The Alternator

What is an alternator?

The alternator powers most car's electronic components while driving around or idling, including headlights, electric steering, power windows, windshield wipers, heated seats, dashboard instruments, and radio. The alternator supplies all of them with direct current (DC) power.

Your alternator is also responsible for charging your car battery while driving. Please keep in mind that the car battery's only purpose is to start the car. When you sit in your car with the radio on, or when you leave your headlights or dome lights on, when you have the auxiliary on, or even if you don't close one of your car doors properly, the battery will drain. If your car battery is drained and you can't start the engine, the alternator cannot recharge the battery. In this case you need a battery boost (or a car jump start).

How does a car alternator work?

The alternator works by turning mechanical energy into electrical energy. When your engine is on, it powers a drive belt that rests on a pulley attached to the alternator. The pulley turns the alternator's rotor shaft and spins a set of magnets around a coil. These rotating magnets generate alternating current (AC) around the coil, which is then sent to the alternator's rectifier. The rectifier converts AC power into DC power, activating your car's electrical systems.

Alternators typically last the lifetime of your vehicle, but that doesn't always happen. General wear and tear, heat damage, overuse, exposure to water, faulty parts, or frayed wires can put your alternator out of commission before your car heads to the scrap yard.

How to test an alternator

If you have a newer battery, but your car won’t start, you may have a bad alternator. Don't test an alternator by disconnecting the negative battery cable. A good alternator may keep the engine running without the negative cable, but this was never a good test.

In the pre-computer days, you could pull it off without damaging anything. Today, you risk frying every electrical device in your vehicle. The second you disconnect the battery, the voltage regulator pegs the alternator to put out maximum power. With no battery in the circuit to act as a buffer, the alternator can put out up to 150 volts, depending on engine rpm. When the smoke clears, that “simple test” could cost you several thousand dollars for new electronics.

Here is how to conduct an alternator voltmeter test:

  • With the engine off, the battery voltage should be between 12.5 and 12.8 volts. If it’s below that, charge the battery with a battery charger before you conduct the test again.
  • Then start the engine and check for increased voltage readings. If you see higher readings, the alternator is good. (More sophisticated testing equipment is needed to detect an open or shorted alternator diode.)
  • Good alternator voltage reading: If you connect the meter leads to the battery terminals and the meter shows that it’s in the 13.8 to 15.3 volt ranges (engine running, lights and accessories off), that means the alternator is working as it should be.

If the alternator is functioning correctly, your dead battery was likely caused by a computer module that isn’t shutting down when you turn off the car. If your alternator passes the voltmeter test, get your vehicle into a shop and pay a professional mechanic to find and correct the defective module.

Beware of the battery management system! Newer vehicles have a battery management system, controlling the voltage coming into the battery when the alternator is running. When the battery voltage drops, the voltage is higher, and when the battery voltage gets higher, the alternator voltage is lower. These battery management systems were introduced in cars in the recent years, to extend the lifetime of the battery and control the battery charging process, so the alternator output might vary over the span of few minutes. Let the engine run for ten minutes and test the alternator output voltage every two minutes. If at anytime you get a reading in the 13.8 to 15.3 Volts range, the alternator is fine, and the battery management system is doing its job properly.

Back to blog