Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

Why is a 12-volt household battery harmless, but the shock from a 12-volt car battery will kill you?

Category: Physics      Published: November 1, 2013

car battery
The shock from a car battery will not harm you. Public Domain Image, source: Christopher S. Baird.

The shock from a car battery will not kill you. In fact, under normal conditions, a 12-volt car battery will usually not even shock you. Car batteries are not harmless, though. There are many ways you can be injured by car batteries:

There are enough dangers present that it is a good idea to be cautious around car batteries and follow the maintenance instructions in the car's manual, even though electrocution by car battery is not going to happen.

Also, this question tacitly contains a common misconception about high voltage being independently dangerous. The ability of electricity to damage biological tissue is dependent on both current and voltage. A very high voltage source providing a very low current does not carry enough energy to harm you. For example, a tabletop Van de Graaff generator (those charged metal balls you see at the science museum) can generate voltages up to 100,000 volts. And yet, children regularly enjoy shocks and hair-raising experiences from these generators without being harmed. In contrast, a high current (even at relativity low voltage) contains enough energy to hurt you. A better indicator of the danger of an electricity source is therefore how much current it gets running through your body, which depends partly on voltage but also depends on resistance and the amount of current the source can provide.

Voltage is a measure of the electrical potential difference between two points and is similar to the amount a river drops as it flows from point A to point B. In contrast to voltage, current measures the total charge flowing past a point on its path per second. Current is similar to how much water in a river is flowing past a point along the river per second. A few drops of water running down a steep hill carries far less energy than a mighty river flowing down a gentle slope. In reality, both voltage and current play a role. A mighty river plummeting over a cliff carries more energy than a mighty river coasting down a gentle slope.

Let us now apply these concepts to the car battery, which is a bit more complicated than it first appears. Car batteries can provide high currents. And yet they won't electrocute you. The key to this curiosity is that it is current running through your body that does the damage, and not the maximum current that a battery can provide. They are different. How much current actually ends up running through an object depends on three things: 1) the electrical resistance of the object, 2) the voltage applied, and 3) the amount of current the source can provide. For a human touching a car battery, the skin has a very high resistance, leading to low current; and the battery has a low voltage, leading to low current. Even though a car battery can provide high current if connected properly, your body does not draw this high current. Voltage does play a role in that it helps limit the total current in your body (along with your body's resistance).

The handbook Auto Electricity, Electronics, Computers states that the "Battery or charging system voltage will normally not produce enough current flow to cause a severe electric shock."

Topics: car, car batteries, car battery, current, electricity, electrocution, electromagnetism, voltage