What voltage is low for car?

In a typical 12-volt car, a low reading is generally below 12.0 volts when the engine is off, and below about 13.8 volts when the engine is running (the alternator should maintain roughly 13.8–14.4 volts).

Resting battery voltage (engine off)

This section outlines the voltage you should expect from the battery when the car is not running, and what those numbers imply about battery health.

• Fully charged resting voltage: 12.6–12.8 V


• Healthy but aging battery: around 12.4–12.6 V


• Discharged or weak battery: below 12.0 V


• Very low or failing battery: around 11.5–11.0 V or lower (starting may fail)

Bottom line: Resting voltages below 12.0 V suggest the battery needs charging or replacement, while readings above 12.6 V indicate a healthy battery when the car is off.

Charging voltage when the engine is running

When the engine is running, the alternator should keep the electrical system charged at a stable level. Keep these targets in mind:

• Normal charging voltage: 13.8–14.4 V (some models tolerate up to about 14.7 V)


• Undercharging: below about 13.0–13.5 V indicates the alternator or wiring may not be charging properly


• Overcharging: 14.8–15.0 V or higher for extended periods can damage the battery and electrical components


• Note: Readings can briefly deviate during startup or under heavy electrical load

Conclusion: A healthy system should maintain roughly 14 volts while running; sustained readings outside the normal range warrant a charging-system check.

What to do if voltage readings are too low

If you see voltage readings outside these ranges, take these steps: check battery terminals for corrosion and tightness, inspect the serpentine belt and alternator pulley, measure charging voltage with a multimeter, and consider a professional load test on the battery and alternator. If the battery is older than 3–5 years, plan for replacement since capacity declines over time.

Summary: Resting voltage around 12.6–12.8 V indicates a healthy battery, while readings below 12.0 V suggest discharge. With the engine running, aim for about 13.8–14.4 V; readings outside this range point to charging-system issues that merit testing or repair.

What is vehicle low voltage?

Low voltage on a car means the battery or electrical system isn't supplying enough power, which can be caused by a failing battery, a faulty alternator, or poor electrical connections. This can lead to symptoms like difficulty starting, dimming lights, or erratic behavior from electronic components, and may trigger dashboard warning lights.
 
Causes of low voltage

• Weak or failing battery: Over time, a car battery's ability to hold a charge diminishes, and a completely discharged battery may not have enough power to start the engine. 
• Alternator problems: If the alternator isn't generating enough voltage to charge the battery while the car is running (typically above 13.5 volts), the system will be low on power. 
• Poor electrical connections: Corrosion on battery terminals, loose wires, or damaged wiring can increase resistance and cause voltage drops throughout the system. 

How to identify low voltage

• Sluggish or failed start: The engine may crank slowly, or the car may not start at all. 
• Dimming lights: Headlights and interior lights may appear dim, especially when the engine is off. 
• Dashboard warning lights: The battery warning light or the check engine light may illuminate. 
• Erratic electrical behavior: Other electronic components like power windows, the radio, or navigation systems may not work correctly. 

What to do about low voltage

1. Check the battery: A simple check with a multimeter can confirm the issue. A fully charged battery should read around 12.612.612.6 volts when off and 13.7−14.713.7 minus 14.713.7−14.7 volts when running. 
2. Inspect connections: Clean the battery terminals and check that all connections are secure. 
3. Test the alternator: With the car running, a voltage reading below 13.513.513.5 volts indicates the alternator is not charging properly. 
4. Replace components if necessary: If the battery won't hold a charge, or the alternator is failing, replace the faulty part. 

How to fix low voltage on car?

To fix a car's voltage drop, first ensure all connections are clean and tight, especially the battery terminals and grounds. If that doesn't work, test the alternator, as a faulty one is a common cause. For persistent issues, upgrading to a larger gauge wire for the "big three" (alternator, negative battery to body, and body to engine grounds) or adding a high-output alternator can resolve voltage drop problems, particularly for high-power systems.
 
This video explains how to identify and fix voltage drops: 1mDiesel LaptopsYouTube · Oct 14, 2024
Step 1: Clean and secure all connections 

• Inspect and clean terminals: Battery terminals, ground straps, and other electrical connectors can become corroded or loose. Disconnect the battery, then clean all terminals to bare metal using a wire brush. Sand the contact surfaces and the bottom of the clamps. 
• Check ground points: Ensure all ground connections are secure and free of rust or paint, as these can impede conductivity. 
• Tighten connections: Make sure all connections are tight. A loose connection, even one that seems hand-tight, can cause a significant voltage drop. 
• Use dielectric grease: Apply dielectric grease to terminals after cleaning to help prevent future corrosion. 

Step 2: Test the alternator 

• Measure charging voltage: With the engine running, a properly functioning car should have a charging voltage between 13.713.713.7 and 14.714.714.7 volts. If the voltage drops significantly (e.g., below 131313V when under load like with a stereo turned up), the alternator may be failing. 
• Inspect alternator grounds: Use a multimeter to test the voltage drop from the alternator's case to its ground point on the engine block. A drop greater than 0.50.50.5 volts when cranking indicates an issue with the ground circuit. 

Step 3: Upgrade components (if necessary) 

• Upgrade wiring: For systems with high power demands (like a car audio system), upgrading to larger gauge wires for the "big three" (alternator to battery, negative battery to body, and body to engine) can significantly reduce resistance and voltage drop. 
• Install a high-output alternator: If the alternator is the main culprit, replacing it with a higher-output version can provide sufficient power. 
• Consider a capacitor: Adding a car audio capacitor can help filter AC ripple and provide a quick burst of power, though its effectiveness can vary. 

This video explains the benefits of upgrading your car's electrical system: 52sTHELIFEOFPRICEYouTube · May 13, 2025

Is 11.7 V enough to start a car?

No, 11.711.711.7 V is likely not enough to start a car, as a healthy battery typically needs to be above 11.811.811.8 V when at rest to start a vehicle. A reading of 11.711.711.7 V suggests the battery is deeply discharged and may struggle to provide the power needed to crank the engine, and a voltage below 11.811.811.8 V indicates a potentially critical state where the battery could be damaged. 
Why 11.711.711.7 V is not enough

• Deeply discharged: A reading of 11.711.711.7 V indicates the battery is significantly discharged and may be on the verge of being unable to start the car. 
• Voltage drop during cranking: The voltage will drop further when you try to start the engine, making it even less likely to start. A healthy battery should not drop below 101010 V during cranking. 
• Potential battery damage: A reading this low suggests the battery is stressed, and frequent deep discharges can compromise its lifespan. 

What to do next

• Charge the battery: Connect the battery to a battery charger to bring it back to a full charge. 
• Jump-start the car: If you need to start the car immediately, you can use a jump-start from another vehicle or a jump starter unit. 
• Check the charging system: After charging, if the battery voltage drops quickly or doesn't hold a charge, have your charging system (alternator) checked. 
• Consider replacement: If the battery continues to show low voltage after a full charge, it may need to be replaced, especially if it's an older battery. 

What voltage is too low for a car battery?

A car battery is too low if it reads below 12.0 volts when the engine is off; this is considered fully discharged and needs to be recharged. A reading between 12.0 and 12.4 volts indicates the battery is partially charged and should be recharged soon, as running it at this level for too long can decrease its lifespan. If the voltage drops to or below 10.5 volts, the battery is essentially scrap and will not be able to start the engine.
 
This video explains what a 12-volt car battery should read when the engine is off: 38sMechanic MindsetYouTube · Mar 11, 2021
Voltage and charge levels (engine off) 

• $12.6$V or higher: Fully charged 
• $12.4$V: Approximately 60% charged 
• $12.0$V to $12.4$V: Partially discharged, needs recharging 
• Below $12.0$V: Fully discharged or "flat" 
• $10.5$V or lower: Critically low, likely damaged and considered "scrap" 

What to do if your battery is low

• Recharge the battery: If the voltage is between 12.0V and 12.4V, a full charge can restore it. 
• Check for problems: If the battery is low again after charging, it could be due to a number of issues: 
  • Alternator problems: The alternator may not be charging the battery correctly. 
  • Parasitic drain: Something in the car might be drawing power even when it's off. 
  • Old age: An old battery is more sensitive to low voltage and may be unable to hold a charge properly. 
• Consider replacement: If the battery voltage consistently falls below 12.0V, or if it shows other signs of failure (like taking multiple attempts to start), it's likely time to replace it. 

You can watch this video to learn how to check your car battery voltage with a multimeter: 1mHSZ GarageYouTube · Aug 22, 2022