How the Alternator on a Car Works

alternator
That lump with a pulley at the end is an alternator. Look important?. Zach Zupancic/Flickr

The alternator in your car is a kind of mini electrical generator, which converts mechanical energy into electrical energy through a process known as alternating current. Without an alternator, the engine in your car would have no spark, your headlights no light, and your heater no way to keep you comfortable in the winter. The process sounds complicated, but the way an alternator works is really pretty simple.

Battery vs. Alternator

Although many people assume that a car's battery powers all those things, the truth is that the battery only does one thing other than keeping the electronics on when the engine is off: It starts the engine. And only for a limited time. Once the engine fires, the alternator takes over and provides the juice.

Automobile engines run on air, fuel, and spark. While the battery supplies the electricity needed for that initial spark, it only has enough power to get the car a few miles down the road, and that's where the alternator comes in—it continually charges the car battery while the car is in motion while also simultaneously operating all the electronic components of the vehicle. This means that while the voltage of most car batteries is 12 volts, an alternator will typically output anywhere between 13 and a half and 15 volts of electricity.

How Alternators Work to Make Electricity

The alternator is comprised of a voltage regulator and three main components: The stator, rotor, and diode. When the battery initially powers the car, the alternator belt, or V-belt, spins the pulley on the alternator, causing the rotor inside the alternator to spin very quickly. This rotor, which is basically a magnet or group of magnets, is situated inside a nest of copper wires, which are called the stator.

The process whereby electricity is generated by spinning magnets at incredibly fast speeds along a set of copper wires is called electromagnetism. The electricity harnessed this way is conducted through the copper wires to a diode, which changes the electricity from AC to DC, the current that the car battery uses.

The next step happens within the voltage regulator—a built-in component on modern alternators—which is basically a gatekeeper that will shut off the flow of power to the battery if the voltage goes above a certain level, usually 14 and a half volts, which keeps the battery from getting overcharged and burning out. As the car battery is drained, current is allowed to flow back into it from the alternator, and the cycle goes on and on.

Signs of a Bad Alternator

When a car alternator is going bad, drivers will notice a reduced capacity for electrical use, often resulting in things like dim headlights. But these clues won't last long, because a partially charged battery usually has enough power to operate things like headlights and power windows, but will fail the next time you try to start the vehicle.

There is also usually a dash board light, also known as the battery light because it's often shaped like a little battery, that will alert drivers to an alternator that is not providing enough charge to keep the system up. Concerned car owners can also check the charging system, or take the car to a mechanic if they are experiencing any sort of electrical issue.