What does your car’s alternator do?


Even if your car isn’t fully electric, it relies on an electrical system that can seem fiendishly complex, especially where maintenance is concerned. But once you start going with the flow, things become noticeably clearer. And in many ways, your alternator is at the heart of that flow. Because while the battery helps start your car, it’s the alternator that keeps it going.

The alternator keeps your car’s headlights on and keeps its battery from draining

A car mechanic holds up an alternator | Jan Sochor/Getty Images

Whether your ignition switch operates via a key or button press, it completes the circuit between your car’s battery, spark plugs, and starter. The starter spins the flywheel, the spark plugs light and voila, your engine is running. However, no battery has an infinite amount of juice. Also, your car battery doesn’t just power the starter. It also allows you to use lights and accessories when the engine is not running.

In short, your car battery regularly consumes a lot of energy. But that’s where the alternator comes in. Once your engine is running, the alternator charges the battery to full power. Plus, rather than the battery, it powers your car’s lights, accessories, and other electrical systems when the engine is on, The reader Remarks.

Remember the previous passage about the alternator being the heart of your car’s electrical current flow? Well, that’s not too far off the mark. Admittedly, blood is not the best metaphor for electricity. And technically, you could start a car with a bad alternator if the battery was fresh and strong enough. But just as you wouldn’t live long without a heart, a car with a bad alternator will only run briefly.

It is an all-in-one belt driven generator and AC-DC converter

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Another reason why the heart analogy is flawed is that your heart does not produce blood. Your car’s alternator, on the other hand, produces electricity. Originally, all cars used generators, called dynamos or magnetos, to charge their batteries and power their few electrical accessories. And although the alternator does all of that today, it is still technically a generator. Only it does its job in a more sophisticated way.

There is a pulley on one end of the average car alternator. The motor’s serpentine belt spins this pulley which in turn spins the attached magnet-covered cylindrical rotor. As the rotor spins, it generates alternating current (AC) in the surrounding stator copper wiring. And if that sounds familiar, that’s because that’s essentially how electric car motors work. So, in a way, an alternator is also an electric motor.

However, we are not finished yet. Although the alternator produces alternating current, the battery and your car’s electrical system use direct current (DC). But that’s why alternators also have semiconductor diodes to convert AC to DC, The reader Explain. These are significantly simpler, lighter, more reliable and more durable than the mechanical dynamo devices used. And after its conversion, the current passes through a voltage regulator, aka rectifier, which ensures that the battery is not overcharged.

Alternators are not interchangeable and not all cars use them

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Speaking of load, alternators don’t just vary in overall size. They are rated by their amperage, and some cars require high-amp versions due to their extra accessories. That’s why police vehicles, with their extra headlights, computers, and other gadgets, require heavy-duty alternators. Technically, you can put an upgraded unit in your car with no problem. But that’s not necessary unless you’re planning other electrical upgrades, like a better sound system.

Also, although modern alternators are built similarly, they are not necessarily identical to each other. So even if the mounting points line up, don’t swap the one on your sedan with the one on your pickup. This is especially true if you have a hybrid vehicle or an electric vehicle.

Hybrid cars and electric vehicles do not have or need alternators. Instead, they recharge their 12-volt batteries using their lithium-ion packs and DC-DC converters, Cars.com Explain. Mild hybrids are a minor exception, however. They also lack alternators, but that’s because their small electric motors serve as all-in-one replacements for the starter/alternator.

So, here is the humble alternator. It may be small, but it’s a vital part of your car.

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