How to Tell if Alternator is Draining Battery

The alternator is one of the most important parts of a car that should not be joked about because the alternator is responsible for charging the battery, if it does not work the way it should, it can lead to a dead battery preventing the car from starting.

Sometimes, the alternator could be the reason why your battery is draining each time you leave the car parked for some days. It has happened to me personally and I managed to get it fixed.

Therefore, if you are consistently having a dead battery and you suspect that the alternator is draining the battery, I will tell you how to find out and how I was able to fix mine after 8 months of a consistent dead battery.

How to Tell if an Alternator is Draining Battery

Below are ways by which you can tell if the alternator is draining the battery:

Step 1: Check the Battery Voltage

Use a multimeter to measure the voltage of your car’s battery when the engine is off. When the battery is fully charged, it should read around 12.6 volts.

Step 2: Start the Engine

With the battery voltage confirmed, start the car’s engine and leave it running while you follow the next step. When the engine runs, the alternator should take over and charge the battery.

Step 3: Check the Alternator Output

With the engine running, use the multimeter to measure the voltage at the battery terminals. The voltage should be higher than the voltage you measured when the engine was off.

It should read between 13.5 to 14.5 volts, indicating that the alternator is charging the battery. If you don’t have a multimeter, you can use an OBD II scan tool.

Step 4: Rev the Engine

While still measuring the voltage, increase the engine’s RPM by revving it a bit. The voltage should rise slightly with increased RPM. If it doesn’t, or if it drops, this could be a sign of a failing alternator.

Step 5: Listen for Unusual Sounds

Pay attention to any unusual noises coming from the alternator area, such as a grinding, whining, or squealing sound. Unusual sounds can indicate a problem with the alternator.

You can check these sounds when the engine is running and when the car is off. Ensure to do the checking in a very quiet environment.

Step 6: Warning Lights on the Dashboard

Some cars may have warning lights on the dashboard that will illuminate if there’s an issue with the charging system or the alternator. If the battery or alternator warning light comes on, it’s a clear indication that there is a problem.

This is how to tell if the alternator is draining your battery. If during the check you notice that the alternator’s output voltage is significantly lower than it should be or if the voltage drops when you rev the engine, it’s likely that the alternator is not charging the battery properly, and it may need to be replaced or repaired.

My Personal Alternator Battery Drainage Experience and How I Got Mine Fixed

Permit me to share my personal experience with you on how I traced and successfully fixed my battery drainage caused by the alternator after 8 months. Imagine having to disconnect your car battery for 8 months. It was overwhelming for me.

I had a battery drainage issue before in my Lexus IS350, it happened that the battery could not last more than 4 days. On the 4th day, the car will crank and not start. If it goes more than that, the battery will be completely dead and I can no longer open the car door with the key fob.

I did everything I could such as replacing the battery twice and cross-checking my installed aftermarket radio. All to no avail. That was when I started suspecting the alternator. I knew the alternator was good because the battery was charging but I suspected that it could be the reason for the battery drainage

One day, while I was at home, I decided to open the car hood and check again. When I opened the car, I started checking to see if there was any wire that was not properly connected around the alternator.

While I was checking these wires, I started hearing a noise, like a quiet whistle sound as though something in the engine was still on and working.

I had to put my ear close to the engine and the more I brought my ear close to where the alternator was installed, the sound/noise became more louder. I quickly called my friend who sells and repairs alternators. He came to my house and noticed the same thing.

The first thing he did was to remove the alternator switch after which the sound stopped. He smiled because finally, we had found what we had missed for 8 months.

Apparently, that noise has been there but I wasn’t able to find out because of how noisy my place of residence is. I lived close to the road at that time.

He opened the socked to access the wires. Lo and behold, the wiring was not properly done. The wires were connected directly to the ignition in the sense that even if the car is off, it will continue to work, hence why it was draining my battery.

He quickly rewired it correctly and ever since then, it has never happened again. I traveled and left my car for 1 week without warming and the battery did not die. The problem of 8 months was finally solved. And the amazing thing is that it did not take him up to 5 minutes to put the wire in the right place.

Therefore, if you are in the same situation as me and you are wondering if your alternator is draining your battery:

  • Go to a very quiet area
  • Turn the car off
  • Open the car hood and
  • Put your ear close to the alternator to check if you’re hearing any noise.
  • If you hear any sound, chances are that it is the culprit.
  • You can also unplug the alternator switch,
  • Open it and see how the connection is done, you might be surprised by what you will see.


Having a car the battery dies every 2 to 3 days can be very frustrating. It will leave you with no choice but to always disconnect your battery if you won’t be going out for some days. The alternator can cause this issue. I know for sure because I’ve experienced it and luckily, I had mine fixed after 8 months. Just follow the steps explained in this article to see if your problem is also the alternator. If it is, then have it fixed immediately.