Bad Alternator vs. Bad Battery: A Quick Guide - Trip Canvas
Bad Alternator vs. Bad Battery: A Quick Guide
Published: October 24, 2025
Whether you are safe in your garage or alone in a deserted parking lot at night, your stomach drops when your car won’t start. “Am I stuck here? Am I safe? How much will this cost me?” These are probably your first thoughts, especially if you aren’t a mechanic. Usually, there are two likely culprits for a car that won’t start: a bad alternator or a bad battery.
The alternator and car battery are part of your car’s electrical system. The battery provides the energy to crank the engine and start the vehicle. Starting a car takes anywhere from 400-2,000 amps depending on your engine's size. Batteries are designed to deliver that charge before being recharged and are responsible for running the electrical system when the car’s engine is off.
The engine starts the alternator, which turns the engine’s energy into electricity to recharge the battery, and directs electricity to the car’s electrical system when the engine is running.
AAA has helped millions of people in this exact situation and knows there are two types of people: The type to immediately call roadside assistance and the kind who tries to handle the problem on their own. If you fall into the latter category, or you just want an idea of what the auto shop will tell you, read on to find out how to diagnose and handle the issue so you can get back on the road.
Table of Contents
8 Signs of a Bad Battery
How to Test Your Car Battery
7 Signs of a Bad Alternator
How to Test Your Alternator
Frequently Asked Questions
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8 Signs of a Bad Battery
If your car doesn’t start, always start by checking the battery. Since it is the first step in your car’s electrical process, it is the most likely culprit. Consider if you’ve noticed any of the following signs in recent weeks.
1. Your Car Struggles To Start Consistently
If your car cranks (makes a clicking noise) and stutters before eventually turning over, your battery is probably losing its charge.
However, excessively low temperatures can also cause your car not to start immediately, so if you live in a cold environment and notice your car has trouble starting, the cold may be to blame instead of the battery.
2. Your Car Won’t Hold a Charge Over Time
You’ve probably noticed that your old cell phone doesn’t hold a charge like it did when you first got it. That’s because batteries naturally degrade over time. The same is true for your car battery.
The battery's age is a big factor here, but another factor is whether you sufficiently recharge your battery each time you use it. If you only drive short distances, it may not have enough time to recharge, damaging its ability to hold a charge in the future.
If you’ve needed a few jump-starts recently, chances are your battery is dying.
3. Your Car’s Electrical Systems Flicker
The battery powers the car’s electrical systems when the vehicle is off. A bad battery will struggle to keep those systems running, and you may notice dim or flickering lights, or the radio signal may go in and out.
4. You Notice Corrosion on the Battery
Like all batteries, car batteries can corrode if acid leaks from it. The acid contains hydrogen gas, which turns into a green, blue or white substance when interacting with the air under your hood.
Corrosion makes it more difficult to start your car because it acts as a barrier between the battery and the rest of your car’s electrical system. This causes two main issues:
Your battery can’t send enough energy to start the engine.
Your alternator can’t send enough energy to recharge the battery.
The good news is that you can clean corrosion off your battery to extend its life until you can get a new one. You’ll need to remove the battery and then use a battery cleaner and a wire brush to scrub the corrosion. You can buy the cleaner at the store or create a mix of baking soda and water.
Ultimately, if you’re seeing corrosion, your battery is leaking and will need to be replaced sooner rather than later.
5. Your Car’s Low Voltage Icon Is Lit
Newer cars have computer diagnostics that can let you know when the electrical system is failing. However, this icon indicates that a problem exists in the electrical system, but it doesn’t necessarily mean it’s the battery. It could also be the alternator or another component of the electrical system.
6. A Rotten Egg Smell Comes From Your Battery
When everything is running smoothly, you shouldn’t notice a smell from your battery. However, if the sulfuric acid in the battery overheats, it converts to hydrogen gas, which has a rotten egg smell.
If you notice this, you should take your car to a mechanic immediately. Not only is your battery dying, it’s at risk of exploding and harming anyone near the car.
7. Your Battery Appears Swollen
Batteries contain dangerous materials like sulfuric acid, which usually isn’t a problem. If it is overheated, though, your battery’s casing will swell.
If you notice this when you pop your hood, do not touch it and call a tow truck immediately. Do not attempt to replace the battery on your own, as this can cause the battery to explode. While batteries are easy to replace and nearly anyone can do it, this is one instance where it’s best to let the pros take over.
8. Your Battery Is More Than 3-5 Years Old
Car batteries have an average life span of three to five years unless you have extra electrical accessories like subwoofers, auxiliary lights, or alarm systems. If you know when you last replaced the car battery, and it’s been longer than its lifespan, it’s a good bet that the battery is the reason your car won’t start.
How To Test Your Car Battery
If those signs indicate the car battery being the problem, you can test your battery at home if you have a multimeter tool.
There are two ways to test your car battery:
Test the battery’s voltage with your multimeter. The reading should be about 12.6 volts with the engine off. If it’s not, your battery is either dead or undercharged.
If the reading is normal, and you have time to investigate further, you can disconnect your battery and plug it into a wall charger. Once it’s charged, let the battery sit for two days before testing it with a multimeter. If the charge is lower than 12.6, it is struggling to hold a charge and on its way out. You can also take your car to an automotive repair shop for further investigation.
An auto repair shop will conduct additional battery tests, including:
Inspecting the battery fluid level if you can — most batteries are sealed, meaning the fluid level cannot be checked.
Looking for corrosion on the battery posts.
Ensuring all cables are snug and the wires are in good condition.
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7 Signs of a Bad Alternator
If your investigation of the battery turns up empty or your battery is relatively new, it’s time to investigate the alternator. A faulty alternator will exhibit the following problems.
1. Your Car Won’t Start With a Jump-start or Starts and Dies Shortly Afterward
The alternator is responsible for keeping the car on after the battery does its initial job. If a jump-start doesn't work, or if it works but the car stalls within 10-15 minutes, the alternator can no longer do its job running the electrical running or recharging the battery.
2. Your Car Vibrates While You Drive
One of the alternator’s jobs is to provide the electricity required to ignite the spark plugs, which are responsible for igniting the gasoline that creates the energy to move the car’s wheels. When a spark plug doesn’t fire, the engine stutters, creating a vibration sensation while you drive.
3. Your Car’s Low Voltage Icon Is Lit on the Dashboard
Like with your battery, newer cars with a low voltage icon may light up when you have a problem with your alternator.
4. You Notice Issues With Your Car’s Electrical System, Especially the Radio and Interior and Exterior Lights
If your car’s electrical system is going haywire, your alternator may be on the fritz. You may notice any or all of the following odd occurrences:
Interior and exterior lights are dimmer or brighter than usual
Exterior lights get brighter the faster you’re going
Radio, especially when set to AM, has a whine or a static sound or cuts in and out even when you are within range of the station
5. Your Engine Squeals or Growls, Especially When the Radio or Headlights Are on
A whine or growl from the engine, especially when running electrical systems like the radio or exterior lights, can indicate a problem with the alternator. However, when you hear this sound, get a professional to investigate. A whine or growl can also indicate other issues such as a worn-out serpentine belt or another bad component, like the power steering pump or air conditioner compressor.
6. You Smell Burning Rubber or Hot Wire When the Engine Is on
A bad alternator sometimes overcharges the battery rather than failing to charge it all together. In that instance, you’ll smell burning rubber or hot wire.
7. Your Alternator Is 7-10 Years Old
Today’s alternators typically last seven to 10 years before they need replacement. If you know you haven’t replaced the alternator during that time and you’re experiencing some of the other signs, have a professional check your alternator before your car starts stalling or not starting.
How To Test Your Alternator
If you’re a DIY-er and want to test your alternator, you’ll need a multimeter.
1. Start the car.
2. Use the multimeter to check the voltage of the battery terminal.
3. If the voltage is 13-14.5 volts, the alternator is correctly charging the battery and your alternator is fine. If you get any other reading, the alternator is either over- or under-charging the battery.
You can also test the alternator by turning on the car and interior lights. If the lights slowly dim while the vehicle is on, your alternator is struggling to do its job and needs replacing.
A professional mechanic can also check the alternator’s diodes, which convert the engine’s alternating current (AC) to the battery’s direct current (DC).
FAQs
Read on to find answers to common questions related to batteries and alternators.
How Do You Jump-start a Car Battery?
You’ll need another car and a set of jumper cables from your car emergency kit to jump-start a car battery.
1. Consult your vehicle’s owner's manual to review the recommended procedure. Some cars require certain steps before attempting to jump start the battery, and if those steps are missed, you can damage your vehicle.
2. Park the car with the good battery close to the car to be jumped. They can be facing each other or side by side, but ensure they are close enough that the cables can connect the two batteries.
3. Find the batteries in both cars.
4. Attach the cables in the following order:
• Red clamp to the positive post of the dead battery
• Red clamp to the positive post of the good battery
• Black clamp to the negative post of the good battery
• Black clamp to an unpainted metal part of the engine
5. Start the engine of the car with the good battery.
6. Start the engine of the car with the bad battery. If it doesn’t start immediately, wait a few minutes for the battery to charge before trying again.
7. Disconnect the cables in the following order:
• Black clamp from the jumped car
• Black clamp from the car with the good battery
• Red clamp from the car with the good battery
• Red clamp from the jumped car
8.. If you can, drive straight to the auto shop or car part store to get the charging system inspected.
Can You Jumpstart a Car With a Bad Alternator?
It’s possible that your car will start even if your alternator is bad, but it will probably stall once the battery runs out of enough power to keep things running the way they should.
Can a Bad Battery Mess up an Alternator?
Fortunately, no. A bad battery will not damage your alternator since the alternator only runs when the car is running.
Why Does an Alternator Go Bad?
Alternators consist of moving parts that suffer wear and tear over time, especially if they get dirty or you drive in climates with extreme temperatures.
Can You Drive a Car With a Bad Alternator?
Yes, but you shouldn’t. A bad alternator can overcharge your battery, damaging it to the point that you’ll need to replace it. Plus, you want to avoid stalling or getting stranded somewhere when the alternator finally dies completely.
Can You Drive a Car With a Bad Battery?
Technically yes, but it’s a bad idea. You’ll need jump-starts to get the car started, and you run the risk of stalling in stop-and-go traffic. The alternator runs the car’s electrical systems when the engine runs, but at low RPMs, the battery takes over because the alternator doesn’t have enough energy to run them.
How Do I Maintain My Car Battery?
Car battery maintenance is relatively simple and can ensure your battery lasts as long as possible.
If you don’t drive your car often or take frequent short trips, your battery may not get fully charged between starts. To prevent this from happening, take a long drive on the highway once a week to let the alternator charge the battery, or use a battery tender when you aren’t using the car.
You should also clean the battery posts and connections with a wire brush to keep corrosion from building up. Adding a bit of dielectric grease to the contact points can also keep corrosion at bay.
You can replace the battery yourself at any auto parts store or take the vehicle to a mechanic you trust. Generally, a new car battery will cost you between $45-$250, depending on the type of voltage your engine needs.
An alternator can be trickier to access, so let the professionals at a AAA Approved Auto Repair facility take care of it. It can cost anywhere from $500-$2500, including parts and maintenance to replace an alternator.
Being stuck with a car that won’t start — regardless of the reason — is never convenient. Protect yourself with a AAA membership so you can take advantage of features like roadside assistance and our car battery replacement assistance.
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