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Can an Alternator Alone Keep a Car Running?

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Close up of a new car alternator installed in the vehicle

You know your battery is dead and you are wondering if the alternator will keep your car running. Having your car not start without a jump is a good indicator that your battery is dead.

The alternator can keep a car running on its own, for a short distance. The alternator will help keep your car running by keeping a charge on the battery. If your battery is low, old, or damaged the alternator will have difficulty maintaining a charge.

Is your battery low or dead and your car will not start? Once jumpstarted your car will run but if the battery is weak or dead, is it pulling most or all of its energy from the alternator? In this article, we will discuss the issues of running your car on the alternator alone.

How Do an Alternator and Battery Work Together to Power a Car?

The alternator produces AC electrical current from the running engine. This current is passed to the battery that stores energy. The stored energy in the battery is released as “DC current” to power everything within the vehicle and is necessary to spark the engine when turning the key on in the ignition.

When you start your car the battery sends the electrical load from the battery is at its peak. It takes more energy to start the car than it does to keep it running. Since it takes this large amount of energy to start the car, the battery will need to recharge.

Once the car is started and the engine is running, the alternator is engaged and sends current back to the battery. The battery then sends a direct current to the parts of the car requiring electricity. It is also banking extra energy for future use.

In order for this system to work smoothly, the alternator and battery must be in good working order. If either is weak or damaged, it will cause issues with how your car functions.

How a car battery charges the engine starting system and alternator

Can You Run a Car With an Alternator and No Battery?

An alternator that is working properly will continue to work in a vehicle that is turned on, even with a battery disconnected battery. Disconnecting a car battery while your car is running may result in damage to the car’s electronics.

Do not be tempted to test your alternator by disconnecting the battery while the car is running, as this can cause damage to your car. If you are wondering if the alternator is working properly you can check your alternator and your battery with a multimeter.

Will A Weak Battery Damage An Alternator?

A weak battery can cause damage to the alternator. This is because the alternator is not meant to run at a high capacity for long stretches of time. As the alternator attempts to charge a faulty battery it works harder than normal and can fry the wiring in the alternator.

All it takes is for one cell in the car battery to have gone bad. The excess stress put on an alternator as it attempts to charge the dead battery will cause the alternator to fail.

Therefore a weak or dead battery should be replaced as soon as possible. If your battery has died due to lights left on, or doors left open the best way to start your car is to jump-start it using a good battery and jumper cables.

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ACDelco Gold 48AGM 36 Month Warranty AGM BCI Group 48 Battery, Black
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How Do You Know If Your Battery is Weak?

Here are some signs of a weak battery:

  • The lights are dim, grow dimmer or blink out when starting the car.
  • The car is slow to start
  • The car may make a clicking sound or no sound at all.

A battery tester can be used to see how much charge a battery has.

If your car starts after being jumpstarted, then there is a problem with the battery. If you have a battery tester, this can be used to determine how much energy is stored in your battery.

If the battery is weak it is not going to register at the full capacity, and it will quickly lose any charge it receives from the alternator or charger.

When you are experiencing problems with your battery, be sure to check for loose cable connections as well as corrosion around the terminals. Tighten any loose cables and clean off the corrosion. Doing these things will help a battery maintain performance.

Sometimes the battery is failing to keep a charge because the alternator is going out. If the alternator is failing then it is unable to send current to the battery. This results in your car running on the battery alone. Without the alternator charging the battery properly the battery will die.

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What Are the Signs of A Bad Alternator?

If the alternator is going out the interior lighting may begin to grow very dim as the car is being driven. The exterior lights such as taillights and headlights will also begin to fade out.  The battery is dead. Unusual sounds may be heard from your car as it accelerates.

If you are experiencing signs of a bad alternator you should have it checked, before it causes hardships for you, as these can lead to extreme inconvenience, or prove to be dangerous in a driving situation.

Your car may make odd noises if the alternator is going out. If you hear:

  • Grinding
  • Squeeking
  • Knocking

Pay attention, as new noises are often your car’s way of alerting you that something is wrong.

One thing that happens with a failing alternator is that you notice your electrical system losing power while you are driving. Your headlights are quickly growing dim and the dashboard lights are becoming difficult to see. 

This is a dangerous situation especially at night and on a freeway! If your lights are dimming, other vehicles will have a difficult time seeing you. You also risk having the car lose power completely as the battery dies.

Get your car off the road as quickly and safely as possible. If you can make it to a mechanic drive there with care. Otherwise, call your roadside assistance such as AAA. DO NOT turn off your car unless told to do so.

Car alternator close up inside the engine bay
Close-up of an alternator inside the engine bay of a car. The location can vary depending on the year, make, and model

Should You Run Your Car On The Alternator Alone?

Though it is feasible that your car will continue to run with only the alternator, it should not be attempted. Doing so can overheat your alternator, and disconnecting the battery while the car is running could result in a power surge that damages your vehicle’s electronics.

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ACDelco Gold 48AGM 36 Month Warranty AGM BCI Group 48 Battery, Black

Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

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