How Alternators Work

By: Josh Briggs & Kristen Hall-Geisler  | 
broken down car
The next time your car dies and leaves you stranded on the side of the road, don't blame just the battery; it could be the alternator, too. Flat vectors/Shutterstock

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 car and crank it up. The engine turns over grudgingly but eventually starts. After warming up the engine for a minute, you begin your 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 lights start to dim ever so slightly, but you keep driving.


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 flickering. Now you're getting worried. Only 15 more minutes and you're home.

But the headlights dim to the point of being dangerous and the unthinkable happens: the engine starts to miss. Less than 5 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 cellphone. It's dead, so you plug it in to call for help. Guess what? No power. It's past midnight, and you're stranded on the side of the road.

This is an example of an alternator breathing its last breath. Your first thought might have been that the battery was dying. In a sense, you would be right because the battery and the alternator are related.

What Is an Alternator?

Your car's alternator works with the battery to generate power for the electrical components of the vehicle. udaix/Shutterstock

In a gasoline- or diesel-powered car, the charging system is made up of three major components: the battery, the voltage regulator and an alternator. The alternator works with the battery to generate power for the electrical components of a vehicle, like the interior and exterior lights and the instrument panel. An alternator gets its name from the term alternating current (AC).

Alternators are typically found near the front of the engine and are driven by the crankshaft, which converts the pistons' up-and-down movement into circular movement. Some early model vehicles used a separate drive belt from the crankshaft pulley to the alternator pulley, but most cars today have a serpentine belt, or one belt that drives all components that rely on crankshaft power.


Most alternators are mounted using brackets that bolt to a specific point on the engine. One of the brackets is usually a fixed point, while the other is adjustable to tighten the drive belt.

Alternators produce AC power through electromagnetism formed through the stator and rotor relationship (more on that later). The electricity is channeled into the battery, providing voltage to run the various electrical systems.

The Components of an Alternator

If your car has a bad alternator, you can still drive it; however, it's best not to. BACHTUB DMITRII/Shutterstock

For the most part, alternators are relatively small and lightweight. Roughly the size of a coconut, the alternators found in most passenger cars and light trucks (including most hybrids) are constructed using an aluminum outer housing because the lightweight metal does not magnetize. This is important since aluminum dissipates the tremendous heat generated by producing the electrical power and since the rotor assembly produces a magnetic field.

If you closely inspect an alternator, you'll find it has vents on both its front and back. Again, this aids in heat dissipation. A drive pulley is attached to the rotor shaft on the front of the alternator. When the engine is running, the crankshaft turns the drive belt, which in turn spins the pulley on the rotor shaft. In essence, the alternator transfers the mechanical energy from the engine into electrical power for the car's accessories.


On the back side of the alternator, you'll find several terminals (or connecting points in an electrical circuit). The most common terminals include:

  • S terminal: Senses battery voltage
  • IG terminal: Ignition switch that turns the voltage regulator on
  • L terminal: Closes the circuit to the warning lamp
  • B terminal: Main alternator output terminal (connected to the battery)
  • F terminal: Full-field bypass for regulator

Cooling is essential to an alternator's efficiency. It's easy to spot an older unit by the external fan blades found on the rotor shaft behind the pulley. Modern alternators have cooling fans inside the aluminum housing. These fans operate the same way, using mechanical power from the spinning rotor shaft.

As we start to disassemble the alternator, we find the diode rectifier (or rectifier bridge), the voltage regulator, slip rings and brushes. The regulator distributes the power the alternator creates, and it controls the output of power to the battery. The rectifier bridge converts the power, while the brushes and slip rings help conduct current to the rotor field winding or wire field. Now let's crack the coconut open.

Inside an Alternator

Opening the alternator reveals a large cylinder with triangular finger poles around the circumference. This is the rotor. A basic alternator is made up of a series of alternating finger pole pieces placed around coil wires called field windings that wrap around an iron core on the rotor shaft.

Since we know the pulley attaches to the shaft, we can now visualize how the rotor spins inside the stator. The rotor assembly fits inside the stator with enough room or tolerance between the two so that the rotor can spin at high speeds without striking the stator wall. On each end of the shaft sits a brush and a slip ring.

As we touched on briefly, alternators generate power through magnetism. The triangular finger poles fixed around the circumference of the rotor are staggered, so the north and south poles alternate as they surround the wire rotor field windings. This alternating pattern creates the magnetic field that in turn induces voltage into the stator. Think of the stator as the catcher's glove as it harnesses all the power created by the spinning rotor.

All these components work together to give us the power we need to run our vehicles.

Understanding Alternator Power Output

In the early days, cars used generators rather than alternators to power the vehicle's electrical system and charge the battery. That's not the case anymore.

As automotive technology evolved, so did the need for more power. Generators produce direct current, which travels in one direction, as opposed to the alternating current used for the electricity in our houses, which periodically reverses directions.


As Nikola Tesla proved in 1887, alternating current becomes more attractive as it generates higher voltage more efficiently, something necessary in contemporary automobiles. But car batteries can't use AC power since they produce DC power. As a result, the alternator's power output is fed through diodes, which convert the AC power to DC power.

The rotor and the stator are the two components that generate power. As the engine rotates the alternator pulley, the rotor spins past three stationary stator windings — or wire coils — surrounding a fixed iron core that makes up the stator. This is referred to as a three-phase current. The coil windings are evenly spaced at intervals of 120 degrees around the iron shaft.

The alternating magnetic field from the rotor produces a subsequent alternating current in the stator. This AC current is fed through stator leads into a connecting set of diodes. Two diodes connect to each stator lead to regulate the current. The diodes are used to essentially block and direct the current. Since batteries need DC current, the diodes become a one-way valve that will only allow current to pass in the same direction.

Three-phase alternators have three sets of windings; they're more efficient than a single-phase alternator, which produces a single-phase AC current. When working properly, the three windings produce three currents that make up the three phases. Adding all three together produces the total AC output of the stator.

The two basic stator winding designs are delta wound and wye style. Delta wound are easily identifiable by their shape, as they're triangular. These windings allow for a high current flow at lower RPM. Wye windings resemble the flux capacitor seen in "Back to the Future." These windings are ideal for diesel engines, as they produce higher voltage than delta stators at even lower RPM.

After the AC/DC conversion, the resulting voltage is ready to use in the battery. Too much or too little voltage can damage the battery, as well as other electrical components. To ensure the correct amount, a voltage regulator determines when and how much voltage is needed in the battery.

One of two types of regulators are found in most alternators: The grounded regulator works by controlling the amount of negative or battery ground going into the winding in the rotor, while a grounded field type works the other way around — by controlling the amount of battery positive. Neither poses an advantage over the other.

The Life and Death of an Alternator

checking alternator
Running a load test on your car's alternator using a voltmeter or multimeter could help you figure out if your alternator is on the fritz. Tricky_Shark/Shutterstock

With so many components working to create the electricity vital for our vehicles, it's safe to say the alternator is a crucial component under the hood. But like many parts on our cars, they fail. Alternators have moving parts, get dirty and are subject to stress from heat and cold. As a result, the internal parts gradually wear out.

One of the most common failures is bearing failure. The needle bearings that allow the rotor to spin freely inside the stator can break down from dirt and heat. When the bearings fail, the rotor will not spin efficiently and can eventually seize. Usually an alternator with bearings failure makes a loud grinding noise. If you suspect this problem, it's only a matter of time before the alternator gives up.


You can take several easy steps to diagnose whether your alternator is on the fritz. First, most cars today have a dashboard light that glows when the ignition is switched on. The light for the electrical system is usually represented by a battery symbol.

Have you ever heard a buzzing noise when the key is on, but the car isn't running? That's the voltage from the battery running through the charging system. If this bulb is burned out, chances are the alternator won't work. The car may start, but it's just a matter of time before the battery drains and the electrical system fails.

As a rule, a three-phase alternator can operate with only one of the stator windings operational, although it's only one-third as efficient. To test whether your car had an issue with one of its stator windings, you'd need to use a voltmeter to check the voltage. (You can buy a basic voltmeter at an electronics store.) This is called a load test.

Since the battery produces DC power, set the voltmeter to DC rather than AC. Connect the red lead (or wire) to the positive terminal and the black to the negative. With no accessories on, start the car and gently press the accelerator with the car in park and the brake on to raise the RPM to around 1,000. The voltage should register around 14 volts. Anything less than 12 may indicate a problem.

Next, turn on the headlights, air conditioner, radio and anything else that draws electrical power. Rev the engine and check the voltmeter. Again, the voltage should register around 14 volts. If you have a failing alternator, the voltage will be well below 14 volts. If so, it's time to replace the alternator.

Before you decide you need to yank the alternator and replace it, make sure you check the serpentine belt. If the belt is worn or loose, the alternator won't function properly. A bad belt is easy to replace and won't set you back much, usually less than $20. But if you have to replace the alternator, you have options.

Replacing Your Alternator

repairing alternator
You can repair or replace your car's alternator at home if you're good with cars, but it might be easier to pay a trusty mechanic to do it for you. ukrit yasuwan/Shutterstock

Typically alternators are less expensive than, say, a power steering pump or air conditioner compressor. Nevertheless, you don't always have to fork out a lot of cash for a replacement. Many automotive stores sell remanufactured or rebuilt alternators at a discount. A rebuilt alternator will usually cost between $100 and $200, depending on how much power your car requires.

While alternators are relatively inexpensive in comparison to some of the other components on your car's engine, some can be expensive depending on the car make. But they often run between $250 and $350.


Sometimes alternators are easily accessible and simple to replace for the at-home mechanic. With a modest amount of automotive experience and the proper tools, you can change an alternator in your garage. But more and more cars don't have much space under the hood, and the alternators can be difficult to reach without first removing several other components. In this case, it's best to take your car to an experienced technician who can do the job quickly.

If you happen to own an alternator that has a repair kit available for sale, you can really save some money. Alternator repair kits are usually less than $30, depending on which components you need to fix. Again, you need the proper tools and a little know-how, but if you're able to find the right kit and know what you're doing, you can rebuild an alternator for a fraction of the cost of even a remanufactured unit.

One thing is certain: A bad alternator will ruin a good battery if you don't address it quickly. Batteries can only be recharged so many times before they'll lose their ability to hold a charge. If the battery isn't relatively old, it should survive. But an older battery that is constantly drained and charged will have a shorter life span.

Originally Published: Jun 9, 2008

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