Bad Alternator vs. Bad Battery [Signs and Symptoms]

It is always frustrating to get into your car and turn the ignition on, only for the vehicle not to start. Almost always, that is because of a failing alternator or battery.

However, between a bad alternator vs. bad battery, how do you know what makes your car not start?

To diagnose your problem, you should understand the functions and operations of both parts in a car. To start and run a car engine, you turn the ignition for the battery to deliver an electric jolt to the starter. The starter powers the engine, which runs the alternator, and the alternator completes the cycle by charging the battery.

Therefore, carefully following this cycle can help you figure out whether you have a battery problem or a bad alternator problem.

How to tell if you have an alternator or a battery problem

The quickest and easiest diagnosis to find out which one between the battery and the alternator is at fault is to use the process of elimination. You can attempt to find the fault by overlooking the battery’s role in powering up your car, as mentioned above. Get jumper cables and jumpstart the vehicle using a different battery from another vehicle.

If your car engine shuts down immediately after it starts or after a short drive, then your alternator has malfunctioned and is not charging the battery the way it should.

If jumpstarting your car works and the car keeps running fine, but the vehicle does not start again using its power, then you have a dead battery on your hands. Once the vehicle started, the alternator kept supporting the car’s power requirements and attempted to charge the battery, but the battery did not pick up any energy.

Alternatively, you can test both the alternator and the battery using the TOPDON 12V load Car Battery and Alternator tester. This easy-to-use tester can test the aging and resistance of your battery and help you figure out whether you have an alternator or a battery problem on your hands.

Read Also: How Long Does it take to replace a car battery?

symptoms of bad battery

However, here are some conventional ways of diagnosing your vehicle’s problem.

Start by checking your battery.

When you face vehicle start-up challenges, you should always start by checking your battery before the alternator. It’s not unusual to wake up to a dead battery that won’t start during the rainy or cold weather season. Start by checking the battery gauge on the dashboard to determine if your battery sent even a slight charge when you switched your car off.

If you notice any dim flickering light at the gauge, then it means that you have something using power from the battery even when the car is off. Check all the connections of the vehicle accessories you commonly use, like the brake lights, wipers, and automatic window openers. Switch off all accessories and try the battery again.

Clean off corrosion from your terminals, jumpstart your car, and run it to see whether the battery will charge up again.

Give it a few minutes, switch off the car, and wait for it to cool. When your car has cooled down, check the battery voltage using a voltmeter. A car battery should read about 12 volts when the car is off. When you turn the engine on, the voltmeter should read around 14-15 volts.

If your battery reads figures that are way over 15 volts, way less than 12 volts, you have a voltage regulator or an alternator problem. If your battery doesn’t register any voltage, you have a dead battery, and you need to visit a car shop to purchase a new battery.

Test the Alternator

In case you tested your battery and found out that it works fine, you need to test the alternator to be sure that it can charge up the battery. If the battery is working correctly, but the car has startup problems, then it’s the alternator that will more likely have a problem.

Another way to tell if an alternator is draining your battery is by running your car and unplugging the battery’s positive connection. If the car immediately stops running, your alternator has malfunctioned because it should depend on the alternator and not the battery after starting.

Another simple way is to test your interior lights. An alternator charges your battery when the car is running. Therefore, if the vehicle’s interior lights start brightly but fade slowly as the car remains on, it indicates an issue with the alternator. You will also notice that the car lights are brighter when you accelerate the car but lose power and become dim when you slow down. These fluctuations show that the alternator cannot keep power when the car is running.

Read Also: How Long Does a Car Battery Last Without Driving?

Symptoms Of a Dying Battery

Many symptoms tell you when your battery is dying or in the process of failure. Some physical signs are conspicuous enough for you to see with your eyes, while others are operational problems related to the battery failing to adequately perform its functions. Here are the symptoms of a bad battery.

Symptoms Of a Dying Battery

Swollen battery case/battery older than three years

When you look at your battery, and it strikes you as old, it is highly likely that it will soon give you problems if it hasn’t already started malfunctioning. The more batteries age, the less they can retain electric charge because the metal inside corrodes the same way the outside terminals corrode.

The level of charge that the battery can retain progressively diminishes to the point where, regardless of how much power the alternator provides, the battery can’t retain enough power to start the car. However, there are a few hacks to keep using your old battery.

You can scrub away corrosion from the battery terminals carefully by using a rag and then have someone jumpstart it. First, run the car for a while and then turn it off. Next, try to restart the vehicle. If the vehicle starts, it means the corrosion prevents it from transmitting power.

A more visible symptom of a bad battery is swollen areas. A bulging or swollen battery case shows a frozen battery or hydrogen buildup inside the battery. An overcharging alternator is the primary cause of a swollen battery case. A battery that has elapsed its service lifespan can be the culprit. If you have a swollen battery, it shows the battery is wearing off.

In some cases, a swollen battery will run fine and provide adequate electrical flow to start the engine. This, however, doesn’t mean you should feel relaxed and ignore this symptom. You need to budget for a battery replacement because the current battery can keep you stranded in the middle of nowhere.

Slow and long crank

When your car is slow to crank, you either have a problem with your battery charging system, the battery is corroded, or you forget to turn off all electrical accessories when your car is parked. You will also experience long and worse cranking problems during cold mornings because your battery has grown weak.

Dim headlights

Your vehicle headlight can help you determine the strength of your car battery.

If it’s challenging to start your vehicle and you suspect the car battery, stop cranking the engine and switch on the headlight. If the headlight is bright, then the battery is okay. But if the lights are dim, you have an undercharged battery.

There are chances that the battery electrodes are wearing off or other issues in the battery. Kindly note that a dead battery won’t hold a charge regardless of how the alternator functions.

You may find it difficult to carry out this test in broad daylight. This is because you may not easily determine if the headlight is shining bright or dim on a sunny afternoon—however, their other methods to test the battery’s strength during the day. See the next symptom.

Horn getting weak

The horn getting weak is a common sign of a weak or failing battery. As a mechanic, the first three things I check whenever I suspect a dead battery is the brightness of the headlamps, the strength of the horn, and the battery light in the dashboard. If these three or two out of them are present at a time, you have a dead battery in place. Recharge or replace it to start your vehicle and commute with your daily activities.

Unable to crank the engine without a jumper

You have a dead battery if your engine won’t crank without a jumper. This means your battery won’t hold a charge anymore. If this is the case, you’ll either hear a clicking or no sound when you crank the engine. If this continues, visit a nearby mechanic garage to have your battery checked.

Battery charge warning indication

The battery charge warning indicator is what you should check without leaving the steering wheel. This light is also known as a battery light. The battery warning indicator will illuminate on the instrument cluster once you turn on the ignition and disappear after starting the vehicle.

If you see this light on the dashboard, it means two things – the alternator or battery is weak. For this reason, if a battery light appears on the dashboard, don’t conclude it is a battery or alternator issue. Instead, inspect the battery terminals, check and clean corrosion on the terminals and determine if the alternator is working properly. Once you confirm your car battery is bad, don’t hesitate to replace it with a good battery brand.

Finally, don’t conclude that a car component is bad without confirmation. Always diagnose the suspected part before concluding. Therefore, you would want to check this well-detailed article on how to test a car battery. This will guide you to test the battery and confirm if the battery or alternator is the culprit.

Read Also: How to Charge a Car Battery Without a Charger

Signs of a Dead Alternator

symptoms of a bad alternator

Here are some common signs of a bad alternator.

Flickering Lights

When your alternator is in good working condition, it constantly maintains bright lights regardless of your driving speed. Therefore when you notice that your headlights have started flickering or dimming as you change speed while on the road, it means your alternator can no longer generate enough power.

Weak or Dead Battery

The alternator has to replenish the car battery’s energy. When you always have a weak battery, it could also mean that your alternator is not adequately charging the battery as it should when the car is running. However, be careful to confirm that you don’t have a weak or dead battery before blaming the alternator. You can check if the battery gauge on the dashboard indicates a strong charge even without turning on your vehicle. If it does, then your battery is fine. Your alternator is the problem.

In this case, your battery is always weak because your car accessories now draw energy from the battery instead of the alternator, thus, draining and leaving it empty.

Unusual Sounds

An alternator has many spinning parts that make strange sounds when one of them starts to malfunction. These sounds resemble a whining or grinding sound. Bad alternator sounds can come from a worn-out bearing or the voltage regulator, demanding that the alternator increases charging frequency.

The grinding or whining and grinding sounds coupled with continually having a weakened battery are an indication that you should check your battery.

Indicator Light

Most vehicles have a warning indicator light on their dashboards labeled “ALT” or “GEN.” When your alternator goes bad, one of the first warning signs is this light turns on. Although this light can turn on because of several reasons, if you pair it up with the other symptoms of a bad alternator listed here, the chances are that your alternator is not providing enough power to support the electrical accessories of your vehicle and to charge your battery.

Electric Failures

The alternator is the primary source of your car’s electrical power. Therefore, whenever you have a bad alternator, all the accessories seem to malfunction. You may have your music player swallowing disks, your power windows getting stuck, your dashboard lights, air conditioning, and other features that depend on electricity malfunctioning.

All the signs listed above warn you that you may need to replace your alternator to keep enjoying using your vehicle adequately. You take certain risks when you drive your car when it has a bad alternator or a bad battery.

Risks of driving with a bad alternator

A car can run with a bad alternator, but it means that your battery will support your vehicle’s power requirements. Since the battery will be using its reservoir energy without charging, it means that you only have limited driving time until the car shuts down. Therefore, you risk getting stranded on the road with a car that cannot power on because of a depleted battery and a dead alternator.

If you wish to prolong your driving distance get to a mechanic to replace the alternator, turn off all the auxiliary accessories like the air conditioner, internal lights, and radio to save enough power to get to the repair shop.

Risks of driving with a bad battery

When your battery dies, you can jumpstart your car and drive it fine because the alternator will support the vehicle’s power requirements while on the road. However, the alternator will generate a lot of heat, and this generated heat will damage the alternator in the long run.

Therefore, to avoid the expenses of buying a new alternator, avoid repeatedly jumping and driving your vehicle and buying a new battery.

Frequently Asked Questions (FAQs)

Q: Can you jumpstart a car with a bad alternator?

Luckily, you can jumpstart your car when you notice that it has a bad alternator. However, you should drive the vehicle to the nearest car dealership to replace the alternator before the battery dies.

Q: Can a bad alternator kill a battery?

Yes, you still have a problem when you have a new battery and a bad alternator because a bad alternator can destroy a working battery when it charges it using too much voltage. Higher than recommended voltages lead to premature cell failure within the battery. It can also destroy the battery if it produces a ripple voltage that surpasses the rectifier.

Q: Do Alternators Fail Suddenly?

Most times, alternator failure occurs gradually over time, and you can notice the symptoms and replace them early. However, there are unique circumstances when an alternator can suddenly stop working. One of these situations is when the voltage regulator or the diodes inside the regulator fails. After noticing bad alternator diode symptoms like flickering headlights, you can test your alternator using a voltmeter. Switch it to a low setting on AC mode and test the battery. If they register any AC voltage, it means your alternator diodes are going bad.

When the voltage regulator fails, the alternator automatically stops transmitting electrical energy.

Q: How long will a car battery last without an alternator?

When you drive without a functioning alternator, your vehicle can only go for around Twenty-five miles before the battery is empty if you have a fully charged battery. Twenty-five miles translates to a maximum of approximately 30 minutes of driving.

Final Words

Batteries and alternators are co-dependent components of a vehicle’s charging and electric system. The battery delivers electricity to the starter to kick-start the engine operation. The alternator powers the vehicle and charges the battery to ensure there will be enough energy to start the car again after switching it off.

Therefore, differentiating vehicle electrical problems between the battery and the alternator can be confusing. However, the symptoms listed are sufficient to diagnose your vehicle the next time you have a bad battery or a bad alternator problem.

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Hi there, I am R. Hasan Tito, a mechanic, and owner of this website. My friend and I created this website to share our knowledge, expertise, and experience with our fellow mechanics' community and car users. I am a specialist and certified automotive mechanic (Both Heavy Commercial and Private Cars). I worked as a Mechanic and Mechanic Supervisor for over fifteen years at Global Rebound Automotive companies - Toyota, TATA, BMW, Nissan, TVs, and Others. Now, I enjoy my new role of leading a team of automotive experts (in their respective fields) and publish new content on a regular basis on my website and social media.

3 thoughts on “Bad Alternator vs. Bad Battery [Signs and Symptoms]

  1. Thank you very much for a well organized and narrated explanation regarding this complementary parts. Definetly follow you and wait the next chapter

  2. Take a multimeter – read voltage at accumulator – voltage < 10 Volt = ACC poor or discharged – voltage 12.8 Volt = Acc is good – voltage 12.8 to 13.8 Volt Acc is full loaded .
    check generator ! – Start engine – voltage should go up from 12 up to 14.xx volt = Generator is good !
    If not – voltage drops below 12 Volt while start process = Generator malfunction.

  3. My uncle’s car is having difficulty starting lately, and now it’s starting to make him worried because it takes him a while before it works. I had no clue that this issue is a sign that it needs to get replaced. I should recommend that he visit an auto parts shop for a battery replacement.

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