Battery vs. Alternator: How to Tell Which One Is Failing

If your car won’t start or keeps dying, measure voltage at the battery: with the engine running you should see roughly 13.8–14.7 volts (some smart systems may dip to ~13.2–14.8 depending on load and temperature). If running voltage stays near battery level (~12.0–12.6 V), the alternator isn’t charging; if running voltage is normal but starting is weak or the battery won’t hold charge, the battery is likely failing. Below is a clear, step-by-step way to confirm which component is at fault and what to do next.

What the Symptoms Usually Mean

The following checklist links common symptoms to the most likely culprit. Use it to form an initial hypothesis before testing.

• Slow crank after sitting overnight, but fine immediately after a drive: points to a weak or aging battery.
• Dashboard battery/charging light turns on while driving, headlights brighten/dim with engine RPM: points to alternator or belt/charging system issue.
• Engine dies while driving or electronics fade out, then the car won’t restart: strong sign of alternator not charging.
• Needs a jump to start, runs while the cables are attached, then stalls soon after removing them: alternator likely bad.
• Starts after a jump and continues to run and charge normally afterward but won’t restart later: battery likely bad or parasitic drain.
• Frequent clicking/no crank, heavy corrosion on terminals: could be battery or poor connections; clean/tighten first.
• Burning rubber smell or squeal, dimming lights: possible slipping/broken belt affecting alternator output.
• “Sulfur/rotten egg” smell from battery or battery hot to touch: possible overcharge (regulator/alternator fault) or internal battery failure.

These signs aren’t definitive on their own, but together with basic tests they can quickly isolate a battery versus alternator problem.

Step-by-Step Diagnosis You Can Do at Home

Tools You’ll Need

These basic tools make diagnosis faster and more accurate.

• Digital multimeter (volts DC and AC millivolts).
• OBD-II scanner (optional, to check charging system codes).
• Wire brush/baking soda-water mix for terminal cleaning; gloves/eye protection.
• Portable jump pack or jumper cables.
• Flashlight; if available, a battery load tester.

With these tools you can check voltages at rest, during crank, and while running, which covers most charging and starting faults.

1) Visual and Connection Checks

Start with simple inspections; many “bad batteries” are actually loose or corroded connections.

• Inspect battery terminals and grounds: clean off white/green corrosion; ensure clamps are tight and cables aren’t frayed.
• Check the serpentine belt: look for cracks, glazing, or looseness; a slipping belt reduces alternator output.
• Look for a battery date sticker: beyond 3–5 years (AGM/lead-acid) often indicates end-of-life.
• Check for warning lights: a red battery/charging icon suggests a charging system fault.

Fixing poor connections or a slipping belt often restores normal charging without replacing costly parts.

2) Jump-Start Behavior

How the car behaves after a jump offers quick clues.

• If it starts with a jump and keeps running, then restarts later on its own: alternator likely okay; battery weak.
• If it starts with a jump but dies shortly after removing cables or after a short drive: alternator likely not charging.

This simple test helps prioritize whether to focus on the battery or the alternator before you break out the meter.

3) Multimeter Voltage Tests (Gasoline/Diesel Vehicles)

These readings are the gold-standard basics. Measure at the battery posts, not on the cable clamps, for accuracy.

• Engine off (after sitting 30+ minutes):
  

  • ~12.6 V = fully charged; ~12.4 V ≈ 75%; ~12.2 V ≈ 50%; ~12.0 V ≈ 25%; <11.9 V = deeply discharged/possible bad cell.

• During crank: voltage should generally stay above ~9.6 V. A drop well below this indicates a weak battery or poor connections.
• Engine idling, no accessories: typically 13.8–14.7 V on conventional systems. Smart/temperature-controlled systems may range ~13.2–14.8 V.
• Engine at 1500–2000 RPM with headlights, blower, and rear defogger on: voltage should remain above ~13.5 V. If it falls near battery voltage (~12 V), alternator or belt/connection issue is likely.
• AC ripple test (meter on AC mV at the battery, engine running): ideally under ~50–100 mV. High ripple suggests a failing alternator diode.

Key takeaway: normal running voltage with poor starting points to the battery; low running voltage points to the alternator/charging system.

4) Smart Alternators and Stop‑Start Nuances

Many 2010s–2025 vehicles use ECU-controlled “smart” alternators and battery management. This can change what “normal” looks like.

• It’s normal for charging voltage to dip into the low 13s, or briefly near 12.8–13.2 V during deceleration/fuel economy strategies.
• After a cold start or heavy electrical load, voltage may rise to ~14.7–14.8 V, then taper down.
• Battery replacement on some cars (AGM/EFB, stop‑start) requires registration/coding so the BMS charges correctly.

If readings are borderline, test under multiple conditions (idle, light throttle, high electrical load) to avoid misdiagnosis on smart systems.

5) Hybrids and EVs (12‑V System Uses a DC‑DC Converter)

Hybrids/EVs have a 12‑V battery but no alternator; a DC‑DC converter charges the 12‑V system when the vehicle is “Ready.”

• With the vehicle in Ready, 12‑V system should read roughly 13.5–14.5 V. If it stays near 12.2–12.6 V, the DC‑DC converter or related fuse/wiring may be at fault.
• Frequent 12‑V failures in hybrids/EVs can be due to short trips, infrequent use, or parasitic draws from accessories.

Treat a hybrid/EV’s 12‑V battery like a conventional one for at-rest testing; just remember charging comes from the DC‑DC converter, not an alternator.

Decision Guide: Battery vs. Alternator

Use this quick decision path once you have basic symptoms and meter readings.

1. Engine off voltage below ~12.2 V after full charge attempt? Battery is discharged or sulfated; load test to confirm.
2. Engine running voltage under ~13.3 V (and not a smart-system decel event), especially under load? Alternator/drive belt/charge wiring issue.
3. Engine running voltage 13.8–14.7 V but hard starts/voltage drops below 9.6 V when cranking? Battery or main cable connection issue.
4. High AC ripple (>100–300 mV) with engine running? Alternator diode failure.
5. Starts and runs only with a jump; dies after jump removed? Alternator not supplying current or belt slipping.
6. Battery passes a load test but goes dead overnight: check for parasitic draw.

Following this sequence typically isolates the fault without guesswork or unnecessary parts replacement.

When It’s Neither: Parasitic Drain and Other Culprits

If the battery keeps dying even though the alternator charges correctly, look for parasitic draws or mechanical issues.

• Parasitic draw test (after modules “sleep,” usually 20–45 minutes): typical draw is ~20–50 mA; more than ~80 mA is excessive in most cars.
• Common sources: glovebox/trunk lights, aftermarket alarms/infotainment, stuck relays, telematics modules.
• Drive-belt issues: a loose or contaminated belt reduces alternator output; inspect tensioners and idlers.
• OBD-II codes like P0620–P0622 can flag generator control faults; BMS codes can indicate sensor/wiring issues.

Addressing drain or belt/tensioner problems can restore normal function without replacing the battery or alternator.

What to Replace, Costs, and Next Steps

Once you’ve identified the faulty part, plan the fix and budget accordingly.

• Battery: $100–$300 for most lead-acid/AGM; more for large AGM/EFB or specialty sizes. Many parts stores will test for free and pro-rate warranties.
• Alternator: $300–$900 installed for many vehicles; premium or tightly packaged models can exceed $1,000. Always check belt/tensioner at the same time.
• Registration/coding: required on many European and newer stop‑start vehicles after battery replacement.
• Good practice: after replacement, verify charging voltage and clear any stored charging system codes.

Confirming results post-repair ensures the issue is truly fixed and helps protect new components from premature failure.

Safety and Good Practices

Electrical work around batteries requires care; follow these tips to stay safe and avoid collateral damage.

• Wear eye protection and gloves; battery acid is corrosive.
• Never disconnect the battery while the engine is running; voltage spikes can damage electronics.
• Connect jumper leads correctly: positive to positive, negative to a good engine/chassis ground.
• Charge AGM/EFB batteries with a compatible smart charger.
• Secure battery hold-downs; vibration shortens battery life.

Taking a few precautions prevents injuries and protects sensitive vehicle electronics during diagnosis and repair.

Summary

Check voltages: a healthy alternator produces roughly 13.8–14.7 V at the battery (smart systems may vary slightly); if running voltage hovers near 12 V, the alternator or its drive/wiring is suspect. If running voltage is normal but cranking is weak and at-rest voltage is low after charging, the battery is likely at fault. Consider smart charging behavior, belt condition, and parasitic drains before replacing parts, and verify repairs by rechecking voltages under load.

Will a car start with a bad alternator?

No, a car cannot reliably start or run for long with a bad alternator because the alternator charges the battery; a failing alternator will drain the battery, eventually causing the car to die. While you might be able to jump-start a car with a dead alternator and run it briefly if the battery is charged, this is only a temporary fix, and the car will quickly fail again as the battery depletes without being recharged. 
Why a Bad Alternator Stops a Car

• Alternator’s Job: Opens in new tabThe alternator generates electricity to power the car’s electrical system and recharge the battery while the engine is running. 
• Battery Drain: Opens in new tabWithout a functioning alternator, the car relies solely on the battery’s charge. 
• Battery Depletion: Opens in new tabOnce the battery’s power is exhausted, it can no longer start the car, or the engine will die shortly after starting. 

Signs of a Failing Alternator

• Battery Warning Light: A lit “battery” or “charging system” light on the dashboard. 
• Dimming Lights: Headlights or interior lights that dim or flicker. 
• Strange Noises: Grinding, squealing, or whining noises from under the hood. 
• Electrical Issues: Problems with other electrical components, such as the radio or wipers. 

What to Do if You Suspect a Bad Alternator

1. Full Charge the Battery: Opens in new tabA fully charged battery might allow the car to start, but this will only be a temporary solution. 
2. Drive to a Service Center: Opens in new tabDrive the car directly to a qualified mechanic, turning off all unnecessary electrical accessories to conserve battery power. 
3. Get a Diagnostic Test: Opens in new tabHave a professional test the alternator to confirm the problem and get it replaced as soon as possible. 

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 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.

How to tell if it’s a bad battery or alternator?

To distinguish between a bad battery and a bad alternator, perform a jump-start. If the car starts with a jump and then dies shortly after disconnecting the cables, the alternator is likely bad. If the car starts with a jump but then refuses to restart on its own later, the battery is probably bad. You can also use a multimeter: a reading of around 12.6 volts with the engine off indicates a good battery, while a reading of 14-14.5 volts with the engine running indicates a working alternator. 
How to Diagnose

1. Check for Dashboard Lights: An illuminated battery or check engine light can signal a problem with either the battery or alternator, so this is a starting point, not a definitive answer. 
2. Test the Battery’s Charge (Engine Off): 
   • Use a multimeter to measure the battery’s voltage. 
   • A good, fully charged battery will read around 12.6 volts. 
   • If it reads significantly lower, the battery may be failing. 
3. Test the Alternator’s Charge (Engine Running): 
   • With the car running, place the multimeter on the battery terminals again. 
   • A healthy alternator will show a voltage between 14 and 14.5 volts. 
   • If the voltage does not increase or stays around 12 volts, the alternator is not properly charging the battery. 
4. Use the Jump-Start Test: 
   • Attempt to jump-start the vehicle. 
   • If the car starts with the cables attached, but then dies soon after they are disconnected, it’s a sign of a bad alternator. 
   • If the car runs on its own after the jump but then fails to start again by itself later, the battery is likely the issue. 

Common Symptoms

• Bad Battery:
  • Engine cranks slowly or makes a clicking sound but doesn’t turn over. 
  • Car is difficult to start, especially in cold weather. 
  • Car only dies after being parked for a long period. 
• Bad Alternator:
  • Dim or flickering headlights and interior lights. 
  • Malfunctioning or underpowered electrical accessories (e.g., radio, power windows). 
  • A growling or squealing sound, especially when electronics are used. 
  • A burning smell, indicating the alternator is overheating.