I am reading a book called "Make: Electronics" and, on page 9, it says:

"Anyone who has dropped an adjustable wrench across the bare terminals of a car battery will tell you that short circuits can be dramatic at a "mere" 12 volts, if the battery is big enough"

They also show this image to exemplify the point above:

enter image description here

I have also watched this video, in which the guy touches both positive and negative parts of a 12V battery with his hand to see if he gets a shock, but, as you can see, nothing happens.

Having said that, I have 2 questions:

1) What does the size of the battery have to do with anything? The first statement says that short circuits can be "dramatic (...) if the battery is big enough". Why? For me the only difference between a small 12V battery and a big 12V battery is how long it can give you a shock for, but both would give you the same shock (ie, both would make the same current pass through your body).

2) Why didn't the guy in the video get a shock? He claimed that it is because the resistance of his hand is too high, which makes sense, but then why does the book quote (and its picture) make it look like you will for sure get a shock?

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    \$\begingroup\$ Nowhere the description of the image or the image itself implies that he gets a shock, it only talks about dramatic things happening. \$\endgroup\$
    – PlasmaHH
    Sep 9, 2015 at 9:40
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    \$\begingroup\$ It's not about shocks. If the 12V battery can start a car, it can supply about 1000A (briefly) or 12kw, which - concentrated in your wrench - will heat it, maybe to melting point, or arc weld it across the terminals. Which is dramatic enough when you don't expect it... \$\endgroup\$
    – user16324
    Sep 9, 2015 at 9:57
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    \$\begingroup\$ There was an accident at the phone company where my dad worked where a man blew up a screwdriver with 6 volts. He dropped it on the power rails (1/4 inch by 6 inch copper plates) connected to a room full of 6v batteries in parallel. \$\endgroup\$
    – Joshua
    Sep 9, 2015 at 15:07
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    \$\begingroup\$ Death has occurred for batteries of 9 Volts, by a user that let the terminals inside his skin. Ultimately, your survival depends solely on the resistance of your skin. Therefore, I wouldn't mess too much with electric shocks, even with low voltages. \$\endgroup\$
    – Fiat Lux
    Sep 9, 2015 at 20:37
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    \$\begingroup\$ In few words. The text of the book is correct. Yes, the effects can be dramatic because of large amount of heat that will be dissipated in the wrench. But not because of electric shock to the operator. The cartoon, unfortunately, might be misleading, because it shows the operator getting an electric shock (unless his hair stood on end because of the sudden \$ OMG I^2R ! \$ realization). \$\endgroup\$ Sep 10, 2015 at 18:16

8 Answers 8


In most ordinary circumstances, 12 V isn't even enough to feel, let alone cause a shock.

However, it's really current that you feel and that shocks you, not voltage. So when we say that 12 V is safe, we're making implicit assumptions about the resistance of your skin. Electrically, you're basically a bag of salt water. Your insides have low resistivity, but the skin has much higher resistance. For current to go thru you, like from one hand to the other, it first encounters the skin resistance on one hand, then gets a relatively easy ride to the other hand, then again encounters significant resistance getting thru the skin to the other electrode.

Skin resistance can vary significantly, but is usually at least a few 10s of kΩ. 12 V applied to that resistance doesn't cause enough current to flow to even notice, usually. However, if the two points where the 12 V is applied to your body are sufficiently wet, the skin resistance is lower, the current is higher, and you can feel the result. A simple demonstration of this is to touch a 9 V battery briefly to your tounge. You will definitely feel it, and 12 V is another 1/3 higher.

I haven't tried this, but I suspect that if you wet two fingers with salt water and then connect 12 V between them, you'd feel it. Doing this between the two hands could even be dangerous because the current will flow near your heart.

Note that the passage you quote didn't really talk about getting electrocuted, but that it is dangerous to drop metal objects across a car battery. This is true, but implying that this proves its dangerous for a human to touch both terminals of the battery is misleading at best. A car battery is a 12 V source that can provide large current, usually a few 100 A. When you put a very low resistance across a car battery, that large current will flow, which will heat something, possibly to the point of melting or even exploding. The reason this isn't relevant to a human touching a car battery is because our resistance is nowhere near low enough to let those kinds of currents flow.

So be extra careful to not accidentally short a car battery. Generally, 12 V is safe to work around, even if you got it across the hands. However, there is no point pushing it, and if you're really sweaty, for example, it could get at least unpleasant.

  • 18
    \$\begingroup\$ 9V batteries already hurt across the tongue...don't think upping the ante is going to go well. \$\endgroup\$
    – Nick T
    Sep 9, 2015 at 14:54
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    \$\begingroup\$ @Jamie: At some level of wetness (skin conductivity), you will feel it. Again, just 9V on the tongue is quite unpleasant. Also, the difference between two fingers on the same hand and different hands isn't much since our internal resistivity is fairly low. Most of the resistance of any path in and out of the body is thru the skin, not internally. \$\endgroup\$ Sep 10, 2015 at 11:24
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    \$\begingroup\$ @Jamie: But soaking your hands with brine is exactly what I'm talking about. As I said, "wet two fingers with salt water". And, it's not about the taste buds on the tounge. Try a 9V battery against the inside of your lip and you'll see what I mean. \$\endgroup\$ Sep 10, 2015 at 12:03
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    \$\begingroup\$ you are plain wrong. you can feel even lower voltages and they can cause a shock like sensation. you can even feel < 5 volts. I know from personal experience as a wacky instructor at school had us test it out. \$\endgroup\$
    – kns98
    Sep 10, 2015 at 19:10
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    \$\begingroup\$ @kns98 agree - yet it'd be nice to mention who is wrong (using @ whoIsWrong maybe?), because now it looks as if you're criticizing Olin ^_^ \$\endgroup\$
    – user20088
    Sep 10, 2015 at 21:54

It isn't just shocks.
A battery can push a lot of current through any piece of metal.
This could melt a wrench and cause a fire.
It could also cause a ring to get red hot and burn the wearer. It is very easy to get a finger with a ring between the positive terminal of the battery and the body or frame of the car.

Always remove metal jewelry when working near the battery or electrical parts of a vehicle.

My brother got a graphic demonstarion of this in an auto shop class in high school.

The shop teacher (stupidly) decided to show the students how dangerous a car battery could be. He did this by short circuiting a fully charged car battery with a short length of solid copper wire.

The high current flow spot welded the wire to the battery terminals. The wire got red hot and sagged down and melted a hole in the battery.

Hydrogen gas escaped and ignited on the hot wire - the battery went "BOOM!" and bits of plastic, lead, and droplets of acid flew all over the place.

All the kids had dived for cover when the teacher headed for the battery with the wire - nobody got hurt.

Most of the kids were from farms and already knew what can happen when a battery shorts out - the rest ducked when they saw the savvier kids heading for cover.

  • 2
    \$\begingroup\$ I always say there is a reason why teachers should do dangerous things at school :-) \$\endgroup\$
    – yo'
    Sep 9, 2015 at 20:17
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    \$\begingroup\$ +1 for warning about metal jewelry. Don't forget that stainless steel watch strap. \$\endgroup\$
    – peterG
    Sep 9, 2015 at 22:17
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    \$\begingroup\$ Was it supposed to explode, to show the students what not to do, or was the teacher being stupid? \$\endgroup\$
    – user253751
    Sep 9, 2015 at 23:14
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    \$\begingroup\$ It sounds like your friend's teacher was Miss Frizzle. \$\endgroup\$ Sep 10, 2015 at 6:21
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    \$\begingroup\$ @immibis The teacher was just stupid. I had him in wood shop the year before, and he was a dumb ass - though he did tell us an interesting story about aluminum, mercury, and the development of the B52 bomber. \$\endgroup\$
    – JRE
    Sep 10, 2015 at 6:59

The text description is right. A wrench will throw sparks and possibly heat up and melt, that's dramatic.

The image, however is wrong. It clearly shows a person and a wrench being electrocuted simultaneously. The picture is wrong on 2 levels:

  • a car battery can't electrocute a human in typical conditions
  • no single source can electrocute 2 targets so different at once

The image was probably added later, by an editor. Editors aren't usually as technically skilled as authors. A book is a product of many people. You and the author are on the opposite sides of long chain of humans, and every one of them had an opportunity to insert his own mistakes.

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    \$\begingroup\$ "no single source can electrocute 2 targets so different at once" is misleading, the two sources would form a parallel circuit. So, current flows through both, but how much current depends on the resistances of both sources. \$\endgroup\$ Sep 9, 2015 at 20:19
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    \$\begingroup\$ @BlueRaja-DannyPflughoeft - No, it's insightful actually. "So different" refers to very different resistance. The wrench protects the human (while it is in the circuit) like a lightning rod protects a building. However, the mains can electrocute two humans at the same time just fine because they are not that different. \$\endgroup\$ Sep 10, 2015 at 10:29
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    \$\begingroup\$ @BlueRaja-DannyPflughoeft Just as Jirka said. In parallel connection, the low-resistance wrench would absorb almost all of the energy - leaving human practically unaffected (at least by current). In series connection, the high-resistance human would absorb most of the energy - leaving wrench practically unaffected. I never said there will be zero current flow - just not enough to qualify as "electrocution". \$\endgroup\$
    – Agent_L
    Sep 11, 2015 at 14:18
  • \$\begingroup\$ @JirkaHanika - The wrench would protect the human from stray current/electrocution (exactly like a lightning rod), but not from massive burns and other secondary hazards. It's protective value is arguably limited. \$\endgroup\$
    – aroth
    Sep 12, 2015 at 10:53

People often assume that 12V batteries and systems are safe because of the low voltage. Some dangers I haven't seen mentioned in other answers are:

  1. Death by electrocution because of other components in the system. A car stereo can kill you and I remember at least 2 stories from the 1980's where men were electrocuted while working on car stereos. I can't find these stories in the web, but lots of car stereo installation guides have warnings like this one:

    An important precaution when wiring car stereo speakers is to never work on the electrical system of your car if the ground wire is still attached to your battery. Failure to abide to this precaution can result in electrocution! Furthermore you could also damage the speakers.

  2. Car batteries used to injure people all the time by blowing up (they can release flammable gas) and spraying acid on people. I used to read about this all the time. Now not so much, but the danger is still there.

  3. Shock and reflex injury due to other components. A common scenario is: get too close to a spark plug wire, get a nasty jolt, slam your hand into some sharp and/or very hot piece of metal.

As for the picture, I've seen guys jump in a very similar way when they accidentally shorted terminals and got a huge set of sparks. It wasn't electrocution that did it but getting the cr*p startled out of them. (^_^)

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    \$\begingroup\$ Electrocution from 12 volts? No. And I don't care who said it or what manual it's printed in. Stories are just that, stories. Now if you have a very high-power amp in the system, the voltage at its speaker terminals when it's playing music at max output could be well above 12 volts, possibly above the 30 volts that it is the upper limit for "low voltage" (below which no precautions need be taken regarding electric shock). But that voltage won't appear anywhere else in the system and it typically isn't even referenced to chassis ground. \$\endgroup\$ Sep 9, 2015 at 23:20
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    \$\begingroup\$ @JamieHanrahan, electrocution from components in a "safe" 12V system. Car audio components can output well over a hundred volts open circuit (they routinely drive 113 Vpk-pk into 8+ ohms). As for the electrocutions, they appeared in reputable newspapers at the time. A great many facts are true, even if they were published before the internet. \$\endgroup\$ Sep 10, 2015 at 0:07
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    \$\begingroup\$ I don't believe the "newspaper" reports. The physics make it impossible, and a great many things in newspapers, particularly in technical fields, are misreported. Re the amplifier, 113 p-p into 8 ohms would make it a 200 watt amp. Not exactly common (in terms of actual, not claimed, amp output). Even then, it will be present only at the speaker terminals and only when the amp is powered on and with high level at its input. The voltage referenced to ground would only be half of that. The unqualified claim that "a car stereo can kill you" is specious at best; the vast majority just can't. \$\endgroup\$ Sep 10, 2015 at 0:24
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    \$\begingroup\$ @JamieHanrahan to kill a person, you need about 100 mA through a heart - that's enough to trigger a fib. Human heart's resistance (from one side to another) is in the range of < 100 ohms (test it on some animal carcass remains if you doubt it). Blood is a fluid ripe with salt and iron, making it an even better conductor than pure water itself. To get about 100 mA though about 100 ohm res, you need about 10 V. People got fibbed by currents as low as 50 mA AFAIR; YMMV. physics.ohio-state.edu/~p616/safety/fatal_current.html darwinawards.com/darwin/darwin1999-50.html \$\endgroup\$
    – user20088
    Sep 10, 2015 at 12:30
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    \$\begingroup\$ @ChrisH nb. the funny thing is that human skin is slightly porous & hygroscopic, mainly due to things like osmotic pressure in the cells, pH etc; the result is that wet/sweaty skin's resistance drops significantly even without visible rupture; DTTAH: put you hand in warm salty water for about 5 minutes, then use two fingers to touch two 5V terminals - that's what happens to me routinely when working with USB connectors in summer. For those who don't want to try it - it vaguely reminds me of the shocks I got from old telephony cabling (48V), which certainly ain't pleasant. \$\endgroup\$
    – user20088
    Sep 10, 2015 at 15:45

The depiction is both correct and incorrect.

12V on your skin, even if you have sweaty fingers, will not do a lot, if anything. You are unlikely to even feel it. The human body is a funny thing, its impedance/resistance goes down as the voltage goes up (that is mostly because of the skin's outermost layer). While it's typically around 1-2kΩ at higher voltages, at 12V, the resistance is well over 70kΩ so you can't expect much more than 0.2mA (which would not even be dangerous if we were talking of 50Hz AC).

With needles stuck through your skin, it would look somewhat different, but I trust you won't try that.

About the wrench... this is something that I've actually seen people lacking a starter cable do, and with surprisingly little injuries... but that highly dangerous, if not insane.

A battery can (depending on model) deliver anywhere from 500 to 1,500 amperes or more in a short circuit. Note that a strike of lightning only has about 20,000-50,000 amperes, which is a mere 15-20 times as much. Lightning, however, lasts only a millisecond whereas a car battery can sustain that current for a quite non-neglegible amount of time. So you can about figure what will happen to a wrench which has close-to-zero resistance.

As an example, take a 100Ah battery, and divide that by a supposed 1,500A. Assuming that the battery is new, and fully charged and doesn't explode in your face, it will (in theory) deliver for (100*3600)/1500 = 240 seconds. That's 4 minutes.
Your wrench will be glowing red after half a second, and it will glow white not long after. Woe if you still hold it in your hands. If you touch the chassis, you'll burn a hole right through it (I've seen that happen to a friend).

This is also the reason why you always disconnect "minus" (ground) first, and reconnect it last when changing a car battery.
In order to fixate the clamps, you obviously must touch them with a wrench. But if you accidentially touch something else while on the "plus" clamp, and ground is already connected, the outcome is desastrous.


If you place your hands across the terminals of a 12V battery very little current will flow between the terminals because your hand has a very high electrical resistance. The current is what you feel and causes damage, and thus you will not notice anything. This is unless you do something to decrease the resistance of the human/battery interface, such as wetting your hands by sweating, and thus cause a non-negligible current to flow (negligible in terms of what you can perceive).

If you place a piece of highly conductive metal between the terminals of a 12V battery there will effectively be zero resistance between the terminals of the battery (compared to the resistance of your body as mentioned above) and as much current as the battery is capable of providing will flow between them. This will dissipate A LOT of energy, very quickly. If the battery is small, the maximum current it can provide is very small (i.e. a 9V battery on your tongue doesn't kill you). If the battery is large a VERY LARGE current will flow (a car battery must provide a lot of current for at least a short time in order to crank the starter motor). This will be very spectacular and exceed the design specifications of all components involved!

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    \$\begingroup\$ I don't think the size of the battery has any impact on how much current will flow across a specific load. $$V = I R \\ I = \frac{V}{R}$$... the battery size isn't mentioned anywhere in there. \$\endgroup\$
    – Chuck
    Sep 9, 2015 at 12:54
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    \$\begingroup\$ @Chuck: Right, but in practice (depending on the battery's construction) the size is likely to have an effect on the battery's internal resistance, which goes to make up R, and that is mentioned in there :) \$\endgroup\$
    – psmears
    Sep 9, 2015 at 15:28
  • \$\begingroup\$ @psmears - Yes, but the internal resistance, typically a fraction of an Ohm, is negligible in comparison to the lowest estimate of internal body resistance I found online - 300 Ohms. So, for a "small" 12V battery with high internal resistance, and your hands soaking wet (no skin resistance), the current through your body would be 12/301 = 39.87mA, and for the biggest baddest battery with no internal resistance, the current through your body would be 12/300 = 40mA. Escortkeel stated, "If the battery is large a VERY LARGE current will flow". \$\endgroup\$
    – Chuck
    Sep 9, 2015 at 15:46
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    \$\begingroup\$ @Chuck An AA cell - fully charged in good condition will giove 5A + from Alkaline and approaching 10A for NimH if short circuited. A typical car battery will give hundreds of amps into a suitably low resistance load and in the 500-1000A range (briefly) into a good short circuit. A spanner dropped across a car battery's terminals may well spit molten steel. \$\endgroup\$
    – Russell McMahon
    Sep 9, 2015 at 15:47
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    \$\begingroup\$ @Chuck: No, the internal resistance for a 9V battery is of the order of 1-2 ohms; for a car battery it is indeed a small fraction of an ohm. Across a short-circut that's potentially the difference between 9A and 1000A, i.e. two orders of magnitude. So it does make a difference to how much current will flow across a specific load. I agree that, in the case of the load being the human body, the difference is negligible - but the post was talking about "a piece of highly conductive metal", which is closer to a short-circuit than to a body :) \$\endgroup\$
    – psmears
    Sep 9, 2015 at 15:58

Normally, dry skin provides enough resistance to current flow that 12V will generally not shock you. If you scrape off some of your skin and expose your wet living tissues to the terminals, though, you can get a significant shock.


As others have said, it's the low resistance at work here. When you short the battery you will dissipate a few thousand watts of power through the short (although some of this will appear in the battery); that can be dramatic.

I have encountered an example of this can can be done safely. I was working with an 8000 µF capacitor charged to 5 volts. That's not a lot of power but the internal resistance is very low so the discharge happens very quickly. Short the leads and they would slightly weld themselves together - it would take a tug to pull them back apart and doing it repeatedly would roughen the leads from where the metal was melting.

  • \$\begingroup\$ I used to have a 4700uF capacitor for this very purpose (when I was 10 or so). I would charge it on 3 9V batteries connected in serial, and then go discharge it on the metal front door. Left lots of little pits in the door. :) \$\endgroup\$
    – BenjiWiebe
    Sep 12, 2015 at 14:12
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    \$\begingroup\$ @BenjiWiebe My case was a high school physics project: The capacitor was our power source and we had to move that capacitor (plus whatever else was needed) as far as possible with that power. My approach was far more effective than anybody else's (I sent the power from the cap through rails to the motors that wound up strings that would unwind off the cap--the cap alone went spinning off the launcher but usually into the walls pretty soon) but lacking metalworking equipment I had accuracy problems because I couldn't hold it straight as it came off my launcher. \$\endgroup\$ Sep 12, 2015 at 22:01

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