Battery or Alternator? How to Tell What’s Failing in Your Car

If the engine cranks slowly or won’t start after sitting, it’s likely the battery; if the car starts but then stalls, or voltage while running isn’t 13.5–14.7 volts, the alternator is suspect. A quick multimeter check—about 12.6 V with the engine off and 13.5–14.7 V while running—usually distinguishes between the two, and jump-start behavior offers additional clues.

Why this matters and what to check first

Starting and charging problems can stem from a weak battery, a failing alternator, poor connections, or even a worn belt. Because the wrong diagnosis can lead to unnecessary parts swaps, a simple sequence of checks can pinpoint the culprit and prevent getting stranded.

Rapid clues you can observe without tools

Before breaking out a meter, these common signs can quickly point you in the right direction based on how the car behaves.

• Slow crank, click-click, or no crank after sitting overnight: More likely the battery (weak/aged or parasitic drain).
• Starts with a jump, runs, then dies while driving or soon after removing the jumper: More likely the alternator isn’t charging.
• Dim lights that brighten with revs: Often alternator output issue or loose belt.
• Battery/charging dash light illuminated while driving: Alternator, belt, or voltage regulator problem.
• Electrical accessories cutting out or fluctuating while running: Alternator or poor main ground/connection.
• Frequent need to jump-start but strong cranking immediately after a long drive: Battery may not hold charge (sulfated/aged).
• Squealing belt at start-up or in wet weather: Slipping serpentine belt can reduce alternator output.
• Burning smell or whining from alternator side: Potential alternator bearing/regulator/diode failure.

These symptoms are directional, not definitive. Quick tests below will confirm whether charging or storage is the core issue.

Do-it-yourself testing with a multimeter

With a basic digital multimeter, you can verify battery state and alternator performance in minutes. Observe safety: work in park/neutral with brake on, keep clothing and tools clear of belts and fans, and never disconnect the battery while the engine is running.

1. Visual check: Inspect battery terminals and grounds. Clean corrosion, tighten clamps, and verify the engine-to-chassis ground strap is intact.
2. Key-off battery voltage: Engine off, lights off for 5 minutes. A healthy, fully charged battery reads about 12.6 V. Around 12.4 V is moderate charge; 12.2 V is low; ≤12.0 V is deeply discharged or failing.
3. Cranking voltage drop: Watch voltage while you crank. It should generally stay above ~9.6 V at 70°F. A drop well below that points to a weak battery or poor connections.
4. Charging voltage at idle: Start the engine. Measure across the battery posts. Typical systems show 13.5–14.7 V. If it stays near the key-off reading (around 12–12.6 V), the alternator isn’t charging. Over ~15.0 V suggests a failed voltage regulator.
5. Load test while running: Turn on headlights, rear defogger, blower on high. Voltage should remain roughly ≥13.2 V at idle and rise slightly with throttle. If it sags into the 12s and doesn’t recover, suspect the alternator or belt/tensioner.
6. AC ripple check (if your meter supports AC volts): With engine running, read AC volts at the battery. Ripple above ~0.3–0.5 V AC often indicates a bad alternator diode.
7. After-drive restart test: Drive 15–30 minutes, shut off, and try restarting after 1–2 minutes. If it cranks strong then, but is weak the next morning, the battery may not hold charge or there may be a parasitic draw.

These measurements separate storage (battery) problems from generation (alternator) issues and can reveal connection or belt faults that mimic both.

Interpreting results: What the numbers mean

Here’s how to match your observations with likely causes to decide what to fix first.

• Battery low at rest (≤12.2 V), cranking below ~9.6 V, but 13.5–14.7 V while running: Battery is weak/aged or not fully charged; alternator appears OK.
• Battery normal at rest (~12.5–12.7 V), running voltage stays near 12–12.6 V or drops with loads: Alternator or belt/tensioner problem; charging isn’t happening.
• Running voltage spikes above ~15.0 V: Faulty regulator (often part of the alternator) risking damage to electronics.
• Good running voltage, but car dies intermittently, especially with squeal or wobbling accessory: Belt slip or failing tensioner; alternator cannot maintain output.
• Good numbers but persistent overnight discharge: Parasitic draw or battery nearing end-of-life; test draw and consider battery replacement if old.

Use these patterns to guide repairs: replace the battery if it fails storage/cranking tests; repair charging components if voltage doesn’t rise while running.

Smart-charging and modern vehicles

Many newer cars use “smart” charging that can vary system voltage from roughly the high 12s into the mid-14s depending on load and battery state. The key is that running voltage should typically be higher than key-off voltage and stable under moderate load. If it never rises above rest voltage, the alternator or its control circuit is suspect.

Start-stop, AGM/EFB batteries, hybrids, and EVs

Start-stop systems often use AGM or EFB batteries that need proper replacement coding and higher charge acceptance. Hybrids and EVs still have a 12 V system for accessories; diagnosis is similar for the 12 V portion, but never touch orange high-voltage cables and follow manufacturer procedures.

Common pitfalls and what not to do

Avoid practices that can damage your vehicle or lead to misdiagnosis when chasing charging issues.

• Do not disconnect the battery while the engine is running; voltage spikes can destroy electronics.
• Don’t ignore corrosion or loose grounds; they can mimic a bad battery or alternator.
• Don’t rely solely on a battery’s “green eye” indicator; use voltage and load testing.
• Avoid short trips only; they prevent full recharging and mask alternator performance.
• Replace belts/tensioners if glazed, cracked, or slipping; they reduce alternator output.

By steering clear of these mistakes, you’ll protect sensitive systems and get clearer diagnostic results.

When to seek professional help

If results are borderline, if there’s a persistent parasitic drain, or if the vehicle has complex energy management, a shop can perform a conductance test on the battery, an alternator bench test, and a parasitic draw test. OBD-II codes like P0562 (system voltage low) or BMS faults can also guide precise repair.

What to fix, at a glance

Use this quick mapping from symptoms to likely solutions to act confidently.

• Weak morning starts, passes running-voltage test: Replace battery and clean/tighten connections.
• Stalls after starting or dies while driving, low running voltage: Replace/repair alternator and inspect belt/tensioner.
• Voltage spikes high: Replace alternator/regulator; verify grounds and BMS wiring.
• Good charging but repeated overnight drain: Test for parasitic draw; fix offending circuit or replace aging battery.

Addressing the right component first saves parts and prevents repeat failures.

Bottom line

If the engine-off voltage is low and cranking is weak but running voltage is normal, it’s the battery. If running voltage never rises above rest or drops under load, it’s the alternator or belt. A few minutes with a multimeter and careful observation will tell you which one to replace.

Summary

Differentiate battery versus alternator by testing both rest and running voltage and observing behavior. Healthy batteries read about 12.6 V at rest and maintain at least ~9.6 V during cranking. A healthy alternator raises system voltage to roughly 13.5–14.7 V and sustains it under load. Starts-then-stalls points to charging failure; slow morning cranks point to a weak battery or drain. Verify connections and belt condition, and avoid disconnecting the battery while running. If in doubt, seek a professional load test and charging system evaluation.

Will a bad alternator drain a battery when the car is off?

Yes, a faulty alternator can drain your car’s battery even when the engine is off. This occurs when the diodes inside the alternator fail, creating a one-way circuit that allows electricity from the battery to flow backward into the alternator, acting like an electromagnet and depleting the battery’s charge. 
This video explains how a failing diode in an alternator can cause a parasitic draw on the battery: 59sproclaimliberty2000YouTube · Nov 11, 2023
Why a Bad Alternator Drains the Battery

• Failed Diodes: Opens in new tabThe diodes within the alternator are responsible for converting the alternator’s alternating current (AC) into direct current (DC) for the battery. If a diode fails, it can create a closed circuit, allowing current to leak from the battery back into the alternator when the vehicle is turned off. 
• Parasitic Draw: Opens in new tabThis backward flow of electricity is known as a parasitic draw, and a failed diode can create a significant enough drain to completely discharge the battery, even overnight. 

This video explains how a bad alternator can drain the battery overnight: 1mAuto Repair GuysYouTube · Apr 16, 2020
What to Do

1. Test the Alternator: Opens in new tabHave the alternator tested by a professional. A failed diode can sometimes be identified even when the alternator appears to be working correctly. 
2. Test for Other Draws: Opens in new tabIf the alternator is not the issue, a professional can help you test for other parasitic draws in your vehicle’s electrical system. 
3. Replace the Alternator: Opens in new tabIf the alternator is found to be faulty, it will need to be replaced to resolve the battery drain. 

How can I tell if I need to change battery or alternator at home?

Warning Lights: Look for the battery or alternator warning light on your dashboard. Dimming or Flickering Lights: If your headlights or dashboard lights dim or flicker, it could indicate an alternator issue. Electrical Failures: Malfunctions in electrical systems (radio, power windows) may suggest a failing alternator.

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.

How to know if a battery or alternator is bad?

To tell if you have a bad battery, your car might have trouble starting, you’ll hear a clicking sound, or you’ll notice dimming lights and a swollen battery case. For a bad alternator, you’ll experience the same slow crank or dim lights, but also frequent stalling after the car is running, a burning electrical smell, or a growling noise from the engine. To differentiate, try jump-starting your car; if it stays running after the cables are removed, the alternator is likely bad. 
This video demonstrates how to identify if your car has a bad battery or alternator: 1mproclaimliberty2000YouTube · Apr 21, 2018
Signs of a Bad Battery

• Slow or No Crank: The engine struggles to start, or it just makes a clicking sound without turning over. 
• Dim Lights: Headlights and interior lights appear dim, especially when the engine is off. 
• Swollen Battery Case: The plastic casing of the battery may be swollen, indicating internal damage. 
• Age of Battery: Batteries have a limited lifespan, and an old battery is a common culprit for starting issues. 

Signs of a Bad Alternator

• Frequent Stalling: The car starts, but then stalls shortly after, or keeps dying even after a jump-start. 
• Dim or Bright Lights: Lights may dim when you press the gas pedal or turn on accessories, or they may be overly bright. 
• Burning Smell: You might notice a burning rubber or electrical smell. 
• Growling or Whining Noise: A squealing or growling sound coming from the engine area can indicate a failing alternator. 
• Battery Warning Light: The battery or check engine light illuminates on the dashboard. 

Quick Test: Jump-Start and Disconnect
This test helps determine if the alternator is supplying the necessary power. 

1. Jump-Start: Safely jump-start your car to get it running. 
2. Disconnect Terminal: While the engine is running, carefully loosen and remove the positive terminal from the battery. 
3. Observe:
   • If the engine stalls immediately, the alternator is not working. 
   • If the engine continues to run, the battery is likely the issue.