Spotting Bad Car Battery vs Alternator: 7 Key Differences

signs of a bad car battery vs alternator

Car troubles are alarming, and diagnosing what’s wrong isn’t always easy when your car gives you issues. One of the most common problems many drivers face is a faulty car battery or alternator. 

To help you know the difference, we’ve created this guide to compare signs of a bad car battery vs alternator. Here are seven tell-tale differences between the two. You can better determine what your car needs to get back up and running if you notice these signs.

The Different Roles of a Car Battery and an Alternator

The car battery and the alternator are two important components of your car’s electrical system. While they serve different functions, they work together to keep your vehicle powered.

The car battery provides power to the starter motor, which kicks the engine into action. The alternator is responsible for maintaining the charge of the battery and providing power to the vehicle’s electrical system while the engine is running.

The car battery is an electrochemical device that stores energy and provides a charge to start the engine. When you turn the key in the ignition, the battery sends a surge of electricity to the starter motor, which starts the engine.

Once the engine runs, the alternator takes over, providing power to the electrical system. The alternator works like a generator. It regenerates energy into electricity to power your car.

Top 4 Signs of a Bad Car Battery

Check Engine or Battery Light

One of the first signs of a bad car battery is that the battery light will come on. This indicates that the battery is not producing enough power to run the car’s systems.

The check engine light may also illuminate when there is an issue with your car’s battery.

Sensors communicate to the computer that the battery is weak and triggers a code that makes the engine light illuminate.

The engine light will also illuminate due to a faulty alternator, so to know the difference, take your vehicle to a maintenance shop to interpret the computer code.

Slow Cranking

Another sign of a bad car battery is slow cranking when you try to start the car. This often happens when the battery has been drained of its power.

Slow cranking is noticeable most when you attempt to start your car for the first time, usually in the morning after the car battery has set for a long time.

You will recognize slow cranking because the car will sound as if it is stalling and won’t turn over even though it does eventually.

A strong battery ensures your car’s turnover is quick without agitation. Always have your mechanic help identify reasons for engine stalling.

Dim Lights

If the car’s headlights or other lights are dimming or flickering, it could signal a bad car battery. This is because the battery is not producing enough power to keep the lights operating at their normal brightness.

In some cases, car lights may remain lit, but once you turn the ignition, the car won’t crank, and the lights will flicker and turn off. If this occurs, your car battery needs attention.

Corroded Terminals

If the terminals on the battery are corroded or filled with white, powdery residue, this is a sign that the battery needs to be replaced.

The terminals are where the battery ports connect with the engine through cables. Corrosion can cause the battery to lose its charge and fail to power the car.

Corrosion happens when gases from the battery terminals evaporate and leave crusty residue behind. Corrosion does not indicate a battery is bad, but a build-up of corrosion residue will inhibit a battery’s function.

Top 3 Signs of a Bad Car Alternator

Burning Smell

The smell of burning wires or plastic from the engine bay may indicate a faulty alternator. This odor is caused by the alternator overheating due to electrical shorting or worn-out bearings.

If this smell is present, shut off your car quickly. The alternator can cause further damage to the engine and other components.

Battery Won’t Stay Charged

Another purpose of an alternator is to regenerate the battery’s energy. A faulty one will not keep the battery charged as the engine runs, causing the battery to be reliant upon its internal energy, which will eventually deplete.

You may first need to test the battery and ensure it’s fine. Your alternator could be the culprit if the battery is good and won’t keep a charge.

Engine Stalls or Shuts Off

If your engine is running and then turns off while driving or idling, it may be because the alternator is not generating enough current for the spark plugs to fire properly. Without the property current, the engine will stall and stop running.

What to Do to Handle a Bad Battery or Alternator?

If you have a bad battery, you should replace the battery as soon as possible. This will help ensure that your car starts correctly when you need it to.

It’s best to take your vehicle to a qualified mechanic for professional advice and to install the new battery. They’ll be able to provide you with all the information you need on the right battery for your vehicle and how to install it properly.

Things to consider when buying a new car battery are:

  • Warranty Replacement Options: Look for car batteries that offer replacement warranties. A replacement warranty will replace your battery within a period if it becomes faulty.
  • Life Expectancy: Car batteries can last up to five years or more. Some batteries are manufactured for a range you can expect them to last. Consider batteries that will give you several years of service.
  • Cranking Amps and Reserve Capacity: Cranking Amps is the current a car battery can provide for 30 seconds at a temperature of 0°F. On the other hand, reserve Capacity is the amount of time a fully charged battery can deliver a minimum voltage of 10.5 volts at 80°F before recharging.
  • Core Return Features: A battery core charge is a rebate feature often provided with new car battery purchases. Parts stores will collect your old batter and give you a discount on purchasing a new one.

You should also have the batter tested to verify if it needs to be replaced. Sometimes it’s not the batter. Other times it could simply be that the terminals are not tight enough or are covered with corrosion. Both of these could make a good battery not perform.

If you suspect the problem is an alternator issue, take your car to a qualified mechanic for a diagnostic and repair. Your mechanic can run a full diagnostic test to determine exactly what the problem is and how to fix it.

Do not drive your car if you believe your alternator is faulty. A faulty alternator can cause your car to shut off while driving which is dangerous (not to mention not being prepared).

By Anita Brown

Anita Brown is our go-to contributor to our emergency preparedness website. Anita brings a wealth of personal experience and professional expertise to the table, having weathered several awful natural disasters. Anita is currently working towards obtaining her Community Emergency Response Team (CERT) certification.