How the Alternator Works in Your Car

Your car alternator, at its most basic, is what works along with your battery and the rest of your charging system to keep your car running. When you start your car, you’re running on battery power. Then the battery transfers power to the alternator to power the vehicle. The alternator is comprised of the following parts:

The alternator rotor

This is simply a coil of wire that is wrapped around an iron core. The wire delivers a magnetic field via DC (direct current) – in other words, a current that flows in only one direction. The current is delivered to the coil by means of slip rings and brushes, and the alternator pulley rotates when the engine runs.

The stator

There is another set of three coils around the rotor – together, they form the stator. The stator is attached to the alternator shell, and it does not rotate. The rotor turns inside the stator and creates an AC, or alternating current.

The output diodes

AC voltage is weak, so before it can be used, it has to be converted to DC. This happens in the output diodes. They convert the current so it moves in only one direction, and blocks the flow of the current in the opposite direction. Six output diodes form the bridge rectifier, which regulates the voltage output of the alternator. The DC current pulsates six times for each rotation of the rotor – it’s not a pure DC current, but for most automotive purposes, it’s good enough.

The diode trio

These three diodes provide the alternator regulator with field current.

The voltage regulator

The voltage regulator is the part of the alternator that senses any drop in the battery voltage, and delivers more field current to the rotor, increasing its voltage output. If the battery voltage goes up, then the voltage regulator sends less field current to the rotor, reducing the output.

The field current supply

Field current is supplied from the alternator by way of the diode trio, and from the battery by way of the alternator warning lamp. When you first start your car, voltage is supplied by the battery. Assuming that your charging system is working properly, the output shifts to the alternator.

The alternator or battery light

The alternator or battery light alerts you to problems with the alternator, or possibly with other parts of your car charging system. A mechanic can perform diagnostics that will identify the problem, which can then be corrected.

Next Step

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42 years of experience
1589 reviews
42 years of experience
Dodge Charger V6-3.5L - Alternator - Houston, Texas
Ben changed the alternator on my 2010 Dodge Charger today with excellent customer service and ease. He arrived right on time and spared no wait to get started on my car. He was done with the repair after about 45 minutes and did his due diligence to make sure the alternator was charging properly. Will definitely be using this service again and will be requesting Ben!
Ford Focus - Alternator - Houston, Texas
Ben was Awesome!!! He was professional and courteous and had a wealth of experience and knowledge about cars. A real life saver!


47 years of experience
416 reviews
47 years of experience
Cadillac Eldorado V8-4.6L - Alternator - Tulsa, Oklahoma
Walter is literally the best mechanic I’ve ever come across. He went to great lengths and waaaay above call on my car. I got a bunch of his “Your Mechanic” cards, and recommended him to three people already. As long as I am in Tulsa, I will use Walter on all future repairs and maintenance for my 1995 Cadillac Eldorado! Thank you, Walter!
Honda Odyssey - Alternator - Sapulpa, Oklahoma
Very knowledgeable, patient, and great advice. Completed the task in a timely manner. Would recommend again.


8 years of experience
187 reviews
8 years of experience
Jeep Liberty V6-3.7L - Alternator - Tacoma, Washington
Very polite and speedy service. Even though the appointment was delayed by 2 hours, he was so quick that it made up the difference! Thank you!
Mazda 5 - Alternator - Puyallup, Washington
Kevin came early and he fixed my battery light issue! He was super fast and he did amazing with explaining what could have been the issue. As a single mom of 3, he took so much stress off my shoulders today!


21 years of experience
168 reviews
21 years of experience
Dodge Grand Caravan V6-3.6L - Alternator - Miami, Florida
Jose was very attentive, patient and methodical. My impression of Jose is that he knows the value of having loyal clients that keep coming back for the quality work and honesty. So he is upfront and frank about what needs to be done.
Dodge Ram 1500 - Alternator - Miami, Florida
Extremely knowledgeable! Very professional an courteous and takes the time to explain things! Excellent mechanic!!!!

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