Battery or Alternator? How to Tell Which One Is Failing

If the engine starts with a jump and keeps running but won’t restart later, it’s usually the battery; if the car dies while driving or shows low voltage while running, it’s likely the alternator. A quick check with a multimeter helps confirm: a healthy battery at rest reads about 12.6 V, and a healthy alternator charges around 13.7–14.7 V while the engine runs. Below is a clear guide to symptoms, simple driveway tests, and precise diagnostic steps to pinpoint the culprit and avoid getting stranded.

Symptoms at a Glance

The following signs commonly point to either a weak battery or a failing alternator. Use them as a quick reference before performing tests.

• More likely the battery:
  

  • Slow crank or rapid clicking when you turn the key, especially after the car sits overnight
  • Starts with a jump, then runs fine, but won’t restart after you shut it off
  • Battery is 3–5+ years old or shows a recent drop in cold weather performance
  • Corroded terminals or loose battery cables

• More likely the alternator:
  

  • Battery/charging warning light on the dash while driving
  • Engine stalls, dimming lights, or electronics flicker while driving
  • Whining or grinding from the alternator area, or a burnt/rotten-egg smell (overcharging)
  • Needs a jump, then quickly dies again while running or after removing the jumper cables

While these symptoms are strong clues, overlapping issues—like loose belts or bad connections—can mimic both, so simple tests are essential.

Quick Driveway Checks

Jump-Start Behavior

Jump the car and observe what happens. If it runs normally after the jump and you can drive for a while but it won’t restart later, the battery likely can’t hold a charge. If it starts but sputters or dies shortly after the jump—or voltage immediately drops while running—the alternator may not be charging.

Headlight Test

With the engine off, turn on the headlights and note brightness. Start the engine: if the lights get noticeably brighter, that suggests the alternator is charging. If brightness barely changes or gets dimmer with the engine running, suspect the alternator.

Battery Age and Condition

Check the date code on the battery (sticker or stamped code). Most batteries last 3–5 years. Heavy corrosion on terminals, swollen case, or damp/acidic residue are red flags for battery issues. Clean and tighten connections before further testing.

Step-by-Step Diagnosis With a Multimeter

A digital multimeter provides the most reliable answer. Follow these steps for definitive readings and interpretation.

1. Engine off, car rested 30–60 minutes: measure across battery posts.
   

   • 12.6 V ≈ fully charged; 12.4 V ≈ ~75%; 12.2 V ≈ ~50%; ≤12.0 V is discharged/weak.

2. Cranking test: watch voltage while starting.
   

   • Should not drop below about 9.6 V at ~70°F (21°C). A deeper drop suggests a weak battery or poor cable connections.

3. Charging test at idle (no major accessories): measure at the battery with engine running.
   

   • Normal alternator output is roughly 13.7–14.7 V. Some modern “smart” systems may vary ~13.2–15.0 V.

4. Load test: turn on headlights, rear defroster, blower, and measure again at 1,500–2,000 rpm.
   

   • Voltage should generally stay ≥13.5 V. If it drops toward battery voltage (~12.x), the alternator can’t keep up.

5. Overcharge check:
   

   • Consistent readings above ~15.0 V (12 V systems) indicate overcharging—an alternator/regulator fault that can damage the battery.

6. AC ripple check (if your meter can read AC): set to AC volts and measure at the battery with engine running.
   

   • Ripple above ~0.3–0.5 V AC suggests failing alternator diodes.

In short: low resting voltage and big cranking drop point to the battery; low charging voltage, inability to hold ≥13.5 V under load, or high ripple points to the alternator. Overcharging also implicates the alternator/regulator.

When It’s Neither (or Both)

Sometimes the charging system is fine but other components cause similar symptoms. Check these common culprits before replacing parts.

• Loose, glazed, or slipping serpentine belt or a bad tensioner reducing alternator output
• Corroded or loose battery terminals, frayed ground straps, or damaged cables increasing resistance
• Parasitic drain (e.g., dome light, module staying awake) discharging a good battery overnight
• Blown fusible link or charging system fuse interrupting alternator output
• Failing starter drawing excessive current, masking as a weak battery

Addressing these issues can restore normal charging and starting without replacing the battery or alternator unnecessarily.

Professional Help and Practical Tips

If you’re unsure, many auto parts stores offer free battery and alternator tests. A load test can confirm battery health, and a bench test can validate alternator performance.

• Safety first: wear eye protection, keep metal tools away from battery posts, and avoid sparks
• Clean and tighten terminals before testing; poor contacts skew results
• Replace like-for-like battery type (AGM/EFB for start-stop vehicles); reprogram/“register” battery if your car requires it
• For hybrids/EVs: the 12 V battery still powers accessories; follow manufacturer procedures and avoid high-voltage components

A quick, safe check and proper installation practices can prevent repeat failures and protect sensitive electronics.

Bottom Line

If a jump gets you going but the car won’t stay running or the voltage won’t hold above ~13.5 V while running, the alternator is suspect. If it runs after a jump and charges normally but won’t restart later—or resting and cranking voltages are low—the battery is likely failing. Confirm with a multimeter, inspect belts and connections, and lean on free professional tests before buying parts.

Summary

Use behavior clues and a multimeter to distinguish failures: 12.6 V at rest indicates a healthy battery; 13.7–14.7 V while running signals a healthy alternator. Dying while driving or low running voltage points to the alternator; starting after a jump but failing to restart later points to the battery. Check belts, cables, fuses, and for parasitic drain to rule out look-alike issues.

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 do I know if it’s the battery or the alternator?

To tell if the problem is the battery or the alternator, check the car’s battery voltage with the engine off and running; with the engine off, a reading below 12 volts indicates a bad battery, while a healthy engine-running voltage should be between 13.5 and 14.5 volts, and if it doesn’t rise, the alternator is failing. A battery will typically show signs like slow cranking or clicking when starting, whereas a failing alternator often leads to frequent stalling, dim or flickering lights, and a “battery” warning light on the dashboard.
 
Symptoms to Watch For
Bad Battery Symptoms:

• Clicking sound: When you turn the key, you hear a clicking noise but the engine doesn’t start. 
• Slow cranking: The engine turns over slowly. 
• Dim or no lights: Headlights, dashboard lights, and other electronics are dim or don’t work properly. 
• Swollen battery case: The plastic case of the battery may look bloated. 

Bad Alternator Symptoms:

• Frequent stalling: The car starts but then dies shortly after, even when driving. 
• Warning lights: A battery or check engine light illuminates on the dashboard. 
• Dim or bright lights: Headlights and other lights may flicker, dim, or become overly bright. 
• Strange noises: A whining, screeching, or growling sound may come from the engine. 
• Burning smell: A smell of burning rubber or wires can indicate a problem with the alternator. 

How to Test Your Car’s System

1. Check Battery Voltage (Engine Off): With the engine off, use a multimeter to test the battery’s voltage. A healthy battery should read over 12 volts. 
2. Check Voltage (Engine Running): Start the car. The battery voltage should rise to between 13.5 and 14.5 volts. 
3. Turn on Accessories: Turn on your car’s headlights, radio, and other accessories. The voltage may drop slightly for a moment but should quickly return to the 13.5-14.5 volt range. 
4. Interpret the Results:
   • If the voltage does NOT rise: with the engine running, the alternator is likely bad and not charging the battery. 
   • If the car still won’t start: after the battery is fully charged, even if the alternator is good, the battery itself is likely the issue. 
   • If the car starts but dies quickly, the alternator is failing to charge the battery. 

What are the signs of a bad alternator?

Signs of a bad alternator include a lit battery or “ALT” warning light, dim or flickering headlights and dashboard lights, difficulty starting, frequent engine stalling, slow or malfunctioning electronic accessories, and strange smells like burning rubber or wires. Unusual growling or whining noises from under the hood can also signal a failing alternator.
 
Visual and Electrical Signs

• Dashboard Warning Light: A flashing or steady battery or “ALT” (alternator) warning light is a common and direct indicator. 
• Dim or Flickering Lights: Headlights, dashboard lights, and interior lights may appear dimmer than usual or flicker, especially when the engine is under load. 
• Slow or Malfunctioning Accessories: Power windows, air conditioning, or other electrical systems may operate slowly or not at all. 
• Failing or Weak Battery: The alternator may not be charging the battery sufficiently, leading to a frequently dead or weak battery and difficulty starting the car. 

Auditory Signs 

• Growling or Whining Noises: A growling or whining sound coming from the front of the car can indicate a worn-out bearing or a misaligned belt on the alternator. 
• Squealing or Screeching: An improperly functioning or worn-out drive belt can create a squealing or screeching sound. 

Smell 

• Burning Smell: A smell of burning rubber or hot wires can be a sign that the alternator is overheating or that its wiring is damaged and melting.

Performance and Starting Issues

• Difficulty Starting or Stalling: Your car may have trouble starting or may stall frequently because the alternator isn’t providing enough power to the electrical system. 
• Erratic Gauges and Instruments: The speedometer and other gauges may start to move erratically due to the unstable electrical supply from a failing alternator. 

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.