A Definitive Guide About How To Check If Alternator Is Bad?

How To Check If Alternator Is Bad

A car battery is an essential component of any vehicle, as it provides the necessary electrical power to start the engine, operate the lights, and power other electronic devices in the car.

There is a delay in your arrival at work. You rush into your car, turn the key, and to your horror, hear nothing but silence. The engine won’t even turn over. The car is lucky if it starts slowly instead of failing to start, but that day is not far away. Your battery is likely wrong, or your alternator is malfunctioning.

You likely have a faulty alternator or battery if your car is slow to start. We will also cover how to check if your alternator is terrible with more information in this article.

How does a car battery work?

Understanding how a car battery works can help you diagnose potential problems and maintain your vehicle’s performance.

A car battery is a type of rechargeable battery that uses chemical reactions to store and release electrical energy. It is composed of a series of cells, each containing a positive and negative electrode, separated by an electrolyte solution. When the battery is connected to the car’s electrical system, a chemical reaction occurs that generates a flow of electrons between the positive and negative terminals. This flow of electrons creates a current that powers the car’s electrical components.

The car’s alternator charges the battery while the engine is running, replacing the energy used to start the engine and power the car’s electrical systems. Over time, a battery can lose its ability to hold a charge and may need to be replaced. Symptoms of a failing battery include slow engine cranking, dimming headlights, and difficulty starting the engine.

Proper maintenance of your car’s battery, such as regular inspections, keeping the terminals clean and tight, and avoiding overcharging or discharging, can help extend its lifespan and ensure reliable performance.

Your car will not run unless you have a working battery. You cannot go anywhere without one. The battery stores the energy needed to run the car, measured in voltage. At least 12.6 volts are required for a fully charged battery. Battery voltages below 12.2 are only 50% charged, and below 12 volts, the battery is “dead” and needs to be replaced. 

How does an alternator work?

The alternator converts mechanical energy into electricity. This electrical energy runs various electrical components like lights, air conditioners, etc., and charges the battery. You hear people talking about “charging-system problems” when referring to alternators. The alternator maximizes your car’s battery life if it is working well. Battery life will be shortened if it is overcharged or undercharged.

How to check if alternator is bad?

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Tools needed


Multimeter measures voltage, current, and resistance, among other electrical properties. It is possible to test a car battery with a multimeter since it can measure how much voltage your battery holds and how fully charged the battery is.

How to check if the alternator is bad?

Start by checking the battery issue with the battery is likely to cause slow cranking in the morning. To test the alternator, you must test the battery first. The battery starts the car; when the car runs, it spins the alternator, keeping the battery charged. Battery failure will prevent the car from starting, so any later alternator test will be useless. 

1. Disconnect the battery for at least one hour

A car battery must be allowed to sit for at least one hour or overnight before you use a multimeter to test it. This is due to the way the battery discharges. A running car when the battery is being tested will result in false readings on the Multimeter because the alternator, which generates electricity and charges the battery, is the component that produces electricity. When the car has recently been running, the battery will likely produce a higher, misleading reading on the Multimeter, resulting in an inaccurate measurement.

2. Multimeter should be prepared

Place the battery to one side of the engine, often in the engine compartment under the hood. If you want your Multimeter to measure within a specific range, you must set the range when you use it to test something.

With a multimeter set to 20 DC voltage, you will be able to accurately measure a battery that holds between 0 and 20 volts. The knob on “DCV” (Direct Current Voltage) should be turned to 20. The solid line above the dashed line on the right is sometimes accompanied by the letter “V.” It should read 0.00 on the Multimeter.  

3. Check the results with the Multimeter attached to the battery

Using the red (positive, marked “+”) meter probe in conjunction with the red (positive) battery terminal and the black (negative, marked “-“) meter probe in conjunction with the black (negative) battery terminal, connect the Multimeter with the battery.

Battery voltage should be at least 12.6 volts when fully charged. It is only 50% charged at 12.2 volts, and below 12 volts, the battery is “dead” and needs to be replaced. Before testing the alternator, you may need to charge the battery or replace it. If the battery is still wrong, check it with a multimeter again.

4. Make sure the alternator is working by testing it with a multimeter

Slow starting may result from a malfunctioning alternator if the battery is in good condition, and if the problem occurs when the engine is hot, that is when the car has run.

To check an alternator with a Multimeter, you must leave the engine running, turn off all lights, including the dashboard and any other accessories that may consume electricity. Use the Multimeter to check the battery as well. Alternators working correctly will generate between 13.1V and 14.6V at a regular idle speed. The reading outside this range means your car is either undercharged or overcharged, which is unhealthy for the battery and your vehicle.

FAQs (Frequently Asked Questions)

  • What does an automobile alternator do?

The car’s alternator generates an electric current to charge the car’s battery. When the battery is charged, the onboard electronics are powered.

  • Is it possible to jump-start a car with a bad alternator?

With a lousy alternator, it is possible to jump-start the car.

  • Will jumping start to damage the alternator?

The alternator is not adversely affected by jump-starting a car. However, a damaged alternator’s coil will affect the car’s electric system.

  • Will the car still run with a lousy alternator?

A vehicle with a bad alternator can run, but once the battery charge runs out, it will no longer move as the engine requires electric power to function.

  • What is the life expectancy of a car alternator?      

An alternator does not have a rated life and is not a consumable item. Most alternator issues occur after a hundred thousand miles.

  • What causes a car alternator to fail?

Poor batteries usually cause alternators to malfunction. A worn-out battery can put more strain on the charging system than the alternator can handle, which causes the alternator to malfunction.

  • What is the best way to determine if the battery or alternator is bad?

With the engine running at 2,000 rpm, check the voltage at the battery terminals. A battery with a voltage higher than 13.5 volts is terrible. Any voltage below that indicates an issue with the alternator.