How Many Volts Is a Bad Alternator?

With the engine running, a healthy alternator typically holds about 13.8–14.7 volts at the battery. If you consistently see less than roughly 13.0 V (undercharging) or more than about 15.0 V (overcharging), the alternator, its regulator, or related wiring is likely faulty. Note that many late‑model vehicles with smart charging can briefly show ~12.5–13.2 V by design, but sustained low or high readings are cause for concern.

What “Normal” Charging Voltage Looks Like

Automotive alternators convert mechanical energy into electrical power and recharge the battery while the engine runs. Voltage varies with temperature, load, and control strategy, but there are well-established ranges that indicate normal operation.

The following ranges summarize typical, healthy readings you should see at the battery terminals in common scenarios.

• Engine off (battery at rest): 12.4–12.7 V (around 12.6 V is typical for a fully charged flooded lead-acid battery)
• Engine idling, minimal accessories: ~13.8–14.7 V (AGM batteries may sit toward the higher end; cold weather can push the target higher)
• 2,000 rpm with accessories on (lights, blower, defroster): ideally ≥13.5 V and generally ≥13.2 V if the alternator is loaded
• Smart-charging systems (many 2014–2025 vehicles): may momentarily drop to ~12.5–13.2 V to reduce drag; voltage should periodically rise into the mid‑14s to recharge

If your readings align with the ranges above most of the time, the alternator and voltage regulation are likely functioning normally for your vehicle and conditions.

Voltage Readings That Point to a Failing Alternator

While one quick snapshot isn’t definitive, patterns in voltage tell a clear story. Below are common red flags and what they often mean.

This list explains the most common voltage-related signs of alternator trouble and their implications.

• Consistently ≤13.0 V with the engine running and accessories off: undercharging; suspect weak alternator, slipping belt, wiring/ground faults, or a bad regulator
• Voltage droops below ~13.2 V and keeps falling under accessory load: insufficient output capacity or failing diodes/brushes
• Consistently ≥15.0–15.5 V: overcharging; often a failed regulator or control issue that can damage the battery/electronics
• Noticeable headlight pulsing and AC ripple >0.3–0.5 V AC at the battery: likely bad rectifier diodes inside the alternator

Any of the above observed steadily over several minutes—especially after a brief drive to stabilize battery state of charge—warrants further testing or replacement.

How to Test an Alternator with a Multimeter

A simple digital multimeter can quickly distinguish normal charging from alternator problems. The steps below assume a 12‑volt passenger vehicle.

1. Set the meter to DC volts and measure battery voltage engine off: expect ~12.4–12.7 V if the battery is healthy and rested.
2. Start the engine, let idle for 60–120 seconds, then measure at the battery: look for ~13.8–14.7 V.
3. Raise engine speed to ~2,000 rpm: voltage should remain stable, generally in the mid‑14s.
4. Turn on major loads (high beams, rear defroster, blower on high) and hold ~2,000 rpm: voltage should stay ≥13.2–13.5 V.
5. Check for overcharge: if readings sit ≥15.0–15.5 V for more than brief cold‑start intervals, the regulator/control is suspect.
6. Measure AC ripple: switch the meter to AC volts and measure across the battery; anything above ~0.3–0.5 V AC suggests a failed diode.
7. Check voltage drop in cables: measure from alternator B+ stud to battery positive with loads on; drops >0.2–0.3 V indicate cable/connection issues. Do the same from alternator case to battery negative for ground-side drop (>0.1–0.2 V is suspect).

If the alternator passes these tests but problems persist, test the battery with a load tester and scan the vehicle for charging/communication fault codes, especially on smart-charging systems.

Important Notes for Smart-Charging Vehicles

Many newer vehicles use ECU/LIN-controlled alternators to improve efficiency. It’s normal to see voltage hover around 12.5–13.2 V during light load cruising, then climb to ~14.4–14.8 V during regen or recharge events. Evaluate averages over several minutes, not split-second readings. A steady failure to reach the mid‑14s during a drive, or chronic over-15 V behavior, is still abnormal.

Common Symptoms and Causes

Voltage readings rarely appear in isolation. The following symptoms often accompany a failing alternator and can help corroborate your measurements.

• Battery/charging warning light or “Check Charging System” message
• Dim or flickering headlights, electrical accessories cutting out
• Dead battery after short drives, hard starting, or stalling at idle
• Belt squeal, burning smell, or whining/rumbling from the alternator

If these symptoms appear alongside abnormal voltage, focus your diagnosis on the alternator, regulator, belt drive, and wiring.

Alternator problems have multiple root causes. The items below outline frequent culprits to check during diagnosis.

• Worn brushes, bearings, or slip rings inside the alternator
• Failed rectifier diodes causing AC ripple and low output
• Bad internal or external voltage regulator
• Loose, glazed, or contaminated drive belt; failing belt tensioner
• Corroded battery terminals, poor grounds, or high-resistance wiring/fusible links
• Control/communication faults on ECU/LIN-controlled systems

Addressing these underlying issues often restores proper charging voltage without guesswork or unnecessary parts replacement.

Safety and Next Steps

Overcharging can boil electrolyte and damage electronics; undercharging can leave you stranded. If your readings fall outside normal ranges consistently, have the alternator, battery, and wiring professionally load-tested. On vehicles with start‑stop or AGM/EFB batteries, ensure replacements are correctly specified and system settings are reinitialized as required by the manufacturer.

Summary

A good alternator typically holds 13.8–14.7 V at the battery with the engine running. Sustained readings below about 13.0 V indicate undercharging; sustained readings above roughly 15.0 V indicate overcharging—both point to alternator, regulator, or wiring faults. Confirm with a multimeter under various loads and speeds, check AC ripple and cable voltage drops, and account for smart-charging behavior on newer vehicles.

Is 14.7 volts charging my alternator?

The normal voltage range for a properly functioning alternator is generally considered between 13.5 and 14.5 volts as long as there is no additional or major load on it. A voltage of 14.7 volts is slightly above the expected range, but it does not always mean it’s a cause for concern.

How do I check if my alternator is bad?

To check if your alternator is bad, you can perform a simple multimeter test by measuring the battery voltage with the engine off (around 12.5V) and then with the engine running (should be between 13.5V and 14.5V). A failing alternator will show a low or stagnant voltage reading, while a malfunctioning voltage regulator may cause an overcharging situation. Other signs of a bad alternator include dimming or flickering lights, slow engine cranking, warning lights on the dash, or a smell of burning rubber. 
Using a Multimeter (Recommended Method)

1. 1. Prepare the Multimeter: Set your multimeter to measure DC voltage (VDC) and set the range to at least 20 volts. 
2. 2. Check Battery Voltage (Engine Off): With the engine off, connect the multimeter’s leads to the corresponding positive (+) and negative (-) battery terminals. A fully charged battery should read around 12.5 volts. Check for any corrosion on the terminals. 
3. 3. Check Voltage (Engine Running): Start the engine and read the voltage at the battery terminals again. 
   • Good Alternator: The voltage should be higher than with the engine off, typically between 13.5 and 14.5 volts. 
   • Undercharging: If the voltage doesn’t increase, or is below 13 volts, the alternator isn’t charging the battery properly. 
   • Overcharging: If the voltage is significantly higher (e.g., above 14.8 or 15 volts), your alternator’s voltage regulator is malfunctioning and overcharging the battery. 
4. 4. Test Under Load: Turn on the car’s electrical components (like headlights and air conditioning) and check the voltage again. The voltage should remain above 13 volts. 

Other Indicators of a Bad Alternator

• Battery Warning Light: An illuminated battery or charging system light on the dashboard often signals a problem with the alternator. 
• Flickering or Dim Lights: Headlights, interior lights, or dashboard lights may appear dim, or flicker, especially when the engine RPM changes. 
• Engine Problems: Your engine may have trouble starting, crank slowly, or stall frequently because the alternator is not supplying enough power. 
• Unusual Noises: You might hear a whining or screeching noise coming from the front of the car. 
• Burning Smell: A burning smell can indicate a problem with the alternator’s wiring. 

Quick Test (Use with Caution) 

1. Start the Engine: Get your car’s engine running.
2. Disconnect Battery Cable: Carefully disconnect the negative battery cable.
3. Observe: If the engine stalls immediately after disconnecting the cable, the alternator is not generating enough power to run the vehicle by itself and is likely failing. If the engine keeps running, the alternator is producing sufficient power. 

• *Disclaimer: Always be careful when working around a running engine, especially when handling car batteries, and consult a professional if you are unsure about any of these steps.*

What voltage is a bad alternator?

A bad alternator typically causes undercharging (voltage consistently below 13 volts) or overcharging (voltage consistently above 15 volts), though a normal range is generally 13.5-14.5 volts when the engine is running. If the voltage is wildly fluctuating, below 13V, or above 15V, the alternator is likely faulty and requires a bench test for confirmation. 
Signs of a bad alternator based on voltage readings

• Undercharging: If the voltage is below 13 volts, the alternator may not be charging the battery properly. 
• Overcharging: A voltage reading above 14.8-15 volts indicates the alternator is charging too much, which can damage the battery. 
• Fluctuating voltage: Wildly fluctuating voltage is a clear sign of a problem with the alternator or its associated wiring. 

This video shows how to test an alternator using a multimeter: 38sAaron HinesYouTube · Mar 23, 2018
How to test an alternator’s voltage

1. Start the engine . 
2. Use a digital voltmeter: to measure the voltage across the battery terminals. 
3. Observe the voltage reading: with the engine running and accessories off, and then with accessories turned on. 

What to do with the reading

• Normal reading (13.5-14.5 volts): The alternator is likely functioning correctly, and the issue might be elsewhere. 
• Low voltage (below 13 volts): This indicates a charging problem, but the issue could be a bad belt, wiring, or connections, not just the alternator itself. 
• High voltage (above 15 volts): This suggests the voltage regulator in the alternator is bad. 
• Professional testing: For a definitive diagnosis, have the alternator bench tested at an auto shop. 

This video explains how to identify symptoms of a bad alternator: 54sAdvance Auto PartsYouTube · Jun 20, 2022

How to tell if an alternator is draining the battery?

You can tell if a failing alternator is draining your battery by observing symptoms like a battery dashboard light, dim or flickering lights, slow engine crank, frequent stalling, and a dead battery after sitting overnight. For a definitive test, use a multimeter to perform a parasitic draw test, measuring the current drain with the engine off and checking the alternator’s voltage output with the engine running. 
Observe the Symptoms
Pay attention to these signs, which suggest a bad alternator or a parasitic drain from it: 

• Dead Battery After Sitting: The car starts fine initially but is dead again after sitting for a period, according to Ultra Bright Lightz. 
• Dim or Flickering Lights: Headlights, dashboard lights, or other electrical accessories become dim or flicker. 
• Slow Engine Crank or No Start: The engine turns over slowly or fails to start, sometimes accompanied by a clicking sound. 
• Frequent Stalling: The engine stalls while driving, often when the battery warning light is on, indicating insufficient power from the alternator. 
• Dashboard Warning Light: A battery-shaped warning light on the dashboard illuminates. 
• Unusual Noises: You might hear a whining, grinding, or growling noise from under the hood. 
• Burning Smell: A faint smell of burning rubber or an electrical burning smell could indicate a problem. 

Perform a Multimeter Test (for a drain)
A multimeter can test for a parasitic draw when the car is off: 

1. Turn off the car: and all accessories. 
2. Set the multimeter to measure amps: (Amperes). 
3. Disconnect the negative battery cable . 
4. Connect the multimeter in series: between the negative battery post and the disconnected negative cable. 
5. Check the reading: A healthy system should show a low draw (typically under 50 milliamps or 0.05 Amps). A higher reading indicates a drain, and you can then remove fuses one by one to find the component causing the drain. If removing the alternator’s connection causes the draw to drop to zero, the alternator is the source of the drain. 

Check the Alternator While Running
If you can get the car started, a voltmeter can help diagnose a bad alternator: 

1. Start the engine . 
2. Check the voltage: with a voltmeter. The voltage should rise as you increase engine speed. 
3. If the voltage doesn’t increase: and stays low, the alternator is not charging the battery properly. 

Professional Diagnosis
If you’re unsure, a professional mechanic can perform an in-depth test of your battery and charging system, often at no cost.