How to Tell if Your Car Battery or Alternator Is Bad
If the engine cranks slowly or clicks and a jump-start gets you going—but it won’t restart after you shut it off—the battery is likely bad; if the car starts then stalls, headlights brighten when you rev the engine, or system voltage while running stays below about 13.0 volts (or spikes above 15.2), the alternator or its regulator is likely failing. A quick multimeter check confirms it: around 12.6 V with the engine off is normal for a healthy, charged battery, and roughly 13.7–14.7 V with the engine running indicates proper alternator output.
Contents
Quick symptom check
The list below highlights common real-world signs that point toward a weak battery versus a failing alternator, plus a few symptoms that can be caused by either. Use these as clues before you test.
- More likely the battery:
– Single or rapid clicking when you turn the key; slow, labored cranking
– Car starts with a jump, runs normally, but won’t restart after a short stop
– Battery older than 3–5 years, swollen case, or heavy corrosion on terminals
– Resting voltage below ~12.2 V after sitting, or cranking voltage dipping below ~9.6 V at 70°F - More likely the alternator:
– Battery/charging warning light illuminated while driving
– Engine starts but then dies shortly after the jump; lights dim at idle and brighten when revved
– Electrical smells (sulfur/rotten egg) or whining/chirping from the alternator area
– Running voltage at the battery stays under ~13.0 V (or spikes above ~15.2 V) - Could be either (or wiring/belt issues):
– Dim or flickering lights, intermittent stalling
– Repeatedly dead battery after it’s been charged
– Wet/loose serpentine belt or failing tensioner, bad grounds, or corroded cables
These symptoms can quickly guide you, but confirmation requires a few simple tests to distinguish between a dying battery, a weak alternator, or poor connections.
Do a safe at-home diagnosis
What you need
Gather a few basic tools to test the battery and charging system without a shop visit.
- Digital multimeter (DMM) with DC and AC voltage modes
- Safety glasses and gloves; a wire brush or terminal cleaner
- Jump pack or jumper cables (for controlled jump-start testing)
- Optional: clamp ammeter for parasitic draw; OBD-II scanner for relevant codes
With these basics, you can perform the same core checks most shops do and make an informed decision.
Step-by-step tests
Follow this sequence to isolate the fault logically, from simple visual checks to definitive voltage measurements.
- Inspect the basics
– Check battery date sticker (month/year). Past 3–5 years is prime failure age.
– Clean any white/green corrosion and ensure terminals are tight; inspect ground connections to body/engine.
– Check the serpentine belt and tensioner. A loose, glazed, wet, or squealing belt can mimic alternator failure. - Listen to the start attempt
– Single click or rapid clicking: very low battery or poor connection.
– Slow crank: weak battery or high resistance at terminals/cables.
– Cranks strongly but won’t start: often not battery/alternator; could be fuel/spark, though low voltage can still be a factor. - Jump-start outcome
– Starts with a jump, keeps running, but won’t restart after you shut it off: battery is weak or sulfated.
– Starts with a jump but stalls shortly after or while driving: alternator likely not charging.
– Important: Do not disconnect a battery cable while the engine is running; voltage spikes can damage electronics. - Measure resting battery voltage (engine off)
– Remove surface charge by turning headlights on for 30 seconds, then off. Wait 2 minutes.
– Healthy, fully charged: ~12.6–12.8 V (AGM may read slightly higher).
– ~12.4 V ≈ ~75% state of charge; ~12.2 V ≈ ~50%; ~12.0 V is very low.
– If resting voltage is low, charge the battery fully and retest to separate “discharged” from “bad.” - Cranking voltage test
– Watch the meter while starting. Voltage should generally stay above ~9.6 V at ~70°F (expect slightly lower in extreme cold).
– If it plunges below ~9.6 V, the battery may be failing even if it was charged. - Charging voltage (engine running)
– At idle with minimal loads: expect roughly 13.7–14.7 V on most cars.
– With headlights, rear defroster, and blower on: voltage should remain above ~13.0 V.
– Over ~15.2 V suggests an overcharging regulator fault; under ~13.0 V indicates weak alternator output, a slipping belt, or wiring/ground issues.
– Note: Some modern “smart” systems may briefly charge lower or higher, but running voltage still should rise above resting voltage. - Alternator ripple/diode check
– Set meter to AC volts across the battery with engine running. More than ~0.5 V AC typically indicates bad alternator diodes. - Voltage drop on cables
– With engine running and loads on, measure from alternator B+ post to battery positive: ideally <~0.3 V drop.
– Measure from alternator case to battery negative: ideally <~0.2 V drop.
– Higher drops point to corroded cables, loose grounds, or poor connections. - Parasitic draw (if the battery keeps dying overnight)
– After the car “sleeps,” typical draw is ~20–50 mA. Anything well above that can drain a good battery.
– Use a clamp ammeter or an in-line ammeter with care; pull fuses one by one to find the culprit circuit.
By the end of these checks, you’ll know if the battery can hold and deliver power, whether the alternator is replenishing it, and if any wiring or drains are undermining the system.
Common scenarios and what to do next
Here are typical outcomes from the tests above and the most effective next steps to prevent repeat failures.
- Battery at end of life
– Resting or cranking voltage is low despite full charging; car behaves normally once running.
– Replace with the correct type (flooded/AGM/EFB) and capacity. Some vehicles (e.g., BMW, VW) require battery registration with a scan tool. - Alternator/regulator failing
– Running voltage is low or erratic; warning light on; AC ripple excessive; stalls after jump.
– Replace the alternator; also inspect/replace the belt and tensioner to protect the new unit. - Connection or cable issues
– High voltage drops, hot/burned terminals, or visible corrosion.
– Clean/replace terminals or cables; ensure solid engine/body grounds. - Parasitic drain
– Battery tests good but goes flat after sitting.
– Track the draw via fuse pull method; common causes include stuck relays, glove box/trunk lights, infotainment modules, or aftermarket accessories. - Cold-weather weakness
– Marginal batteries fail first in cold. Consider an AGM battery where specified, keep the battery fully charged, and check CCA ratings if you live in subfreezing climates.
Matching the remedy to the root cause—battery age, alternator output, wiring resistance, or parasitic draw—prevents repeat no-starts and protects electronics.
Safety and what not to do
Electrical testing is straightforward but can be hazardous without precautions. Keep the following in mind.
- Wear eye protection; charging batteries can vent hydrogen gas.
- Never disconnect the battery while the engine runs; you can damage the ECU and other modules.
- Observe polarity when jump-starting; make the final connection to a clean engine ground away from the battery to limit sparks.
- Before working on the alternator, disconnect the negative battery terminal; the alternator’s B+ stud is always live.
- Use jack stands if you must work underneath; never rely solely on a jack.
A few careful habits prevent injuries and avoid turning a simple diagnosis into an expensive electronics failure.
When to see a professional
Some situations call for a scan tool, specialized equipment, or OEM procedures beyond DIY scope.
- New battery dies repeatedly or the charging light stays on despite normal voltages—could be control-module or sensor related.
- Start-stop or smart-charging systems that require coding/registration after battery replacement.
- Suspected parasitic drains you can’t isolate, especially intermittent draws.
- Hybrid/EV systems (high voltage) or integrated starter-generators—DIY is not recommended.
Professional diagnostics can save time and parts when software, networked modules, or intermittent faults are involved.
Costs and time expectations
Knowing typical costs helps you decide whether to DIY or book a shop appointment.
- Battery: roughly $120–$250 for most cars (AGM and start-stop types can be higher); 15–45 minutes to install and register where required.
- Alternator: commonly $350–$900 parts and labor depending on vehicle and access; add $30–$100 for a belt and more if a tensioner is needed.
- Cleaning/repairing cables and grounds: often under $50 in parts; significant improvement for minimal cost.
Prices vary by vehicle and region, but budgeting within these ranges is realistic for most modern cars.
Summary
If a jump gets you rolling and the car runs but won’t restart, suspect the battery; if it starts then dies or running voltage won’t rise above resting voltage, suspect the alternator. Confirm with a multimeter: ~12.6 V engine off and roughly 13.7–14.7 V running are healthy targets. Check belts, cables, and grounds, and don’t overlook parasitic drains. A few simple tests at home can pinpoint the problem and prevent an avoidable breakdown.
How does a car act when the alternator is going out?
A car with a failing alternator will display signs of inconsistent electrical power, such as dimming or flickering lights, slow-operating accessories like power windows, and a battery warning light on the dashboard. You might also hear whining or grinding noises from the engine area, smell burning rubber, or experience difficulty starting the car or frequent stalling due to a depleted battery.
Common Symptoms of a Failing Alternator
- Electrical Problems:
- Dim or Flickering Lights: Headlights, dashboard lights, and interior lights may appear weak or flicker because the alternator isn’t providing consistent voltage.
- Malfunctioning Accessories: Power windows may operate slowly, the radio might not work correctly, or other electrical features may become sluggish or erratic.
- Warning Lights: The battery warning light on the dashboard is a common indicator of a failing alternator, though a check engine light or an “alt” indicator can also illuminate.
- Noises and Smells:
- Whining or Grinding: These sounds can indicate a problem with the alternator’s internal bearings or the serpentine belt that drives it.
- Burning Smell: A smell of burning rubber or wires can signal that the alternator is overheating or its components are wearing out.
- Performance Issues:
- Difficulty Starting or Stalling: As the alternator fails, it can’t recharge the battery, leading to a depleted battery and eventual engine stalling or difficulty starting.
- Repeated Dead Batteries: If you find yourself frequently needing a jump-start, the alternator may not be adequately recharging the battery.
What to Do if You Suspect a Bad Alternator
If you notice any of these signs, it’s essential to have a professional check your alternator promptly. Continued driving can lead to the battery running completely dead and the engine stalling, which can leave you stranded and potentially damage other components.
What can mimic a bad alternator?
“Faulty or corroded battery cables or connections can also mimic a failing alternator or battery,” Mock says. A mechanic or battery technician can test the system to determine the cause of the problem.
Will a new battery start a car with a bad alternator?
Yes, a new battery can start a car with a bad alternator because the battery’s sole job is to start the engine, not to keep it running. However, once the engine starts, the alternator is responsible for charging the battery and powering the car’s electrical systems. With a bad alternator, the new battery will gradually lose its charge as it powers the car, and the car will eventually stall and stop running.
This video explains why a new battery will start a car with a bad alternator: 48sMaxTheCarGuyYouTube · Jul 28, 2024
Why a New Battery Won’t Fix a Bad Alternator
- The battery starts the car, the alternator runs it The battery provides the initial power to turn the engine over and start it, but it’s the alternator’s job to generate electricity once the engine is running.
- Battery drain Since the alternator isn’t recharging the battery, the battery will be the sole power source for the car’s electrical components, like the lights, radio, and wipers.
- Eventual stall As the battery’s charge depletes, the car will eventually lose power and stall.
What to Do
- Get the alternator fixed: The only way to resolve the issue is to repair or replace the bad alternator.
- Drive carefully: If you must drive with a bad alternator, conserve battery power by turning off all non-essential electrical components.
- Don’t delay: The longer you drive on a dying battery, the harder you’re working it and the faster it will drain, leading to a stall.
What are the common signs of a failing car battery?
This post will teach you how to tell if your car battery is failing so that you can get it replaced timely.
- Your Battery is Likely Old.
- Your Car Doesn’t Start as It Used to in the Morning.
- You Frequently Have to Jump Start Your Car.
- Slow Engine Turnover is Another Sign.
- Your Battery Smells Bad.
- Your Battery Case is Bulging.
