Struggling to start your car or noticing it stalls more often?
That’s usually a sign something’s wrong.
But is it your alternator or battery causing the trouble?
Both play crucial roles in getting your vehicle running. The battery supplies power to the starter motor to crank the engine, while the alternator keeps the battery charged and runs your car’s electrical system.
When either one fails, you could be left stranded.
In this guide, we’ll discuss the role of each component, break down the common signs of failure, and give a quick way to identify the culprit.
Let’s begin!
This Article Contains:
- What’s the Role of a Battery and Alternator?
- 9 Evident Signs of a Bad Battery
- 8 Telltale Symptoms of a Bad Alternator
- How to Check If It’s a Bad Alternator or Battery Problem
- How to Prevent an Alternator or Battery Issue
- 5 FAQs About Alternator and Battery
What’s the Role of a Battery and Alternator?
Both the alternator and battery keep your vehicle powered — one starts the car, and the other keeps it running.
Here’s what each component does:
A. Battery
Your car battery converts chemical energy to electrical energy to provide the initial power needed to crank the engine. It supplies direct current (DC) to the starter motor, which engages the flywheel to turn the engine over and start combustion.
The car battery also supplies electrical power to accessories such as headlights, cabin lights, radio, and power windows.
B. Alternator
Once the engine cranks up, the alternator takes over the job of supplying electrical power. It converts your engine’s mechanical energy to electrical energy, which helps power the car’s electrical system and recharges the battery while you drive.
That’s why ignoring a battery or alternator problem can lead to unexpected breakdowns and expensive repairs.
Let’s first check out the battery failure signs you need to watch out for.
9 Evident Signs of a Bad Battery
If your engine won’t turn over or there’s an electrical problem, the initial blame usually falls on the car batteries. However, before you get your jumper cables, you must verify whether your vehicle’s battery is causing the problem.
Here are the bad car battery signs to look for:
- Active battery warning light or check engine light
- Dim dashboard lights or headlights
- Slow movement of power windows
- Slow engine start or no start
- Battery doesn’t hold charge for long
- Battery corrosion
- Odd or rotten egg smell from the battery
- Warped or swollen battery
- It’s an old battery
Now that you know the signs of bad car batteries, let’s move on to faulty alternator symptoms.
8 Telltale Symptoms of a Bad Alternator
If you don’t have a bad car battery, then startup issues could be caused by alternator trouble.
Here’s how a malfunctioning alternator flags its problems:
- Active battery light
- Cranking troubles and frequent engine stall
- Dimming or overly bright headlights
- Dimming interior lights
- A dead battery
- Malfunctioning electrical components and accessories
- Growling or squealing noises
- Burning smell from the alternator
Still not sure which one’s causing the electrical problem?
Up next is a simple test to help you confirm it.
How to Check If It’s a Bad Alternator or Battery Problem
Here’s a quick way to find out whether it’s a failing battery or a malfunctioning alternator:
- Connect jumper cables from a running car to your battery.
- Let it charge for a few minutes.
- Start your car.
- Remove the jumper cables.
- If your car stalls, it points to alternator failure.
- If your car keeps running, it’s probably a battery issue.
Knowing the underlying issue helps — but prevention is key. Let’s look at how to steer clear of these electrical system issues altogether.
How to Prevent an Alternator or Battery Issue
Here are a few simple habits that can help you avoid a costly battery or alternator problem:
- Get your battery and charging system checked regularly.
- Clean each battery terminal to prevent corrosion buildup.
- Inspect drive belts for cracks, wear, or looseness.
- Avoid frequent short trips that strain the battery.
- Turn off electronics before shutting off your car.
- Watch for warning signs like dim headlights or slow cranking.
- Use quality batteries and alternators designed for your vehicle.
Got more questions?
We’ve got you covered!
5 FAQs about Alternator and Battery
Here are a couple of questions (and their answers) on these charging system components:
1. How Urgent Is an Alternator or Battery Replacement?
A bad battery won’t damage the alternator, but a failing alternator can damage a battery.
The car battery isn’t made to deliver electrical energy for extended periods, so both components need to be functioning optimally.
Fortunately, conventional flooded batteries are comparatively cheap, typically falling around $50-$150. On the other hand, alternator replacement can cost a bit more, running anywhere between $200-$1200, with labor included.
NOTE: You may be able to repair the alternator instead of replacing it, and a rebuilt alternator might be a little more cost-effective. However, just like a new alternator, it’ll depend on the make and model of your car.
2. How Do You Check the Alternator or Battery Output?
Use a voltmeter or battery tester and connect its wire set to each battery terminal:
- With the engine off, a healthy battery voltage should fall around 12.6V.
- With a running engine, the battery voltage should go up to 13.5V-14.4V.
- Turn on the stereo, AC, and headlights. A battery voltage that stays around 13.5V indicates good alternator output.
NOTE: Your vehicle may also have a gauge that measures volts or amps, which can also help you determine your battery or alternator output.
3. How Long Can You Drive with a Failing Alternator or Battery?
It depends on which one is failing — the battery or the alternator.
- If you have a failing battery but your alternator is working, you can usually drive until the battery fully drains, which might be a few starts or short trips. However, a weak battery puts extra strain on the alternator and could leave you stranded.
- If you have alternator failure, your car will only run as long as the battery has power. That’s typically 30 minutes to 1 hour or 25-50 miles, depending on your vehicle and electrical load. Once the battery drains, your engine will shut off.
Remember, a faulty alternator won’t charge the vehicle’s battery, so you’ll end up with a dead battery at your next crank attempt.
In either case, driving is risky and can cause more damage. It’s best to get it checked by a mechanic and replaced immediately.
4. What’s the Average Lifespan of a Car Battery and Alternator?
On average, a car battery lasts 3 to 5 years, depending on driving habits, climate, and maintenance.
A car alternator typically lasts 7 to 10 years or 80,000 to 150,000 miles. However, factors like poor battery health, worn belts, or heavy electrical use can expedite alternator replacement.
5. How Does a Car Alternator Work?
Your vehicle’s alternator consists of several parts: the stator, rotor, diode, and voltage regulator.
An alternator pulley is connected to the engine and drives the alternator belt. The belt spins the rotor, creating a magnetic field that the stator uses to generate voltage. The diode converts the voltage from alternating current (AC) to direct current (DC) for the battery, and the voltage regulator regulates this electricity output.
Don’t Guess — Get Your Battery and Alternator Tested
The battery needs the alternator to stay charged, and the alternator needs the battery to start charging. Neither works well without the other.
So, if you have an alternator issue or a dead car battery, get them tested ASAP to avoid any further troubles.
Fortunately, with AutoNation Mobile Service, you don’t need to drive to an auto shop.
Our mechanics can come to you to test and fix your car’s charging system. They’ll even help you sort out a new alternator or new battery (if that’s what you need).
Just contact us, and our expert mechanics will be at your doorstep, ready to help you out!