Is My Car Alternator Undercharging the Battery?

Sometimes knowing if a car alternator is working as designed is not clear cut.

There are several considerations – is the battery drained when the voltage is measured, at what RPM is the car engine running, and others.

So in this post, I go over some basic tests to helo you determine if the car alternator may be undercharging, possible causes and how you can fix the undercharging.

Can an Alternator Undercharge a Battery?

Yes, it is possible for an alternator to supply a lower than expected voltage to the battery thereby undercharging it.

There are other possible reasons why your car battery may undercharge such as loose battery or alternator connections including the ground connections.

Therefore, you need to conduct additional tests to confirm if the reason the battery is undercharging is the alternator.

Signs that an Alternator is Undercharging

Though these signs are not confirmation of an alternator undercharging the battery, they can signal that there is a good chance that the alternator is undercharging the battery. Note:

Additional tests need to be done to confirm where the fault is.

If you are unable to start your car say in the morning after driving it yet the car battery drain appears normal when measured and the disconnected battery is able to hold charge and start the car once fully charged with an external charger.

The car loses power and stops running when driving combined with the battery warning light on the dashboard flashing signaling a problem with the car’s charging system.

Note, this may be as a result of a problem with the electrical wiring in the car or a bad battery too.

What Tests to Confirm if an Alternator is Undercharging

If you can, take the car or alternator to an Autozone and have it bench tested to confirm if it is working well and what the problem might be.


# 1. You can start the car engine and with the engine at idle, measure the battery voltage (no accessories switched ON). The meter should read between 13 – 14V if the alternator is working well.

# 2. You can take it a step further and switch on the headlights with the car engine running and measure the battery voltage again.

The battery voltage should drop by no more than 0.2V. If the voltage you measured in step 1 above was 13.5V, the voltage measured should be no less than 13.3V.

If the voltage is lower, then there is a good chance that the alternator or wiring has a problem.

Why an Alternator May Undercharge the Battery (5 Reasons)?

If your alternator appears to be supplying a lower than expected voltage to the battery, here are 5 common reasons why this might be the case.

Alternator Faults

#1. The alternator voltage regulator is faulty. If the voltage regulator is faulty, then it is not able to regulate and supply the necessary voltage required to keep the engine running and the car battery charged.

#2. The alternator’s stator windings or diodes (open or shorted). If the alternator windings or diodes are open or shorted then the alternator will not be able to supply the necessary voltage to fully charge the battery and keep the engine running.

#3. The alternator’s belt is loose or slipping. If the alternator belt is loose then the alternator will not run at the necessary speed to generate enough power to top up the battery within the expected time, leading to a lower battery being undercharged.

Beyond the problems associated with the alternator, there are other factors that can lead to the car battery being undercharged.

#4. Weak or loose battery or alternator connections. If the cables connecting supplying power to the battery do make firm, clean contact then there will be limited current flow to the battery.

This may be because they are loose or there is corrosion at the battery terminals that need to be cleaned.

How to Fix an Undercharging Alternator

Whereas some car owners choose to repair faulty alternators by replacing faulty regulators or diodes, for reliability and peace of mind, in my opinion, it is probably best that you replace it with the manufacturer’s recommended part. Some people have had their alternators repaired

Final Thoughts

If your car suddenly stops when driving or your car is not able to start in the morning after a drive then it is possible that you have a case of an undercharging alternator if the drain on the battery is low and the battery tests confirm that it is in good condition.

It is best though to have the alternator and battery checked at an Autozone.

The common causes of an undercharging alternator range from faulty voltage regulators or diodes, loose or slipping alternator belts, and open or shorted alternator stator windings.

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