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While a car alternator can work reliably for many years and keep the battery topped up while powering the car’s electrical accessories as you drive, you can still face an issue that affects its performance.
For example, the output voltage from the alternator may be lower than expected.
This post lists what signs you can watch out for that may point to a low output voltage, what checks you can do to identify the cause, and how you can fix the underlying fault faster.
What Causes an Alternator to Put out a Low Voltage?
If the output voltage from the alternator is lower than the 13.5-14.5V range, check the following:
1. The alternator belt is firmly connected to the alternator pulley and is not cracked on the sides, slipping, or loose.
A loose or worn alternator belt can lead to a lower output voltage from the alternator.
2. The Battery terminals are corroded, or the battery positive, negative, or ground cable connections are damaged.
Confirm that the cabling from the battery negative to the car chassis is firm with no corrosion and that the connector plug is firmly fitted into the back of the alternator.
Loose, damaged, or corroded terminals can cause voltage drops at the battery terminals.
3. The car alternator is faulty. It is possible that the alternator is faulty and needs to be replaced too.
What is the Voltage of a Working Car Alternator?
The voltage of a car alternator connected to a car battery, measured across the car battery terminals should read between 13.5 – 14.5V even when the engine is idling.
A lower voltage reading when the car engine is running is a sign that there’s a problem probably with the alternator, it’s wiring or even the battery connections.
When is a Car Alternator Voltage too Low?
If the alternator voltage is lower than 13.5 V then the car battery will not be able to charge fully. With the battery only partially charged, the battery can appear to drain faster.
The car may not be able to start or in extreme cases, the battery may fail prematurely from not being maintained at a sufficiently high voltage level.
Why a Car Alternator may be putting out only 12 or 13 Volts?
An alternator voltage as low as 12V or 13 V measured at the battery terminals when the car engine is running may be caused by:
- damaged battery positive, negative, or ground connections,
- the connector plug into the alternator or the wires connecting the alternator to the battery be loose or damaged.
Contact a qualified mechanic or auto repair shop to conduct additional tests and confirm the fault.
Signs an Alternator Voltage is Low
Though not confirmatory, the signs listed may point to a low output voltage from the alternator.
Additional tests by a qualified mechanic need to be done to check if indeed it is a case of a low alternator voltage and the fault is with the alternator.
1. Red battery warning light on the car dashboard. This is a sign that there’s a problem with the car’s charging system that may be caused by a bad battery, loose wiring such as the bad alternator or battery connections, and the low alternator voltage.
2. The car battery condition is confirmed as good yet appears to drain faster than usual or you experience starting problems associated with a low battery voltage.
While this can also be caused by a parasitic load that drains the battery charge, it could also be that the battery is not fully charged because the alternator voltage is low.
3. A voltage reading of 13V or below with the engine running. This may be caused by low voltage from the alternator, a bad battery, a loose or broken alternator connector, or damaged battery cables.
Additional testing needs to be done to confirm the actual cause of the fault.
Tips to Check the Car Alternator Voltage
Measure the alternator output voltage by taking a voltage reading using a good quality digital voltage meter set to read DC volts when the car engine is running.
Note: Do not disconnect the battery terminals when the car engine is running as it can damage the alternator!
To check the alternator voltage when the car engine is running, you can use either of two methods.
1. Alternatively, use a good quality cigarette lighter car battery voltmeter. Simply plug it into the cigarette lighter socket the and display shows the alternator/ battery voltage.
2. Or, use the car’s dashboard voltmeter ( if it has one).
Can you Increase the Output Voltage of an Alternator?
Yes, you can increase the output voltage of an alternator up to a limit by revving the engine (RPM). Keep in mind that there are other factors at play that contribute to the actual alternator voltage.
These include engine temperature, alternator design, and battery voltage.
Related Topics
1. Why an Alternator may be only 12 Volts?
If the alternator is only putting out 12Vs, it might be that:
- the battery terminals are corroded,
- the battery positive, negative, or ground connections are damaged,
- the connector plug into the alternator or the wires connecting the alternator are loose
2. Why an Alternator Voltage May Still Be Low with a New Alternator and Battery?
If the battery voltage is still low even with a new alternator and battery fitted, double-check that:
- the alternator belt is still in good condition,
- is firmly fixed, the battery terminal and ground connections are free from corrosion and firmly fixed,
- the alternator connector is free from corrosion and firmly plugged into the back of the alternator.
Do not hesitate to contact a qualified auto mechanic.
Closing Thoughts
Your car alternator output voltage may be low for several reasons ranging from a loose or damaged alternator or battery wiring, broken or loose ground cable at the battery terminals, loose or worn alternator belt, or a bad alternator.
Contact a qualified auto mechanic unless you have the skills to conduct the repairs yourself safely.
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