Can 12v kill you
Can a 12V battery cause a shock or not?
I'm reading a book called "Make: Electronics" and on page 9 it says:
"Anyone who has pulled an adjustable wrench over the bare terminals of a car battery will tell you that shorts to" only "12 volts can be dramatic if the battery is big enough."
They also show this picture to illustrate the point above:
I also watched this video where the guy was hand touching both the positive and negative parts of a 12V battery to see if he was going into shock, but as you can see nothing happens.
That being said, I have 2 questions:
1) What has the size of the battery got to do with anything? The first statement says that short circuits "can be dramatic (...) if the battery is big enough" . Why? To me, the only difference between a small 12v battery and a large 12v battery is how long it can shock, but both would deliver the same shock (i.e. both would allow the same current to flow through your body).
2) Why didn't the guy in the video get a shock? He claimed it was because the resistance of his hand was too high, which makes sense, but then why does the book quote (and his picture) make it look like you're sure to be in shock?
In most cases, 12V is not enough to feel, let alone cause, a shock.
However It is really current that you feel and that shocks you, not voltage. So when we say 12V is safe, we are making implicit assumptions about your skin's resistance. Electrically, you're basically a bag of salt water. Its interior has a low resistivity, but the skin has a much higher resistance. In order for the current to flow through you, as if from hand to hand, it first encounters skin resistance, then a relatively easy ride to the other, and then again a significant resistance flowing through the skin to the other electrode.
The skin resistance can vary considerably, but is usually at least a few 10 s kΩ. 12 V applied to this resistor did not cause enough current to flow nor notice usually . However, if the two points where the 12 V is applied to your body are sufficiently moist, the skin resistance will be lower, the current will be higher and you can feel the result. A simple demonstration of this is to hold a 9V battery to your tongue for a moment. You will definitely feel it, and 12V is another third higher.
I haven't tried it, but I suspect you would feel it if you wet two fingers with salt water and then plug in 12V between them. Doing this between your two hands can even be dangerous as the current flows near your heart.
Note that the passage you quoted didn't really talk about electric shock, but it is dangerous to throw metal objects over a car battery. This is true, but it is misleading at best if it is found to be dangerous for a human to touch both poles of the battery. A car battery is a 12V source that can deliver a high current, usually a few 100A. If you put a very low resistance across a car battery, that high current, which heats something, may flow to the point of melting or even exploding. The reason this is not relevant to a person touching a car battery is because our resistance is nowhere near low enough to allow these types of currents to flow.
So be extra careful not to accidentally short a car battery. Generally speaking, 12V is safe to handle even if you get it across your hands. There's no point in pushing for it, however, and if you're really sweaty, for example, it could at least get uncomfortable.
It's not just shocks.
A battery can push a lot of current through any piece of metal.
Doing so could melt a wrench and cause a fire.
It could also cause a ring to get scorching 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 a vehicle's battery or electrical parts.
My brother got a graphic demonstration of it in an auto business class in high school.
The shop teacher (stupid) decided to show the students how dangerous a car battery can be. He did this by shorting out a fully charged car battery with a short piece of copper wire.
The high current point welded the wire to the battery terminals. The wire got red hot and sagged, melting a hole in the battery.
Hydrogen gas escaped and ignited on the hot wire - the battery went "BOOM!" Plastic parts, lead and acid droplets flew everywhere.
All the children had taken cover when the teacher took the wire to the battery - no one was injured.
Most of the children came from farms and already knew what could happen if a battery ran out - the rest ducked when they saw the savvy children take cover.
The text description is correct. A wrench will spark and potentially heat and melt, which is dramatic.
However, the picture is wrong. It becomes clear that a person and a wrench are electrocuted at the same time. The picture is wrong on 2 levels:
- A car battery cannot supply people with electricity under normal conditions
- No single source can hit two targets so differently at the same time
The picture was probably added later by an editor. Editors are usually not as tech-savvy as authors. A book is the product of many people. You and the author are on the opposite side of a long human chain, and each of them has had an opportunity to insert their own mistakes.
It is often believed that 12 V batteries and systems are safe because of the low voltage. Some dangers I didn't see in other answers are:
Death from electric shock due to other components in the system. A car stereo can kill you, and I remember at least two stories from the 1980s in which men were electrocuted while working on car stereos. I can't find these stories on the web, but many car stereo installation guides come with warnings like these:
An important precaution when wiring car radio speakers is never to work on your car's electrical system with the ground wire still connected to your battery. Failure to observe this precaution could result in electric shock! You could also damage the speakers.
Car batteries that are used to injure people by exploding (they can release flammable gas) and spraying acid on people. I've read about it all along. Well not so much, but the danger is still there.
Impact and reflex injury from other components. A common scenario is: if you get too close to an ignition cable, get an uncomfortable jolt, slap your hand on a 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 together and got a huge amount of sparks. It wasn't an electric shock that did it, it scared her. (^_^)
The representation is right and wrong.
12V on your skin, even if you have sweaty fingers, doesn't do much, if anything. You are unlikely to even feel it. The human body is a fun thing, the impedance / resistance of which decreases as the voltage increases (this is mainly due to the outermost layer of skin). At higher voltages the resistance is normally 1-2 kΩ, but at 12 V it is significantly more than 70 kΩ. So you can't expect much more than 0.2mA (which wouldn't even be dangerous if we were talking about 50Hz AC).
With needles stuck through your skin, it would something look different, but I trust you won't try.
About the wrench ... that's something I've actually seen people with no jumper cables and surprisingly minor injuries ... but that's very dangerous, if not insane.
A battery can (depending on the model) deliver between 500 and 1,500 amperes or more in the event of a short circuit. Note that a lightning strike is only about 20,000-50,000 amps, which is only 15-20 times that. However, the flash only lasts one On for milliseconds, while a car battery can maintain this current for a not insignificant amount of time. This gives you a rough idea of what will happen to a wrench that has near zero resistance.
For example, take a 100 Ah battery and divide it by an assumed 1500 A battery. Assuming the battery is new and fully charged and not explode in your face , it delivers (theoretically) (100 * 3600) / 1500 = 240 seconds. That's 4 minutes.
Your wrench will glow red after half a second and white after a short while. Woe if you still hold it in your hands. If you touch the chassis you will burn a hole in it (I saw this happen to a friend).
This is also why you always first Disconnect "minus" (ground) and reconnect last when changing a car battery.
To fix the clamps, you can of course have to touch them with a wrench. However, if you accidentally touch something else while sitting on the "plus" terminal and the ground is already connected, the result will be catastrophic.
When you place your hands over the terminals of a 12V battery, very little current flows between the terminals because your hand has a very high electrical resistance. The flow is what you feel and what harms you and you will not notice anything. This does not apply if you are doing something to decrease the resistance at the human / battery interface, such as by pushing a button. B. to wet your hands by sweating, and thus to let a non-negligible current flow (negligible in terms of what you can perceive).
If you put a piece of highly conductive metal between the terminals of a 12V battery, there is virtually no resistance between the terminals of the battery (compared to the resistance of your body as mentioned above) and as much current as the battery can handle will flow Provision will flow between them. This will use up a lot of energy very quickly. When the battery is small, the maximum current it can deliver is very small (i.e. a 9V battery on your tongue won't kill you). If the battery is large, it will have a VERY LARGE current (a car battery must deliver a lot of current for at least a short period of time to start the starter). This will be very spectacular and will exceed the design specifications of all the components involved!
Usually, dry skin offers enough resistance to the flow of current that 12V generally won't shock you. However, if you scrape off any part of your skin and expose your moist living tissue to the terminals, you could end up in significant shock.
As others have said, it is the low resistance that is at work here. If you short the battery, you will use a few thousand watts of power through the short (although some of it will appear in the battery). that can be dramatic.
I've found an example of this can be done safely. I worked with an 8000 µF capacitor charged to 5 volts. That is not a lot of power, but the internal resistance is very low so that the discharge takes place very quickly. Short the leads together and they will easily weld together - it would take a jerk to pull them back apart and doing this repeatedly would roughen the leads from where the metal melts.
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