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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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Josh Beam
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    Since nobody is saying it in the answer, current, even at very low voltages, can be deadly – Scott Seidman Sep 23 '14 at 18:59
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    @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? – Josh Beam Sep 23 '14 at 19:01
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    If the resistance is high, the current is low. If the current reaches dangerous levels, it is dangerous, regardless of the voltage. – Scott Seidman Sep 23 '14 at 22:09
  • 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. – sergiol Feb 22 '15 at 18:13
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    @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) – Rohit Gupta May 22 '23 at 13:31

2 Answers2

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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.

horta
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  • Good, and people can get a little shock from that equalizing charge... (getting car body to same potential as the earth ground.) The car can charge up due to the rolling friction in the tires. (Think of rubbing a balloon on your sweater.) So you sometimes see ground straps that come off the car body and scrap along the road surface. – George Herold Sep 23 '14 at 17:53
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    Even closing the circuit, 12 V are not enough to produce a shock in usual conditions. – Martin Petrei Sep 23 '14 at 17:54
  • @MartinPetrei Agreed, unless you're licking the positive terminal :D – horta Sep 23 '14 at 17:55
  • If you're touching both terminals of the battery at the same time while the car is on, then charge is continually flowing through your body, correct (because the battery is constantly producing electromotive force)? But if you just touch the negative terminal (or the chassis) and the concrete you're standing on, charge would only flow through you momentarily, right (because the voltages would balance)? – Josh Beam Sep 23 '14 at 18:00
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    @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. – horta Sep 23 '14 at 18:03
  • I don't really agree with this answer. While the theory is accurate, you don't get shocked because the voltage is so low. Even if you dunked your hands in dirty water then grabbed both terminals of the battery you wouldn't get shocked. 14.5V even if the car is on is just too low. – ACD Sep 24 '14 at 14:18
  • @ACD what do you mean "you wouldn't get shocked" ? The few times that I have touched car batteries, it was sufficient to get quite strong shocks (e.g. battery-hand-leg-car chassis). 12 V is safe enough to avoid *fatal* shocks, but strong enough to spark through clothing and cause rather unpleasant shocks. – Peteris Sep 24 '14 at 14:24
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    @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. – ACD Sep 24 '14 at 14:25
  • @ACD see edits, pressed Enter too early. – Peteris Sep 24 '14 at 14:26
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    @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. – ACD Sep 24 '14 at 14:29
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    @ACD Go ahead and lick a 9V battery. I guarantee you'll feel it. You need special conditions to feel it, but there's ways to make it 'work' if you're after a shock. – horta Sep 24 '14 at 14:55
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    @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. – ACD Sep 24 '14 at 15:36
  • @ACD I was referring to this part of your quote: "Seriously, you can't feel a shock from 12VDC." I just showed you a single counter example. The rest of what you say, I agree with. – horta Sep 24 '14 at 17:11
  • @horta Okay, I concede my wording was poor there. I guess I should've clarified that I was talking about only parts of your body that aren't usually in your mouth but I'm sure other loopholes exist anyways. Eyeballs might have low resistance! – ACD Sep 24 '14 at 17:14
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    @ACD Lol, yes, please don't attach your car battery to your eyeballs. :D – horta Sep 24 '14 at 17:25
  • So you're saying that since he won't be closing the circuit himself he wont get shocked, but I have put my finger on a single AC wire and I've been shocked, how does that work? – php_nub_qq Mar 05 '22 at 20:42
  • @php_nub_qq It's probably best that you ask this as a new question to get a full answer, but the short of it is 2 ways. 1) You're acting like a capacitor (to ground) so current is "filling you up and getting sucked from you 60 times per second". 2) You actually were poorly connected to ground or a return line and so you had a "real" current passing through you - very dangerous. Note that the question/answer were referring to DC and therefore (1) doesn't apply to it, unlike your question. – horta Mar 06 '22 at 05:28
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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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