How Long Will a Battery Last With a Bad Alternator? – Faulty Battery or Bad Alternator

Every person in their life may have faced the situation of not having their car turned on when they put the key in. This happens due to a prevalent reason. The reason is that the battery or the car’s alternator may be dead or have a low battery. What are the common signs of understanding that the vehicle’s battery is low or quiet? We will let you know about the situation and identify when the battery or alternator becomes low.

What is an alternator in your car?

The alternator is an electrical generator in your car that converts mechanical energy to electrical energy. The alternator produces electricity and charges the car battery.


What is a car battery?

A car battery represents a rechargeable battery used to start a motor vehicle. The alternator is responsible for recharging your car battery.

car battery

Now let us see the whole charging system in your car:

car charging system

How Long Will a Battery Last With a Bad Alternator?

Your car can last with a bad alternator from 25 minutes up to 2 hours. Does time depend mostly on battery charge level and driving speed. Do not turn off the engine once you start the engine, and try to turn off all unnecessary lights in the car and the air conditioner.

Once I had a problem with the faulty alternator. I took my car battery and charged up to the maximum level. Then I went into the car, and I drove 100 miles on the highway (very fast). It was a stupid decision, but I wanted to go to my favorite car electrician in another town instead of visiting the nearest car electrician. However, I tested my car and realized that my car didn’t lose power for 95 minutes (100 miles). Unfortunately, I lost car power 1 mile before my destination. I was lucky that my car didn’t turn off on the highway.

How long can a car run without an alternator?

Usually, cars can run 60 miles without an alternator on average. However, if your car battery is maximum charged with a slight energy loss, you can run your car up to 100 miles.


In theory, if you want to calculate how far you can drive with a bad alternator, you need to monitor:

  • Alternator state
  • Battery state
  • Speed
  • Current Electrical Load such as AC, Headlights
  • Your Car Age

What to do when the car battery doesn’t charge?

A car battery won’t usually charge if the car battery is old (more than five years) or a leaked battery (or any other damage). So the best solution is to buy a new car battery and replace the old one.

Signs you need a new car battery are:

  • Car battery recharges slow
  • The age of your car battery is above 5 years
  • Leaked battery


What is the difference between a dead battery and a bad battery?

If you have a dead car battery, you can not start the car; there is no ignition. However, if you have a bad battery (falling battery), you will not be able to fully power your car’s electrical components such as dim headlights, slow crank, no dome light, etc.

For this, we first need to understand how the car engine works. When you put the key in the keyhole of the car and ignite it, the engine starts to run, and this then passes electricity to the starter of the vehicle. The machine will begin to start once it receives the electricity jolt. The alternator will then have a cycle completed and charge the car’s battery.

Using the jumpstart technique, you can identify whether it is a low battery or a dead battery. You may have heard of this technique. In this, what you do is open the car’s bonnet and pass the electricity through a jumper cable. If on doing this, your engine is ignited but does not sustain and dies immediately, it means that the battery is low.

Alternatively, if your car starts by the jumpstart and keeps on running but does not start on its power, the battery is dead.

Sometimes we can have two problems:

    • The battery light is on, but the alternator is charging. In this case, we can have a problem with the battery, alternator, or cables between the alternator and car battery. Usually, an error can be a faulty alternator or even the most common faulty diode in the alternator.
    • The alternator is not charging the battery, but the alternator is good. In this case, you need to check are their problems with carbon brushes. First, you need to look at the voltmeter to see if it displays an increase in volts. If you can not see increased volts, worn carbon brushes create problems.

Does a dead battery make noise?

Yes, dead batteries make noise. Dead battery sound is “click sound,” and clicking sound appears when you turn the key and comes from the starter motor. It is when the battery doesn’t have enough power to start the engine.

Can you jumpstart a car with a bad alternator?

Yes, you can try to jumpstart your car with a bad alternator, and your vehicle can run if the battery is charged. However, if the alternator is faulty, it can not produce the required voltage, so your car can run purely off the battery.

However, if your car battery is empty and the alternator is faulty, your car can not run.

If the car battery is dead, will windows work?

Yes, if the car battery is dead, you will not open windows automatically, only manually. This is because the windows opening system needs the power to work, and if your car battery doesn’t charge and it is empty, windows will not work.

How to tell if the battery or alternator is bad in your car?

A quick way to diagnose and determine whether the alternator or car battery is causing the problem is to start the engine with the help of cable jumpers and then disconnect them quickly. If there is a problem with the battery, the car will continue to run, but if it stops suddenly, the problem is with the alternator.

However, this test you can use if you can not start your car. This cable jumpers test is valid only if your car battery is empty. If you have a full car battery and a faulty alternator, your vehicle will work until losing power because the alternator will not recharge your car battery.

I had a problem a few years ago with oil spillage in my car, and I didn’t know why my vehicle kept burning out alternators. After that, I saw that my car had a problem with overheating, belts, and oil spillage, and I fixed the problem very fast.

What causes an alternator to get hot?

The alternator can be hot if your car is overheating (broken water pump, faulty radiator fan, or clogged coolant hose ); faulty belt (loose wiring), worn brushes, oil spillage, or falling battery.

What happens when a car battery dies?

When a car battery dies, there can be no response at ignition or the vehicle starts, but the engine immediately dies, the starter motor can crank, but the engine will not turn over, and there are no door chimes, and there are no door chimes no dome lights.

There are some of the signs of the dead battery, and these can be identified by looking at the following:

  • Dashboard lights: You should check the gauges of the dashboard battery to know about the charge. This should also be limited by turning the vehicle off. If you notice that the dashboard lights are not bright and dim, then this means that the car’s battery is dead. You should also check for the button of opening and closing the windows and turning on the vehicle’s wiper as well. These will either work slowly or not if there is a dead battery.
  • Battery corrosion: it might also look like corrosion in the battery. The battery will not work fully if this is the case so. You should ensure that the battery is wiped clean before starting the engine.
  • A car battery is worn out: The car battery is ideal to be used for between two to five years. After that, however, the car battery can be worn out and therefore unable to produce the electric charge required for the car to start working.

How does a car act when the alternator is going out?

Some signs tell you that the car’s alternator is not working correctly. These include the following:

  • Lights of the car: The car’s insides will be dim and not bright as they usually are.
  • Headlights are dim: When the headlights are not shining bright, this also means that the car’s alternator is not working correctly.
  • Noise: Notice the sound of your car. If the vehicle is making a lot of growling noises that it does not normally do, then it means that something is wrong with the alternator.
  • Burning: Notice if you can smell some burning smell, such as burning plastic or rubber-like incense.

Let us think about complicated problems.

New battery and alternator, but the car keeps dying

If you have a new battery and new alternator and your car keep dying, car coils or corroded battery connections are typical. It is good practice to change the alternator and car battery to change coils and battery connections. It is not expensive, and you will not need to worry more about the ignition system.

Robert Aksamit

Robert Aksamit

Robert Aksamit is a mechanical engineer and automotive industry expert. Robert was born in Minnesota and worked in the US automotive industry for 25 years. He is highly regarded for his passion and dedication to continually improving vehicles in response to customer feedback. Robert has a keen eye for sourcing the best vehicle components and materials on the market and is always looking for ways to enhance the user experience. As a writer, Robert covers automotive-related topics. Read more on Robert Aksamit's about page. Contact Robert:

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