Battery Light on In Your Car? Low Voltage Codes? How to Tell If It’s the Battery or Alternator

Is the battery light on in your car? Did you get a reading for low voltage codes? Before diagnosing the codes, you’ll want to check the condition of the battery and the charging system. Find out how to tell if the problem is with the battery or the alternator in this article and video.

How to Tell If the Battery or Alternator Turned on the Battery Light or Triggered Low Voltage Codes

Mechanic testing if the problem is with the battery or the alternator after the battery light came on and the vehicle read for low voltage codes

How to tell if the battery or alternator is turning the battery light on or making low voltage codes appear in your car

  1. Inspect the Battery for Age and Its Condition

    Look on the battery for a sign of its age. Sometimes you can find a date on the battery, and they tend to last up to 5 years. If your car is old and it looks like the battery is the original, you’ll likely need a new battery.

    Check the condition of the battery terminals. See if they are loose or corroded, meaning they have a powdery build-up. This can decrease the battery’s performance and it’s a sign you’ll need to clean them.

    Read more on how to clean a car battery of corrosion

  2. Test the Battery with a Battery Analyzer or Multimeter

    A battery analyzer can tell you the condition of the battery and offer more information about it than a multimeter. If you have a battery analyzer or tester, hook it up to the battery terminals and get a reading of the battery’s condition.

    You can also test the battery with a multimeter. A multimeter will not give an as in-depth reading as a battery analyzer, but it will let you know what the surface charge is.

    Place one prong on the negative terminal and the other prong on the positive terminal. You want a reading of 12.6 volts. In this example, our multi-meter recorded 12.47 volts, so our battery needs a charge.

    Read more on how to use a battery analyzer

  3. Charge the Battery If Needed

    If the battery needs to be charged, charge it with a charger to 12.6 volts. The battery will probably be okay if it is charged above 12.3 volts, we charged it to be safe.

    Read more on how to charge a car battery

  4. Inspect the Alternator for Damage or Loose Connections Before Starting the Car

    Before starting the vehicle to see if the battery is working fine, check the alternator. Make sure the alternator is there and that it wasn’t stolen. Confirm the belt is attached to the alternator, and check the wires and electrical connections for damage or looseness.

  5. With the Battery Charged, Connect a Voltmeter and Start the Car

    With the battery charged, hook up a multimeter to the battery and start the car to get a reading. The voltage will temporarily drop with the car started, but the reading should be above 9.5 volts. Then, with the car running, the reading should be above 13.5 volts.

    In our case, the reading is below 12 volts. This means there is a problem with the alternator, but there could also be a problem with the wire traveling from the alternator to the battery, or the signal that turns the alternator on.

  6. If Replacing the Alternator, Test the Voltage After It Is Replaced

    The alternator is known to fail on this vehicle, so we replaced it with a new one. After replacing the alternator, the voltage read way over 13.5 volts with the car running, so it is all set.

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Battery Light on in Car? Low Voltage Codes? - DIY Tips - 1A Auto
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Battery Light on in Car? Low Voltage Codes? - DIY Tips - 1A Auto
If the battery light is on in your car or you got a reading for low voltage codes, find out how to tell if the problem is with the battery or the alternator with tips from our expert mechanics
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1A Auto
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