Bad Alternator Diode Draining Battery [Explained]

Alternators are the sole component in your car responsible for charging the battery. But did you know a bad alternator, especially a faulty alternator can drain the battery?

So, is a bad alternator diode draining battery even possible?

Yes, a bad alternator diode can and will drain the battery. The job of the diodes in your alternator is to keep the current flow from the alternator to the battery. But a faulty diode does not do its job and starts draining power from the battery.  If the alternator diode is draining the battery, 

bad alternator diode draining battery

you will find your car stalling or rough idling and your car may not even start. On top of that, the battery light will turn on and your car’s electronics will malfunction.

In this article, we will talk about how this problem can be fixed and how to prevent it from happening in the future.

What Causes An Alternator Diode To Go Bad

Before we tell you how to fix a bad alternator diode, you should know why this happened in the first place. There are mainly two reasons.

Note: Read our ultimate guide on can a bad alternator cause loss of power.

1. Overheating And Overloading

Alternators generate electricity under a load, which can cause the rectifier diodes to overheat or overload. The heat produced by the alternator can damage the diodes, ultimately leading to their failure.

overheating and overloading

Also, if the alternator is exposed to excessive current, it can cause the diodes to burn out.

2. Poor Connection

Another common cause of diode failure is a poor connection between the alternator output and the battery-positive terminal. If the connection is loose or corroded, it can lead to arcing, resulting in faulty diodes.

Symptoms Of A Bad Alternator Diode Draining Battery

Pinpointing a bad alternator diode is difficult because the symptoms can mean a lot of things. Regardless, here is how to check if the alternator is draining the battery:

1. Continuous Battery Draining

We have heard people ask this question a lot: will a bad alternator drain a new battery? Well, the answer is yes and this is the primary symptom of a bad alternator draining the battery.

So, you find your battery drained and you just recharge it without checking anything else. But guess what, you found your battery drained overnight, again. Now, you are clueless.

If this feels familiar then it is highly possible that you have a bad alternator diode. A bad alternator diode will even drain a new battery and it can drain a battery overnight. 

2. Battery Warning Light

The first thing to look for when you are suspecting a faulty alternator diode is the battery warning light.

battery warning light

A red battery warning light will go on when the ECU detects any abnormal voltage fluctuations, such as low or high voltage. But the battery warning light doesn’t necessarily indicate a problem with the diode specifically. 

3. Stalling Then Not Starting

One of the symptoms of a bad alternator diode that drains the battery is the engine stalling and running rough. A blown diode in the alternator drains the battery, which affects the engine’s performance. As a result, you will find your car stalling and running rough. 

The car will not start again after stalling, and you may also experience misfires. 

4. Malfunctioning Accessories

Another symptom of a bad alternator diode draining battery is dim lights and electronic issues. The low voltage caused by a failing diode can cause the headlights and interior lights to become dim. However, the problem does not stop there.

Modern car infotainment systems, which are becoming increasingly common, can also be affected by low voltage. The screen may randomly turn on and off or not turn on at all. Or, the navigation system may lose the GPS signal or malfunction.

How To Test Alternator For Parasitic Draw

As we have said earlier, the symptoms of a bad alternator draining battery are so common that it can be confusing. That’s why even after finding the symptoms you need to perform a test to really make sure that the alternator diode is the problem.

Performing this test is very easy and straightforward, you will only need a multimeter. Here’s how you can perform the bad alternator diode test-

  • Set your voltmeter to a low setting on the AC voltage scale.
  • With the engine running, touch the meter probes to the battery terminals. Make sure to connect the positive (red) meter probe to the battery positive terminal and the negative (black) meter probe to the battery negative terminal.
  • If the meter shows more than 0.5 ACV, it’s likely that the alternator’s rectifier has one or more bad diodes.

Even a small amount of AC voltage will indicate a bad diode, this indicates the alternator diode has failed and is draining the battery. 

How To Fix A Bad Alternator Diode

What should you do now that you know your alternator diode is bad and draining your battery? Well, you have to get it fixed, right?

Since this is a serious issue you should address it as soon as possible. But if it’s not possible to do so, having a battery charger will help a lot as a temporary measure. If that’s not possible either, you can disconnect the battery from the car when you’re not driving. This will keep the battery from draining completely.

As for fixing the issue, unfortunately, a bad alternator diode cannot be repaired. You will have to replace the rectifier, rebuild the alternator or just get a new one. Here is the process for alternator diode replacement- 

  • Start by removing the alternator from the vehicle. While you’re at it, remove the terminal insulator.
  • Next, remove the end cover and terminal plate. With the end cover removed, you’ll have access to the diode rectifier. 
  • In order to replace the alternator rectifier, you’ll need to separate the rectifier from the regulator. Now, install the new rectifier onto the alternator by compressing the brushes until the rectifier slips onto the alternator. 
  • Slip the end cover onto the alternator and move the terminal plate into place.
  • Lastly, install the terminal insulator by pushing it around the stud.

For detailed guidance, you can watch the video

If you want to opt for the alternator rebuild, it is not a task for an amateur. But if you are confident in your skills and want to save some money, you can find alternator rebuild kits on Amazon and rebuild it yourself. Here is a video guide to help you out.

You can also just choose to buy a brand-new alternator. Here is how to replace an alternator in your car

  • Disconnect the battery. Also, detach electrical connections to the alternator.
  • Loosen the belt tensioner and remove the belt. Remove the bolts securing the alternator in place. Then extract the old alternator from the engine bay.
  • Inspect the replacement alternator for any damage.
  • Install the new alternator in place and reconnect electrical connections.
  • Put the belt back in place and adjust the tension and reconnect the battery and test the alternator.

If you decide to have a mechanic over for the replacement, obviously you will have to spend some extra money. The alternator replacement cost depends on the alternator that your car needs and the labor cost in your area.

Usually, in the US, the cost to replace an alternator at a dealership can vary between $350-$900, all-inclusive. 

How To Keep Alternator From Draining Battery

As we have mentioned earlier, investing in a battery charger or leaving the battery disconnected overnight can work as a temporary prevention. But how to prevent this from happening again? Here are some tips that may be useful.

1. Regular Maintenance

Subjecting your vehicle to regular maintenance is key to preserving the health of your alternator and preventing it from draining your battery. Regular maintenance includes checking your vehicle’s charging system for repairs or replacements.

2. Don’t Ignore Warning Signs

If you’ve noticed any of the signs of a bad alternator, such as dimming lights or a dead battery, it’s important to address the issue promptly. Ignoring warning signs can lead to more costly repairs down the road. So, book a service appointment as soon as you can.

3. Cleanliness Is Key

Keeping your alternator clean is essential to preventing overheating and avoiding potential damage. Keep your alternator in good condition by regularly checking for dirt, dust, and debris around it. On top of that, remove any obstructions that could cause it to overheat.

4. Use High-Quality Oil

Using high-quality oil is important for lubricating the engine’s moving parts and preventing excessive wear and tear. Low-quality oil can cause more wear on parts, including the alternator. So, make sure to use the recommended oil type for your vehicle.

5. Check Your Belt

The alternator belt plays a critical role in driving the engine and keeping the alternator running smoothly. It’s recommended to replace your belt every 60,000 miles or so. 

check your alternator belt

You should also check it at least once a year for signs of wear, such as cracks or fraying. Use an OEM belt recommended by your car’s manufacturer when that happens for the best results.


Can a bad alternator drain a battery while the car is off? 

Yes, a bad alternator with a failed diode can create a closed circuit that drains your battery, even when the car is off. This can cause your battery to die and leave you stranded. 

Can a bad alternator drain a battery overnight?

Yes, a bad alternator can drain a battery overnight. When a diode has failed it creates a closed circuit that drains the battery even when the engine is off. 

How fast can a bad alternator drain a battery? 

A bad alternator can drain a car battery quickly, in less than thirty minutes in some cases. Battery age, alternator condition, and other car components influence how fast the battery drains.


If your bad alternator diode is draining the battery, you should try and solve the problem ASAP. The alternator diode can drain the battery even when the car is off. That’s why preventive measures must be taken or else you may get stranded in unwanted places.

Hopefully, we have discussed the problem in detail so that you can solve this problem on your own. If you are not confident about your skills, you should not hesitate to call a mechanic.

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

Leave a Comment