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What Is an Alternator?

mazda cx 5 2021 11 engine jpg 2021 Mazda CX-5 | Cars.com photo by Christian Lantry

An alternator is a generator that produces the electricity stored in a vehicle’s 12-volt battery and powers the starter motor for the engine and the various computers, lights, power windows and locks, wipers and other electrical accessories.

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The name comes from  the alternating current that gets converted to direct current, which is what the battery and the electrical accessories use. Without an onboard generator to continuously recharge the 12-volt battery, the battery would quickly run out of power and the car would stop running.

Alternators replaced DC generators in the 1960s because they were lighter, able to produce more power and proved to be more reliable. 

They are typically mounted on the front of engines and connected to the crankshaft by an accessory drive belt (often along with the air-conditioning compressor and water pump). Whenever the engine is running, the belt turns rotating parts inside the alternator to generate electricity and charge the battery.

Although alternators often last 10 years or longer, the serpentine belts that drive them can wear out sooner and break. When a belt starts slipping, the alternator won’t be able to generate enough electricity for the battery to keep up with the electrical demands of the vehicle. That often will trigger a dashboard warning light or, worst case, prevent the vehicle from starting.

Electric vehicles and hybrids don’t have an alternator. Instead, they have a DC-to-DC converter that recharges the 12-volt battery using power from the high-voltage battery pack (the “traction battery”) that powers the vehicle. 

On a gasoline or diesel vehicle, the alternator generates electricity from mechanical power supplied when the engine is running. In an EV, there isn’t an engine to provide that mechanical power, so it comes from the main (traction) battery. 

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