How Do I Know If It's The Battery Or The Alternator?

Updated October, 2020
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Testing your battery and alternator tells you which one is the problem. If your car doesn't start or isn't running at full power, it may be your battery. When your vehicle runs fine at first but begins to fail over the course of minutes, you have a bad alternator.

How to know when you might have a problem

If your car's battery isn't holding a charge, you have a problem with your alternator or battery. If your battery can start your vehicle but dies as the car runs, you have a faulty alternator. If your battery has been charged and can't start your vehicle, it's a battery issue.

Culprit #1 : The Alternator

What does it do?

The alternator charges your battery while you drive. If your alternator fails, your battery doesn't charge and eventually dies.

How long will a car run without an alternator?

If your alternator isn't working, your car can barely do more than start before you begin losing power. At most, you should get around 5 to 10 minutes of driving before your car dies.

Culprit #2: The Battery

What does it do?

The battery is responsible for generating all the electrical power your car needs. Your car's battery charges while you drive by your alternator. Without your battery, your car wouldn't be able to start or run.

How long will a car run with a bad battery?

How much charge your battery can take determines how far you can get with it. A battery that's missing .4 volts can only run at 50% power. Anything less than that and your car isn't even able to start.

Signs of a bad battery vs. alternator

Both of these car parts control your electrical system. However, your alternator charges your battery while your battery supplies power to your car. The failures of a battery are sudden, while failures of a failing alternator are slow. Always check your battery first, it can generally tell you which of these two is the culprit.

Sign #1 - Interior Lights Fading

  • What it looks like - Your interior lights slowly fade from being bright and get dimmer.
  • The Cause - This is caused by your alternator not being able to charge your battery enough.

Sign #2 - Dim Headlights

  • What it looks like - Your headlights are dim but aren't fading.
  • The Cause - Your battery isn't working at full power.

Sign #3 - Speed Dependent Lights

  • What it looks like - Your lights are dim but get brighter when you accelerate.
  • The Cause - Your alternator is only able to put out proper charge when it gets excess power.

What to do about a bad battery

A bad battery causes all of your electronics to fail. Before you replace or attempt to repair your battery, make sure your battery is an issue. A fuse may have blown or, you could have a short in your circuits. Make sure you follow the proper safety procedures when checking your battery.

How to check your battery

When checking your battery, make sure you are in a well-ventilated area. Also, make sure you wear proper safety equipment

  • Step 1: Check your battery gauge

If the light is flickering, there is something in your car draining the battery.

  • Step 2: Check commonly used connections

Check your power windows, power locks, and your clock.

  • Step 3: Check your battery voltage with the car off

It should read 12.6 volts.

  • Step 4: Check the battery voltage with the car on

It should read between 13 and 14 volts.

How to replace your battery

To replace your battery, make sure your car is off and in a safe workspace.

  1. Disconnect the old battery
  1. Safely remove the old battery and put the new battery in its place
  1. Connect the new battery to the leads securely and adequately
  1. Properly dispose of the old battery

What to do about a bad alternator

Before you touch your alternator, make sure it isn't any other issue. If you suspect your alternator, check it before you remove it. Testing an alternator at home is relatively simple.

How to check your alternator

There are two simple tests for checking your alternator.

  • Test 1: While your car is running, disconnect the positive battery cable. If the vehicle continues to run, your alternator is properly charging the battery.
  • Test 2: When you start up your car, look at your interior lights. If they don't fade over time, your alternator is working fine. If they slowly fade, you have an issue.

How to replace your alternator

Replacing the alternator in your car can be done at home. Finding an alternator can be expensive, but buying one second-hand can save you some money. Before you start, make sure your car is off.

  • Step 1: Remove the wiring plug from the back of the alternator
  • Step 2: Remove the screws holding the alternator in place
  • Step 3: Remove the alternator and replace it with the new one
  • Step 4: Replace the screws and then the plug

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Call an insurance expert
Our team will help you find the lowest insurance prices.
866-996-3156