The scene is familiar if you’ve watched a lot of spy dramas or thrillers: the hero has been captured, restrained, and is helpless to resist as his captor hooks up a pair of jumper cables to a car battery. As dutiful consumers of media, we’ve been conditioned to know that means our hero is about to be tortured, possibly to within an inch of his life.
But that’s in the movies. Here in the real world, can a car battery actually electrocute you?
The full answer to that question is predictably complex, but at the root of things, this is just another one of the many fibs Hollywood tells in the service of offering a more engaging story and a bigger spectacle.
While there are certain aspects of automotive electrical systems that are dangerous, and batteries themselves can also be dangerous, the deck is stacked against your car battery electrocuting you, let alone killing you.
Why Can’t Your Car Battery Electrocute You?
The math can get a little complicated, but the main reason that you can safely touch the positive and negative terminals of a typical car battery, and walk away unscathed, has to do with the voltage of the battery. While car batteries technically have the amperage to kill you, the voltage is a different story.
Car batteries have a nominal voltage of 12V, which can vary up or down a little depending on the level of charge. Alone, that just isn’t enough to pose a problem. If you wired many batteries in series, you could potentially reach a voltage high enough to reach dangerous territory.
Traditional car batteries are capable of delivering a lot of amperage in short bursts, which is the main reason that ancient lead-acid technology is still in use. Starter motors require a lot of amperage to run, and lead-acid batteries are good at providing short, intense bursts of amperage.
However, there’s a world of difference between the coils of a starter motor and the high contact resistance of the human body.
Simply put, voltage can be thought of as “pressure,” so while a car battery may technically have enough amperage to kill you, the paltry 12 volts DC simply doesn’t provide enough pressure to push any significant amount of amperage through the contact resistance of your skin.
That’s why you can touch both terminals of a car battery without receiving a shock, although you may feel a tingle if your hands are wet. Certainly nothing like the confession-inducing, potentially-deadly, electrical torture you may have seen in the movies or on television, though.
Car Batteries Are Still Dangerous
Your car battery, in and of itself, may not be capable of delivering a deadly—or even noticeable—electric shock, but that doesn’t mean it isn’t dangerous. The main danger associated with car batteries is an explosion, which can occur due to a phenomenon known as “gassing,” where the battery releases flammable hydrogen gas.
Don’t douse yourself in saltwater and hook yourself up to jumper cables, or insert electrodes into your fingertips and touch them to a car battery, to test this. The math says you would probably be just fine, but the human body is a complicated thing, and these aren’t experiments worth doing.
If the hydrogen gas is ignited by a spark, the entire battery can explode, showering you with sulfuric acid. This is why it’s so important to follow the correct procedure when hooking up jumper cables or a battery charger.
Another danger associated with car batteries has to do with accidentally bridging the terminals, or accidentally bridging any +B wire or connector, like the starter solenoid, to ground. While a car battery can’t pump a dangerous amount of amperage into your body, a metal wrench has far less resistance, and will tend to grow extremely hot, and may even become welded in place, if it bridges battery positive to ground. That’s pretty much bad news all around.
Some Automotive Electrical Systems Are Dangerous
Remember when we said that the main reason car batteries can’t electrocute you is because they’re only 12V? Well, that’s true, but the problem is that not all car batteries are 12V. There was a huge push in the early 2000s to move from 12V systems to 42V systems, which would have been much more dangerous to work with, but the switch never really materialized for a variety of reasons.
However, hybrid and electric vehicles often come with two batteries: a traditional lead-acid battery for the starter, lighting, and ignition (SLI) functions, and a much higher voltage battery or battery pack to run the electric motor or motors. These batteries often use lithium-ion or nickel-metal hydride technology instead of lead-acid, and they are often rated at 200 or more volts.
The good news is that hybrid and electric vehicles typically don’t keep their high voltage battery packs anywhere that you’re likely to run into them on accident, and they almost always use some type of color code to warn you about high voltage wires.
In most cases, high voltage wires are color-coded orange, although some use blue instead, so it’s a good idea to verify what color your vehicle uses before you try to work on it.
When 12 Volt Electrical Systems Actually Can Shock You
Although you can’t be electrocuted by simply touching the terminals of a regular car battery, due to the low voltage, you can receive a nasty shock from other components of a traditional automotive electrical system.
For instance, in ignition systems that use a cap and rotor, an ignition coil is used to provide the tremendous amount of voltage that’s required to push a spark across the air gap of a spark plug. If you run afoul of that voltage, typically by touching a spark plug wire or coil wire with frayed insulation, while also touching ground, you will definitely feel a bite.
The reason that you can be shocked by touching a worn spark plug wire while touching the battery terminals won’t do anything, is that the voltage pumped out by the ignition coil is high enough to push through the contact resistance of your skin.
Getting zapped like this probably still won’t kill you, but it’s still a good idea to steer clear anyway, especially if you’re dealing with the higher voltage of a distributorless ignition system.
So What about the Persistent Car Battery Torture Trope?
There’s actually a kernel of truth hidden in the scene we opened with. If a villain starts with a car battery, which he hooks to ateranother device, and then uses that device to torture the hero, that’s a situation that’s grounded in reality.
There’s a very real device known as a picana that, powered by a common 12V car battery, is capable of delivering electric shocks of very low amperage at high voltages, which, like grabbing a hold of a bad coil wire, is extremely unpleasant.
So while grabbing the terminals of your battery isn’t likely to provide even the weakest of shocks, let alone kill you, this is a trope you can more or less chalk up to artistic license.
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