CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Why aren't car batteries more deadly?

So, I’ve been reading up on car batteries, because I’ve always been somewhat terrified of the things and have generally stayed away from them. I’ve known for a while now that it takes just a few dozen milliamps to severely injure/kill a person, and a car battery has several hundred amps. Thousands of times the amperage necessary to kill a person. But then again, it’s only 12v, which is barely even a tickle. And if our cars contained such a deadly, deadly thing under the hood, surely its handling would be restricted and regulated in some way to prevent silly people from doing dumb things. But it’s not, so handling a car battery must be at least somewhat safe.

So, I guess I’m wondering, why is it safe for a person to touch both terminals, but if a piece of metal does, it’ll get white-hot, weld itself to the terminals, and cause the battery to explode? Why does that much energy melt metal but not harm a person even slightly?

Thanks much for educating this person that knows FA about electricity!

I just measured the resistance between my two wet thumbs as about 1,000,000 ohms resistance (couldn’t get a reading with dry thumbs). Since volts = amps X resistance, touching both terminals with wet thumbs would allow only 0.000012 amps to flow through me. Totally harmless.

The piece of metal shorting the terminals has a very low resistance so lots of current flows. Your body has a very high resistance and 12 volts is not enough to push a lethal amount of current through your body, with dry skin, it’s not even enough to feel.
Most people even survive touching 115, 230, and even 460 volts briefly but it’s not at all pleasant.
I have even met a survivor of a 4160 volt contact, but he needed skin grafts and was missing an ear.

I went looking for a well written explanation and found this one from the University of Hawaii. I thought I’d pass it along. The biggest thing to be aware of isn’t electrical shock.
http://www.chem.hawaii.edu/uham/bat.html

Your body will only allow a very minute amount of current (amps) to pass through on such a low voltage. But you will conduct electricity. The small amount you will conduct is inconsequential to your health though.

A simple test of fuel control systems on early computer controlled cars was to disconnect the wire from the oxygen sensor to the computer. Then lick your fingers, place one finger on the input wire and the other on battery positive. The engine computer would then see about 1 volt on the sensor input, think the car was running rich, and then compensate by taking away fuel.

Batteries are dangerous though. Drop one on your foot and you’ll regret it. Spill battery acid on your jeans and you’ll wish you hadn’t. And, seriously, a battery blowing up in your face can blind you.

Be glad cooler heads prevailed and the manus didnt adopt the proposed 48 volt system,I think the big problem with DC is the effect on the heart(12volts hurts through the nipples,real sensitive areas if dampened will respond to 9 volts on some people,so electricity is nothing to fool around with) however some folks seem to be happily immune to some types of electricity(one Guy could stall a flathead eight by putting his fingers on the sparkplugs and grounding his johnson-hope thats not too lewd-Kevin

I heard of a case where a Navy sailor managed to kill himself with the 9 volt battery of a multimeter. When he wanted to measure his body’s resistance just for kicks, he poke through his skin with those probes. A person’s bodily fluid has much less resistance than the outer skin. Thus, enough current was able to pass through the sailor’s body to drop him before he could read the multimeter. If a 9v battery can kill, a 12v car battery definitely is lethal.

@chunkyazian That sounds like one of those “stories” that get told by instructors to keep their students from doing stupid things with multimeters.

Now, would someone please explain how a defibrillator works. I had one used on me once, but i truly don’t remember it.

I feel the most dangerous thing about a Lead Acid Battery is the hydrogen that is released when it is charged. The percentage it takes for flammability of hydrogen in a room is roughly 4% to 95%

For comparison, gasoline in air is flammable roughly between 1.5 - 7%.

All it takes is a spark and you can blow the battery up in your face.

30V is generally considered the threshold of danger, although lower voltages can give a shock on wet, salty skin or mucous membranes (try touching a 9V battery to your tongue).

A defibrillator puts out a short (~0.1 sec.) pulse of voltage/current which can restart a heart, as opposed to a steady or slowly decaying voltage (found to be ideal for the electric chair) which can disrupt the operation of a beating heart.

I have an electric rat/mouse trap (RatZapper) which delivers ~3,000V for 15 seconds, pauses 30 seconds, then gives another 15 second jolt.
Why?
A rat heart can restart itself.

The overwhelming majority of injuries from electrical shock come not from the electricity but from the secondary effect… banging one’s head against the hood latch, or falling off the roof, or whatever. Respecting that, I like to wear leather or rubber gloves when working with electricity.

Chunky, that sounds to me like a legend. However, it takes very little electricity through the heart to interfere with the signals operating the heart muscles, and if one were to apply a signal directly to the nervous system in that area it could cause fibrillation. The maximum capacity of the human nervous system is in millivolts (current depending on circuit resistance). You could get hit by enough electricity to blow your arm off without killing yourself (think: lightening strike), but don’t interfere with your heart’s signals.

I’ve been zapped by 12vDC, 24vDC and 110vAC. It gets your attention but it won’t kill you. Cars ar 12v, big trucks and some aircraft are 24vDC, I believe. Electric forklifts are commonly 24, 36, or 48VDC in the US and as much as 96vDC in Europe.

AC is safer than DC because the voltage makes your muscles contract so you can yank your hand back but batteries can’t be made AC, only DC. DC will lock your muscles in place making it more likely to kill you. About 50 volts is the danger point. Just google Edison vs Tesla for the DC vs AC debate. Earlier posts in this thread explain things very well.

Bottom line, 12v car batteries aren’t really dangerous although really stupid people could hurt themselves with one. Stupid people could hurt themselves with a twinkie, too…

Here’s one description:

AC's alternating nature has a greater tendency to throw the heart's pacemaker neurons into a condition of fibrillation, whereas DC tends to just make the heart stand still. Once the shock current is halted, a "frozen" heart has a better chance of regaining a normal beat pattern than a fibrillating heart. This is why "defibrillating" equipment used by emergency medics works: the jolt of current supplied by the defibrillator unit is DC, which halts fibrillation and gives the heart a chance to recover.

Good question OP. I think the poster’s explanations above are excellent. In summary the amount of power the battery is able to deliver is not the most important factor in determining whether it can hurt you by getting an electrical shock. The voltage, 12 volts, is the most important factor.

Batteries have a big power rating b/c they need to turn the starter motor. If you got your hand stuck in the starter motor, THAT could do a lot of damage. If you were designing a starter motor, then you’d want maximum electrical power transfer, so electrical theory says you’d try to match the motor resistance to be in the range of the resistance of the battery. Much less than an ohm in other words. Since your body’s dry skin resistance is usually in the millions of ohms, while a lot of power is delivered to the starter motor, not much power is delivered to you if you accidentally grab hold of the terminals.

All that said, even 12 volt batteries could present a shock hazard to people in certain situations. Besides dropping them on your foot I mean. Wet hands, broken skin, bad heart, pacemaker, etc, an electrical shock could possibly do damage. I try to avoid grabbing hold of the battery terminals myself.

I can only tell from my experience with today’s batteries compared to old is, they are more powerful and last much longer. On cars in the last 25 years that I have kept for as long as ten years, they never needed replacement. So generally, no one is touching them. The OEM batteries are much better in the cars I own. So, if there is less contact, there are fewer chances for things to go wrong.

Sure, you could hurt yourself with a twinkie

But it’s more of a cumulative thing . . .

Yeah, you could kill yourself with twinkies . . . but it takes years to happen

And a diet of several twinkies a day, I would think

"you could kill yourself with twinkies . . . but it takes years to happen"
Oh yah !

Or you could lose weight with Twinkies:
http://www.cnn.com/2010/HEALTH/11/08/twinkie.diet.professor/

AC is far more dangerous than DC. The human central nervous system uses electromagnetic energy to transmit signals, and that electromagnetic energy is between 50 and 60 Hz. When you get zapped by AC, it interrupts the CNS and is far more likely to result in death.

Back during the current wars between Westinghouse and Edison, the Edison people would take a dog and zap him with 500 VDC. It would hurt the dog, but it didn’t kill it. Then they would hit it with 100 VAC and that would always kill the dog.

The first electric chair was invented by the Edison camp and intentionally used AC to demonstrate to the public just how dangerous AC was compared to DC.

I have been shocked by a 500 VDC power supply that I was repairing once. I got burned pretty good but it did not kill me. In fact the only thing I felt was the burn, I did not feel the shock.

And as a general rule, 60 VDC is generally considered the required voltage to penetrate the skin. I think we will soon see either a 42 or 48 volt system in cars. There are 40V tools out now, I have a 40V chain saw (Ryobi) and it is impressive, about as strong as a small (14") 2 cycle gas saw, and a whole lot easier to start.

I bought an old electric mousetrap from an auction, still in the box with the original bait.
I came down stairs, after setting it up the night before, to find the kitchen filled with the smell of fried mouse… unfortunately, the mice never went near it again after that…