You probably know that you have an alternator under the hood of your car, and that when it quits working, it’s a bad thing. Before figuring out how it works, let’s ask “Why is it there in the first place?”
Alternator is just another word for “electrical generator.” When the engine is running, the alternator produces electricity to power your lights and radio, and recharge your car’s battery. So when it quits working, your battery is running your car, lights and radio, and pretty quickly the battery dies.
If you look under your car’s hood, you can find the alternator by looking closely at the fan belt. The fan belt wraps around several pulleys to transfer the spinning motion of the engine to spin the cooling fan and a few other accessories. One of those pulleys the fan belt wraps around is connected to the alternator. You can recognize the alternator because it usually has vent holes in the cover through which you can see copper wiring.
The alternator works kind of like an electric motor in reverse. When you connect electricity to an electric motor, the electricity makes the motor spin. Using the opposite approach, when you make an alternator spin, it produces electricity.
Danish physicist Hans Christian Oersted discovered in 1820 that electricity flowing through a wire creates a magnetic field around the wire. A flurry of other experiments by scientists soon led to the discovery of the opposite effect; if you passed a magnet over a wire, you could make electricity flow. These are the basic principles of electromagnetism.
Without getting into all the technical engineering involved, an alternator is basically a shaft with a magnet on it spinning around inside coils of wire.
The alternator’s design causes electricity to flow in the wires first in one direction as the magnet makes one-half of a revolution, then as the magnet’s north and south poles flip over, the electrical current reverses and flows the other direction for the second half of the revolution. This produces an alternating current just like the AC current from your electrical outlets at home. Hence the name alternator.
One difference, though, is your home electrical current is 120 volts, while the alternator produces only about 14 volts. This is perfect, however, for recharging the 12-volt battery in your car, except for a small remaining problem.
Electrical current that changes direction (alternating current) is not good for charging a battery. The battery needs the current to flow consistently in one direction for recharging. To solve this problem, the alternator contains diodes, which are like small electrical valves. The diodes allow the current to flow through them when it is going one way, and stop the current when it tries to flow the other way. The resulting output is current flowing in only one direction called DC or direct current.
So the next time you start your car and it sounds like your battery is getting low, check your headlights while the engine is running. If they’re dim, it probably means they’re running off of the battery instead of the alternator. It’s time to drive straight to your favorite auto shop before your battery dies completely.