Signs of a Bad Alternator: 7 Symptoms and How to Tell It From the Battery
Published Oct 23, 2026 · 6 min read · By Hassan, Owner of Roadmate Services
When a car will not start, the battery gets the blame. Sometimes it is innocent. The 7 signs of a bad alternator are a battery or charging light while driving, dim or flickering lights, a battery that keeps going flat, slow electrical accessories, hard starting or stalling, a whining noise and a burning smell.
The alternator charges the battery while the engine runs, so most of these signs look like battery trouble. The quick way to tell them apart: a bad battery stops the car from starting, while a bad alternator lets the car start and then lets it die.
Quick answer: is it the battery or the alternator?
| What you notice | Battery | Alternator |
|---|---|---|
| Won't start, starts on a jump and keeps running | Likely | |
| Starts on a jump, then dies again while driving | Likely | |
| Battery or charging light on while driving | Sometimes | Usually |
| Lights dim at idle and brighten when you rev | Likely | |
| Slow crank on cold mornings, battery 3 to 5 years old | Likely | |
| Whining noise or burning smell from the engine bay | Likely | |
| Still not sure | Test the battery and the charging system together | |
The battery is a bit like the coworker standing closest when something breaks. It gets blamed first, even when the real problem is under the hood somewhere else.
What does an alternator do?
The battery starts the car. Once the engine is running, the alternator takes over: a belt from the engine spins it, and it makes the electricity that runs your lights, fans and radio while topping the battery back up.
When the alternator stops charging, the car keeps running on whatever is left in the battery. That buys you some time, but not much. The battery opens the shop; the alternator keeps the lights on all day.
7 signs of a bad alternator
1. Battery or charging warning light while driving
The light shaped like a battery is really a charging-system light. If it comes on while you drive, the car is telling you the battery is not being charged. Our guide to what the battery light means covers the light itself in more detail.
2. Dim, flickering or overly bright lights
Headlights or dash lights that dim at idle and brighten when you press the gas are a classic alternator sign. Lights that get unusually bright can mean the alternator is overcharging, which is hard on the battery too.
3. A battery that keeps going flat
If you have replaced the battery and it still dies, look at what charges it. Putting a new battery on a bad alternator is like refilling a bucket with a hole in it. For the other causes of repeat dead batteries, see why your car battery keeps dying.
4. Slow power windows, gauges or accessories
Windows that crawl, a radio that cuts out, gauges that jump around or heated seats that never quite warm up can all mean there is not enough power to go around.
5. Hard starting or stalling
A battery that has not been properly charged will crank slowly the next morning. If the alternator quits completely while you drive, the engine can stall once the battery runs out.
6. Whining or growling from the engine bay
A whine that rises with engine speed, or a growl or grinding sound, can come from worn bearings inside the alternator. Of all the complaints a car makes, this is one worth listening to.
7. Burning rubber or electrical smell
A slipping belt can smell like hot rubber, and an overheating alternator can give off a hot electrical smell. If you smell burning while driving, pull over somewhere safe, turn the engine off and do not keep driving. Call a mechanic or a tow.
How can you tell if the battery or alternator is bad?
The jump test
A jump start is a useful clue. If the car starts on a jump and keeps running fine, the battery was the weak link and the alternator is doing its job. If it starts on a jump and dies again within minutes, or the battery light comes on as you drive away, the charging system is the likely cause.
Do not test this by pulling a battery cable off a running car. It is an old trick that can damage the electronics in a modern car. Our guide to jumping a car safely covers the cable order.
The voltage check
With a simple multimeter on the battery terminals, AutoZone gives these market figures: about 12.6 volts with the engine off, and about 13 to 14.5 volts with the engine running. A reading under 13 volts with the engine running, or one that swings around, points to the alternator. It is the closest your car gets to letting you take its temperature.
Source: AutoZone, bad battery vs bad alternator. Market guidance, checked October 2026, not a Roadmate figure.
How do I know if my alternator is bad for sure?
The signs and a multimeter narrow it down. A proper test of the battery and charging system confirms it, because a weak battery, a loose belt or a corroded terminal can fool a quick check. AAA’s guide to a bad alternator vs a bad battery makes the same point: test, then replace.
After every jump, our technician tests the battery, the alternator and the charging system, so you leave knowing which one let you down.
What to do next: roadside or mechanic?
Not everything is a roadside job, and we will say so. We test and replace batteries on site. We do not repair or replace alternators. If the test points to the alternator, the car needs a mechanic.
- Battery is weak, alternator is fine: a jump, or a new battery on the spot.
- Alternator is failing but the car runs: drive straight to a mechanic on local roads, with the radio, heater fan and other extras off.
- Car will not stay running: do not keep jumping it. Get it towed to your mechanic.
If the car dies on I-70, I-71 or I-270, get safe first: hazards on, off the roadway if you can, and stay buckled in if you cannot. Then call.
Help with a battery or alternator problem in Columbus
Our jump start service in Columbus is typically $75 to $150, including the battery and charging test. If the battery is the problem, mobile battery replacement is quoted by phone.
If it is the alternator, we can tow the car to your mechanic. A local tow of about 1 to 15 miles is typically $125 to $275; longer tows are quoted by phone.
We work 6 AM to 8 PM daily, Eastern Time, and arrive in about 32 minutes on average, depending on traffic, weather and location. If your car is just dead and you are not sure why yet, start with what to do when your car battery dies.
Frequently asked questions
What can be mistaken for a bad alternator?
A weak or old battery, loose or corroded battery terminals, a worn or loose drive belt, and a bad ground cable can all look like alternator failure. That is why a test of the battery and charging system beats guessing. Check the simple things first: clean, tight terminals and a belt that is not cracked or slipping.
What happens before an alternator goes out?
Usually a few warnings: lights that flicker or dim at idle, the battery light coming on and off, power windows or the radio acting up, and sometimes a whine from the engine bay. Then the battery runs down, and the car stalls or will not restart.
Is it my battery, alternator or starter?
Rapid clicking with dim lights points to the battery. One loud click with bright lights points to the starter. A car that starts on a jump but dies again while you drive points to the alternator, because the battery is not being recharged.
Can a bad alternator cause a car not to start?
Yes. The alternator recharges the battery while you drive. If it stops charging, the car runs on the battery alone until the battery is flat, and then it will not start again. A new battery will not fix that for long.
Can I keep driving with a bad alternator?
Only as far as a safe place to stop, and not onto a highway if you can avoid it. The car is running on what is left in the battery, and when that is gone the engine and lights can cut out without much warning. Get off the road, then call a mechanic or a tow.
The bottom line
A bad battery stops the car from starting; a bad alternator lets it start and then lets it die. A battery light while driving, lights that dim at idle and a battery that keeps going flat all point to the alternator. A test of the battery and charging system settles it, and a failing alternator is a job for a mechanic, not a roadside fix. If you are not sure whether a new battery is worth it yet, our car battery cost guide has the numbers.
Battery or alternator trouble in Columbus?
Call (380) 288-2822, 6 AM to 8 PM daily, Eastern Time. A dispatcher confirms the price and an arrival estimate before anyone heads your way, and tells you honestly whether you need a jump, a battery or a mechanic.
May your battery light stay off and your alternator stay boringly quiet.