid of it, thus causing the battery ... – Majenko Oct 29 '11 at 09:05
  • ... to explode causing massive injuries and even death. So, touching the + and the car body *could* be nasty. (note the use of *could* not *will* to denote that the circumstances have to be right) – Majenko Oct 29 '11 at 09:06
  • @Majenko: Shorting the battery is certainly bad, especially if you short it with something like a metal ring on your finger. But I'm skeptical that touching it would ever cause problems, even with broken skin. – endolith Oct 31 '11 at 13:50