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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.sstatic.net/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.sstatic.net/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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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.

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  • \$\begingroup\$ 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? \$\endgroup\$
    – IDLacrosseplayer
    Commented Oct 28, 2011 at 19:51
  • \$\begingroup\$ 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. \$\endgroup\$
    – Majenko
    Commented Oct 28, 2011 at 20:03
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    \$\begingroup\$ @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) \$\endgroup\$
    – Jason S
    Commented Oct 28, 2011 at 20:11
  • \$\begingroup\$ @Majenko: Why would it be nasty to touch a 12 V battery? \$\endgroup\$
    – endolith
    Commented Oct 29, 2011 at 1:07
  • \$\begingroup\$ 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 ... \$\endgroup\$
    – Majenko
    Commented Oct 29, 2011 at 9:02

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