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According to this answer to a question about car batteries:

[...] so the entire chassis is an extension of the minus terminal of the battery.

If this is the case, I would expect that you'd get shocked when you touch something metal on a started vehicle if you're grounded.

I've read that 10mA passing through the heart at a high enough voltage is enough to kill a human, but only if the voltage is high enough (which in the case of most car batteries is only 12V.)

Is the voltage simply too low to produce an electric shock?

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    \$\begingroup\$ Since nobody is saying it in the answer, current, even at very low voltages, can be deadly \$\endgroup\$ Sep 23, 2014 at 18:59
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    \$\begingroup\$ @ScottSeidman, But because the body has such high resistance in normal conditions, it takes a high voltage in order to pass enough current through the body for it to be damaging, right? \$\endgroup\$
    – Josh Beam
    Sep 23, 2014 at 19:01
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    \$\begingroup\$ If the resistance is high, the current is low. If the current reaches dangerous levels, it is dangerous, regardless of the voltage. \$\endgroup\$ Sep 23, 2014 at 22:09
  • \$\begingroup\$ I am one of the lucky persons that never got a shock when touching a car. But I know LOTS of people that usually gets them when trying to open their cars. I usually get shocks on some metallic stair handrails, and also touching the office chair, because there the carpet accumulates electrostatic charge. \$\endgroup\$
    – sergiol
    Feb 22, 2015 at 18:13
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    \$\begingroup\$ @sergiol - that shock is because of static electricity, not the battery. Some cars have grounding straps which prevents it. If you do get a shock it is a combination of the car having been driven (losing electrons) and you wearing synthetic material, possibly getting charged and whether your shoes are insulating. If I am wearing certain shoes, I get a shock from every door handle (house and car) \$\endgroup\$ May 22, 2023 at 13:31

2 Answers 2

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You don't get shocked because you're talking about DC with no connection path back to the positive terminal of the battery.

If you touch something metal on the vehicle you're now at the potential of the negative terminal of the battery (or the car's ground voltage). If you're also touching earth ground, then a very slight current will flow to balance out earth ground and car ground. But after that's balanced, there's no difference in potential across you and no current flows.

It's kind of akin to a bird resting on a high voltage power line. They don't get shocked much because there's no completed circuit.

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    \$\begingroup\$ Even closing the circuit, 12 V are not enough to produce a shock in usual conditions. \$\endgroup\$ Sep 23, 2014 at 17:54
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    \$\begingroup\$ @JoshBeam Yes, you're pretty much right. Concrete isn't earth ground, it's relatively insulating unless it's wet so in that case, you're not even earth grounded, just battery grounded. And as Martin mentions, even if you do hold onto both terminals of a 12V battery, you probably won't feel much because of the low voltage. Btw, the car doesn't need to be on for the battery to produce that 12V. \$\endgroup\$
    – horta
    Sep 23, 2014 at 18:03
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    \$\begingroup\$ @Peteris I'm having trouble parsing what you wrote. If you are saying you touched a 12V battery and got shocked, I find that almost unbelievable. \$\endgroup\$
    – ACD
    Sep 24, 2014 at 14:25
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    \$\begingroup\$ @Peteris Now I am even more confused. There's absolutely zero chance touching across a 12V battery with two parts of your body would cause arcing (You said "strong enough to spark"). Seriously, you can't feel a shock from 12VDC. That's just false. \$\endgroup\$
    – ACD
    Sep 24, 2014 at 14:29
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    \$\begingroup\$ @horta that is different. Right now I have a 24V lithium bank. Putting two fingers across it and I feel nothing. Dipping my hand in water and repeating, still nothing. No way will you get a "spark through clothing" off of 12VDC. I stand by that. \$\endgroup\$
    – ACD
    Sep 24, 2014 at 15:36
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The battery is forcing a difference of potential between the - terminal and the + terminal. The chassis (- terminal) is somewhat isolated from the Earth because of the tyres so theoretically the potentials are different however as shown on the attached simplified equivalent schematic, there is nothing in particular forcing a voltage across your body (you will simply put the - terminal potential to a potential closer to Earth after a small equalisation current has settled).

schematic

simulate this circuit – Schematic created using CircuitLab

Even if you touched by accident the + terminal, you're right in thinking that what matters is the current going through your heart (there is also the duration of that current but nevermind that for now), however as I said in another thread that current depends on resistance for a fixed voltage. Your body resistance depends on conditions of contact and your humidity, so that's only when you're soaked wet that it would start to get dangerous (60mA with a foot immersed); however engineers never trust users and the + terminal is always covered (rarely the -, at least not on any of my cars).

Edit: [Disclaimer: Warning, the following (and above) is for information only and should not hold me responsible for any harm caused to you or others] When disconnecting the battery before holidays, I always pull the - plug bare handed... And I'm still here to write these lines.

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