How Alternators Work

Your car’s alternator generates the electricity required to charge the battery and run all the electrical components of the vehicle. It will be mounted towards the front of the engine, and you’ll see the front of the alternator with its pulley connected to a long belt called a serpentine belt. The alternator will be mostly grey or silver with lots of holes in its housing to allow air through for cooling, and if you look closely, you can usually see the copper windings of the stator or rotor inside the housing. The stator is the set of copper windings that stay in one place, while the rotor is the set of copper windings that rotates on a shaft in the middle of the alternator. The pulley that attaches to the serpentine belt is connected to the rotor with that shaft.

What Does an Alternator Do?

It may be helpful to think of your vehicle’s alternator as a generator, because that is essentially what it is. In fact, automobiles up to the 1960’s used a generator (which produces electricity but does so differently with different internal parts) and some vehicle manufacturers (many in Europe) still refer to a modern-day alternator as a generator. In simple terms, your car’s engine delivers power to the wheels. The engine also spins the alternator through a drive belt, or serpentine belt, and pulley system. This causes the alternator to create electrical current that is fed back in to the electrical system, and battery.

How does an Alterntor Work?

The alternator converts the rotational mechanical energy of the engine into the electrical energy your vehicle’s components need to function. It receives mechanical energy from the pulley, which is rotated by the engine belt, and uses magnetism to generate alternating current. Furthermore, it changes this alternating current (AC power) into direct current (DC power) that your vehicle can use.

  • Stator: The stator is a set of fixed or stationary wire windings. Several of these precisely spaced windings work with the rotor to generate an alternating current.
  • Rotor: The rotor is the part of the alternator that spins. It is supported on both ends with bearings and spins with the assistance of a pulley and drive belt. The rotor features an iron-core center that has a copper wire wrapped around it. Energizing the wire with an electrical current causes a magnetic field with north and south poles to develop.
  • Bridge Rectifier: The bridge rectifier converts the alternating current (AC) from the stator into direct current (DC) that your vehicle can use. To accomplish this task, the rectifier has several diodes. These diodes allow current to flow in one direction and prevent it from flowing backward. This changes the alternating current into direct current.
  • Voltage Regulator: The voltage regulator is the nerve center of the entire assembly. It is an electronic component and is responsible for maintaining the charging system voltage within a specified range.

The Parts of an Alternator

The charging system in your vehicle includes the alternator and battery, but there are also other important parts that are essential to the proper operation of the alternator. 


The alternator housing will have many holes and will be constructed of aluminum, so it will look silver or grey, if it’s clean. Manufacturers use aluminum because it does not magnetize. 


This is the part that you will see at the front of the alternator, and it will have the serpentine belt running around it. This is what provides the spin to the rotor that generates electricity in the alternator.


The stator is the set of copper windings that sits just inside the housing and doesn’t move when the alternator is running.  


The rotor is the set of copper windings that is attached to the shaft of the alternator and creates the magnetic field when an electrical charge is applied to the windings. 

Slip rings

The slip rings are two rings that sit at the end of the rotor shaft and provide a contact point for electricity to travel into the rotor and power the electromagnet. 


The brushes are spring loaded pieces of metal that contact the slip rings so that electrical current can be transferred to the rotor for the electromagnet. 


At each end of the shaft, there is a bearing that allows the shaft to spin freely. 


The rectifier converts the alternating current (AC) that your alternator produces to the direct current (DC) that your car needs to run its electrical systems. 


The regulator measures the voltage that is coming out of the alternator and if it is too high, it will reduce the amount of electricity going to the electromagnet in the rotor, causing the alternator to generate lower voltage. If the output voltage is too low, it will increase the flow of electricity to the rotor, causing the alternator’s output voltage to increase. It keeps the output voltage at between 13 and 14.5 volts. 


The fan is located inside the housing and it draws airflow into the alternator to cool the parts and avoid overheating. 

Serpentine Belt

The serpentine belt isn’t really part of the alternator, but it is a vital part of the electrical system because it turns the rotor in the alternator to generate electricity. The belt takes its power from a pulley at the bottom of the engine called the serpentine drive pulley or crankshaft pulley.

Idler Pulley

Idler pulleys are also not actually part of the alternator. They are pulleys bolted to the front of the engine that don’t turn any shafts. They just help to route the serpentine belt to all the important pulleys, so if you trace the travel of your belt, you’ll often see that an idler pulley makes it “turn a corner”. 

Tensioner Pulley

The tensioner pulley is another important part of the system that isn’t attached to the alternator. You will typically have one tensioner pulley and its job is to keep the serpentine belt running with the correct tension. If the serpentine belt is too loose it can cause some of the accessories to “slip” and not produce enough power. The belt is too tight, you can prematurely wear out the belt. Some Tensioners are spring loaded and others have to be set manually. If you are replacing your serpentine belt, the first step will be to loosen the tensioner pulley.

What Causes an Alternator to Go Bad?

Most modern alternators are well-built and can function for a long time. Still, they are not invincible. Eventually, your vehicle’s alternator may require repair or replacement. Typically, when alternators fail, the culprit is one of the following:

  • Bearing failures: The alternator’s rotor relies on bearings to keep it spinning freely and smoothly. Like the bearings in other parts of your vehicle, the alternator’s bearings can eventually wear out. If this happens, you are apt to hear a loud, grinding noise coming from under your car’s hood. These sounds should get louder as you listen near the alternator, if that is truly the problem. Furthermore, if the alternator’s bearings are well past their prime, the belt may whine, chirp or squeal.
  • Damaged regulator or rectifier: If your alternator has a damaged regulator or rectifier, you may face a variety of problems. These could include a dead battery, an lit battery warning or message on the instrument cluster, and dim lights when driving at night. Your vehicle may have an instrument readout that tells you how many volts the alternator is delivering. This should be between 12.3 and 14.4. If your car does not have a voltage gauge, you can use a voltage meter to test alternator output.
  • Bad belts: Your alternator needs the pulley and belt system to function optimally. Eventually, drive belts may fray or otherwise wear out. Regularly inspecting the drive belt for signs of wear is a good way to ensure your alternator’s rotor keeps spinning properly.
  • Worn Brushes: The brushes in the alternator are wear items and eventually they do wear out. This will cause the alternator to not charge properly and can also lead to a dead battery or lit CEL on the instrument cluster.
  • Over Heating: Heat is a major cause of failure in alternators. The alternator has vent holes that allow heat to escape and help to prevent overheating. When vent holes are plugged by 60% or more, failure is likely to occur. Also, if the battery does not hold a charge of 12.4 volts or more, the alternator will constantly try to recharge a depleted battery. This constant charging can cause an alternator to overheat.

Because the consequences can be significant, you do not want to ignore problems with your vehicle’s alternator. After all, if the alternator fails, you may find yourself stranded somewhere you would rather not be. You may also cause additional damage to your vehicle by putting off essential alternator repairs.

Alternator Maintenance

If your it fails, you probably need to replace the alternator. You can, however, take certain steps to keep this part in tip-top shape.

First, keep a good battery under your car’s hood. Because car batteries and alternators go hand-in-hand, you should replace a old, damaged, weak, or ineffective battery. After all, a bad cell inside your battery can ruin your alternator. If you have a vehicle that doesn’t get driven often, consider using a trickle-charger to keep the battery maintained during the down time. The opposite is also true. If you have a failing alternator but a good battery, your battery may disguise alternator performance issues. Eventually, though, both components are likely to require replacement. It is important to note, an alternator should NEVER be used to charge a dead battery. Any time your battery is suspect of being dead, or low on charge, always use a battery charger to complete this task. Charging a dead battery via the alternator will lead to alternator failure. Many times, this is why both failure of the battery AND the alternator can occur within weeks of each other. First, the battery fails. Then, after frequent jump starts and driving around with a dead battery, the battery is replaced, only to find out the new battery goes dead after several days, because the alternator died during this same time. It is a frequent occurrence.

If you think your vehicle’s alternator or battery may be suspect, you can seek out one of our preferred shops in your area to help. Also, you have several testing options that AutoZone can do. We can do a charging test in the parking lot which tests the battery, and will determine whether or not your system is charging properly. This test may not pinpoint whether the alternator is bad or not, but you can also remove the alternator from your vehicle and have it tested at the nearest AutoZone auto parts store.

Coming Out on Top

Now that you have answered the question, “What does an alternator do?” you likely understand the importance of keeping your alternator in tip-top shape. If you want to come out on top, you cannot leave the health of your vehicle’s alternator to chance.

By watching for common problems, you know when to repair or replace your alternator. If something goes wrong, you need high-quality components and the right tools to achieve success. Take a look at your alternator today. If you find a problem and need any parts or tools, stop by your local AutoZone.

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Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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