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Can a Bad Alternator Drain a Battery? (A Helpful Guide)


It’s really frustrating to wake up in a nice mood and face the starting issue in your car. Should you blame your battery or alternator? If you are here, then most probably you are suspecting your alternator. So, can a bad alternator drain a battery?

As a general rule, a bad alternator can drain a battery if the alternator has a defective diode. Ideally, the alternator should pass the current to flow in one direction only. However, when the alternator is faulty, it can start draining the battery.

Read on to learn in detail about why a bad alternator drains a battery, plus how to figure out if it is really bad or not!

A Quick Overview of Alternator

Before we learn the exact reason why a bad alternator drains a battery, let’s refresh the basics quickly to get a better understanding.


When the car’s engine is off, it needs electricity to start again. That initial push of electricity comes from the battery inside your car.

So, when the engine is off, the battery offers full power to the engine to start again. It provides a large current to the starter motor for starting the vehicle.However, by doing this, the battery also drains.

As the engine starts, an alternator is used to recharge the battery. This ensures the next time you start your car; the battery has enough energy stored in it to power the starting process again.

Functioning of Alternator

The normal function of an alternator is to convert the mechanical energy from the engine into electrical energy. When the car is running, and the engine is on, the alternator keeps on charging the battery and powering the other electrical devices of the car.

An alternator produces an alternating current, and hence it is called an alternator. This type of electric current is similar to the type used in our homes.

But the thing is, the various electrical components of the car also use a different type of electricity, known as DC or Direct Current.

In this type of electricity, the electric current flows only in one direction (and the electricity is the same as the one obtained from a battery)

Now to power those devices, the alternator converts the alternating current into a direct current by the usage of a rectifier.

Moreover, the output voltage of an alternator varies depending on the speed of the car. Hence, to maintain a near-constant output voltage, the alternator also has another component called a regulator.

The alternator has small devices called diodes that transfer the electricity from the alternator to the car battery.

Why Can A Bad Alternator Drain A Battery?

In ideal conditions, the current from an alternator is rectified into a direct current, and it is made to flow in one direction only.

However, if the diode in your alternator goes bad, then it would cause the reverse thing – it will let the car battery charge flow back into the alternator of your car. And gradually, this way, it will discharge your car battery.


And this is how a bad alternator drains your battery. Now there can also be other reasons why your battery keeps on draining even though it is in good condition.

If there is any sort of wiring problem between the alternator and the battery, then despite the alternator being in good condition, your battery can’t remain charged fully.

The faulty wire connection could potentially cause the alternator to provide insufficient electric current to your battery.

Also, if you don’t have a good-quality and powerful alternator that is needed based on your car’s specification, then also your battery may not be charging properly.

So these are some of the things you should keep in mind while suspecting your alternator.

How To Know If You Have A Bad Alternator?

So to identify if you have a bad alternator or not (means to essentially check if you have a bad diode or not), you need a voltmeter. You can check out my recommended voltmeter here (available on Amazon).

Set the voltmeter and arrange its settings accordingly to check the diodes because we will be testing the diodes of our alternator.

Cracked and old used alternator for the car

Assuming that you already have the alternator out of your car in an isolated state, here’s what you need to do:

  • Your alternator will have different posts from which different electrical connections go through. Usually, different terminals will be located on the backside of your alternator.
  • You need to touch the red wire of your voltmeter on the battery post and the black wire anywhere on the alternator housing and observe the reading. (Ideally, you should get no communication)
  • Now switch the whole process and touch the black wire of the voltmeter and the red wire on the alternator housing; you should get some reading.
  • If you switch the process again, usually, you shouldn’t get any reading.

If you do all the above-mentioned processes and get a similar type of reading, then it shows your alternator is good and has good diodes.

However, if the current passes both ways when you do the above-mentioned alternator test, then it’s a sign of a bad diode and thus a faulty alternator.

How To Know If You Have A Bad Diode In The Alternator?

Also, if you pull out the cover of your alternator, you should be able to see all the diodes in your alternator.

And you can also do the voltage test on each of the individual diodes. The setting and the procedure are almost the same as we did earlier.

With each diode, you can touch the red wire of the voltmeter with the diode and the black wire with the alternator housing to observe the reading (once again, the ideal reading observation for this should be no communication).

And doing the same procedure in a vice versa way, you should get some reading.

In this process, if you find one of the diodes giving a reading in both ways, then most probably it is a faulty one.

Final Thoughts

In short, a bad alternator can drain a battery if it has a bad diode. There could also be other reasons like the wiring problem between the battery and the alternator.

Or, maybe your alternator is incapable of charging the battery fully. So depending on the root cause, you can fix the problem accordingly.


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