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Car Battery vs. Alternator: Which One Is the Problem?

A battery warning light on the dash is never a good thing, but what does it actually mean, and how can you diagnose some of the more common problems that might creep up in your vehicle’s electrical system? To start with, it’s important to note that the red battery light provides warnings related to the entire electrical system, and that will almost always mean that the problem is with your battery or your alternator. 

What is your car battery responsible for?

Your vehicle’s battery has one primary responsibility: to provide enough energy to start the car. Aside from this, it will also provide backup power to your vehicle accessories for instances when your alternator is not running at full spin because the engine RPMs are too low. This may happen frequently in city driving, especially in stop and go traffic, or a traffic jam where you’re stuck in one spot for a while. It also provides all the power to any accessories that need to be on while the car is not running, like a security system or dashcam. 

Starting a car takes anywhere from 400 to 600 amps normally, and can range up to 2,000 amps, depending on the size of the engine. Smaller car engines will be on the lower side of that scale, whereas large commercial vehicles may have a big diesel powerplant that requires the upper end of the scale. Because of the incredible strain put on automotive batteries to start the car, their design is optimized to discharge a lot of current in the starting phase and then recharge immediately. If a battery is started repeatedly without fully recharging, it can start to lose its ability to retain a full charge and you may begin to see battery issues. 

What symptoms indicate a battery problem?

The battery’s main function is to start the vehicle, so the most obvious symptom of a bad battery will be failure to start the car. If you turn the key or press the start button and nothing happens, it’s a good bet that your battery is completely depleted. If you get a clicking sound but the engine doesn’t turn over, it’s often a sign that the battery is just about dead. Another sign of a bad battery is very slow cranking when you’re turning over the engine to start the car. Very slow cranking also happens in northern locations where the super-low temperatures cause the engine components to be stiff and the battery voltage to drop. A block heater or oil pan heater can help to mitigate that problem. 

What is your alternator responsible for? 

Your alternator is a small generator that produces the electricity your car needs to run all the accessories and recharge your battery at the same time. The output voltage of an alternator will be between 13 and 14.5 volts and the amperage output can vary, depending on the vehicle. Most passenger vehicle alternators have a maximum amperage between 130 and 200 amps, but you can get alternators that are rated up to 400 amps, depending on the application. If you install a high-power stereo system with massive subwoofers you may want to upgrade your alternator because those subs will pull a lot of amps and reduce the amount going to recharge your battery.  

Alternators aren’t always running at maximum output, either. When your car is idling, like in heavy traffic or at a stoplight, the engine RPMs are too low to maximize alternator output, so it will focus on providing power to the engine ignition and the essential accessories like cooling fans, water pumps, and HVAC. Your battery won’t be receiving a full recharge in that scenario.  

What symptoms indicate an alternator problem?

If you’re concerned that your alternator is beginning to fail, there are some early warnings to look out for. One potential warning of a bad alternator is if your headlights or interior lights are running dim or are dimming and then getting brighter, depending on how fast you are driving. This indicates that the alternator is providing inconsistent voltage. If other electrical accessories in the car are starting to do weird things, like shutting off unexpectedly or operating poorly, that’s another sign that you need to check the alternator. 

If your car won’t start because of a dead battery, try to jump start it. If it starts but dies again after a short time, that may be a sign that your alternator isn’t functioning properly because your alternator provides the power to the engine ignition. 

How can I tell which piece of the system is causing a warning light? 

You can test your battery’s voltage with a multimeter available at AutoZone. A good battery will have a voltage reading of about 12.6 volts and if it’s below that, you may want to consider having the battery tested at your local AutoZone, or replaced. If you’re suspecting that your battery is not holding a charge, use a battery charger to fully charge it and then let it sit for a day or two before testing it again. If there is a big drop in voltage from the time you tested it, you know there’s an issue there. 

You can test your alternator by starting your car and using a multi-meter across the battery terminals to measure voltage If the voltage is between 13 and 14.5 volts, then your alternator is charging the battery. Further tests can determine if the alternator is supplying the correct amperage under load, which can also be done at any AutoZone location. If the voltage is below 13 volts or if it is fluctuating wildly, the alternator is likely suspect and will need to be removed and bench tested. This can also be performed at any AutoZone to determine if the fault is in the alternator, or the wiring circuit between the alternator and battery.   

How can I prevent problems with my battery or alternator? 

If your driving habits include infrequent short trips and many days of not driving, you may deplete your battery and damage its ability to recharge. You can either make a point of taking the car for a longer drive on the highway once a week, or you can install a trickle charger to keep the battery topped up. It’s also a good idea to clean your battery terminals and connections with a wire brush periodically and apply some dielectric grease to the contact points. Dielectric grease helps to improve the electrical connection and also protects the surface from corrosion.

Whether you’ve settled on the car battery being at fault or the alternator needing to be replaced, or if you simply need tools to accurately diagnose the concern, AutoZone can help. Ask an AutoZoner for Trusted Advice or choose the parts and tools you need online for quick delivery or pickup. Also, if at any time the job appears to be too big, take a look at our list of local Preferred Shops in your area that can help you tackle the job.  

Advice, how-to guides, and car care information featured on AutoZone.com and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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