How Alternators Work

By: Josh Briggs  | 

This diagram shows the diodes, rotor assembly and stator, all of which you wouldn't normally see without opening the alternator. You'll hear more about them in a little bit.

It's 11:45 p.m. and you just finished working a double shift. It's late, you're tired and you have class in the morning. As you fumble for your car keys, you can't help but shiver. It's about to snow.

You hop in your Honda Civic and crank it up. The engine turns over grudgingly but eventually catches and fires. After warming up the engine, you pull out and begin your half-hour journey home. Just as the heater begins to remove the frosty conditions inside the car, the radio begins to cut out. Soon after, the dash light starts to dim ever so slightly. You keep driving but the anomalies with your ever-reliable car pile up.


Eventually, the radio and the heater quit altogether, and the comfortable warmth is slowly replaced by the outside chill. Your headlights are the next thing to start going. Now you're getting worried. Only 15 more minutes and you're home.

­Those next 15 minutes never come. The headlights dim to the point of dangerous, and the unthinkable happens: The engine starts to miss. Less than five miles (8 kilometers) from home, your engine dies, along with everything else in the car. You coast to a stop on the side of the road and pull out your cell phone. It's dead, so you plug it into the cigarette lighter so you can call for help. Guess what? No power. It's past midnight, and you're stranded on the side of the road with snowflakes for company.­

What you've just experienced is an example of an alternator breathing its last breath. Your first thought might have been the battery is dying. In a sense, you would be right because the battery and the alternator are related, but the battery tends to get all the press. This article aims to explain the mechanics of alternators, how you can diagnose problems and what you can do if you have a bad alternator. Read the next section to learn some background information about alternators and the war of the currents.