Why the Car Alternator Output Voltage may be Low (How to Check)

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Good quality car alternators can work reliably for many years.

One in a while, you may be faced with a puzzling scenario – where the car alternator voltage measured at the battery terminals with the engine is low (13V and below) regardless of whether you increase the RPMs.

There are several possible reasons why the car alternator voltage may be low and fortunately several of them are easy to check.

In this guide, I will go over what voltage a good working alternator should put out, the symptoms of an alternator with a low output voltage, and the possible reasons why your alternator has a low voltage.

How Many Volts Should a Good Alternator Put Out?

A good working alternator should put out about 14.5 volts DC when the engine is running at about 1,500 RPM. When idling, the alternator voltage output reads about 13.6V. Note that all these voltages are measured at the battery terminals.

In fact, you should always keep the battery connected if the alternator is running to avoid damage to the alternator.

Should you measure sub 13V and below across the battery terminals when the engine is running then there is a problem with the alternator or wiring system that needs to be investigated.

How You Can Check the Car Alternator Voltage

To check the alternator voltage when the car engine is running, you can use either of two methods.

#1. The most convenient one is to use a good quality cigarette lighter car battery voltmeter. You simply plug it into the cigarette lighter socket the and display shows the alternator/ battery voltage.

#2. You can read it if your car’s dashboard voltmeter ( if it has one).

Symptoms of an Alternator with a Low Voltage

What shows that your alternator voltage is low? There are some signs that can point to a low voltage output from the alternator.

Note that these symptoms do not necessarily confirm that the alternator has a low output voltage. You need to conduct additional tests.

  • A red battery light on the dashboard comes on as you drive. This may be caused by low voltage from the alternator or a bad battery, loose wiring such as bad alternator or battery connections.
  • A voltage reading of 13V or below when the engine may be caused by low voltage from the alternator. Note this may also be caused by a bad battery, loose or broken alternator, and battery cables.
  • If a good quality charged car battery is found regularly discharged in the morning. This is usually a sign that the battery is not charging properly.

Whereas there are other possible causes such as the bad battery, loose or broken alternator, and battery cables, it may also be a sign that the car alternator is faulty and has a low voltage output.

What Causes an Alternator to Put out a Low Voltage?

There are several possible reasons why your car’s alternator voltage output may be low.

  • The alternator belt may be loose or worn. Confirm that the belt drive to the alternator is firm, is in good condition and does not slip as the engine runs.
  • Check for loose or broken wiring. This may be ground cabling from the battery terminal, the connector plug into the alternator or the wires connecting the alternator to the battery.
  • Inspect and confirm that the battery terminals and posts are clean, with no corrosion and make firm contact with the battery cable connectors.
  • High alternator temperatures. As the alternator runs and supplies current combined with the buildup of heat under the hood, the alternator output voltage will automatically reduces sightly.

It should therefore not come as a surprise when the voltage reading when the car engine has started reduces slightly after several minutes of the engine running.

Does Heat Affect Alternator Output?

Yes, it does. It is normal for the alternator output voltage to reduce slightly (up to a limit) after a few minutes of the car engine running as the alternator temperature increases.

The alternator has temperature compensation that adjusts the alternator voltage with an increase in temperature to maximize the battery life.

Can You Increase the Output Voltage of An Alternator?

Yes, you can increase the output voltage of an alternator up to a limit. To increase the alternator output voltage, increase the engine speed (RPM) and the output voltage of a car alternator will increase.

Keep in mind that there are factors at play that contribute to the actual alternator voltage such as engine temperature, alternator design, and battery voltage.

Related Questions

Why Does My Alternator only Put out 12 or 13 Volts Only?

If the car alternator is only putting out 12V or 13 V measured at the battery terminals when the car engine is running then it may be because of:

  • a faulty alternator with a damaged voltage regulator or short in the the internal wiring of the alternator
  • break in the alternator wiring.

Additionally, check out other possible causes of why the alternator voltage is low.

Final Thoughts

Your car alternator output voltage may be low for several reasons ranging from a loose or broken alternator or battery wiring, broken or loose ground cable at the battery terminals, loose or worn alternator belt, a high alternator temperature, or a bad alternator.

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