What is a bad voltage for a car battery?
A bad voltage means the battery isn’t charging properly or isn’t able to hold a charge. For most 12‑volt car batteries, resting voltage should be about 12.6–12.8 volts, and when the engine is running the alternator should deliver roughly 13.8–14.8 volts. Readings outside these ranges point to a potential problem with the battery or charging system.
Understanding normal voltage ranges
To interpret voltage readings, it helps to separate resting voltage (engine off) from charging voltage (engine running) and to account for battery age and temperature.
- 12.6–12.8 volts: healthy, fully charged resting voltage.
- 12.4–12.6 volts: about 75% charged.
- 12.2–12.4 volts: roughly 50% charged.
- 12.0–12.2 volts: around 25–50% charged or approaching discharged.
- Below 12.0 volts: battery is discharged; may indicate a failing battery or one that needs a professional recharge.
Understanding these resting ranges helps you gauge how much charge the battery is holding when the car is not running.
Charging voltage and what it means
When the engine is running, the alternator charges the battery at a higher voltage. The commonly recommended range for most vehicles is 13.8–14.8 volts. Readings outside this band can stress the battery and suggest a problem with the charging system.
- 13.8–14.8 volts: normal charging range for a healthy system.
- Above 15.0 volts: potential overcharging, which can gas the battery and damage electronics; indicate a faulty voltage regulator or alternator.
- Below 13.0 volts: undercharging, meaning the battery may not be getting enough charge from the alternator.
- If the engine runs and the voltage remains at around 12–13 volts, the charging system is not functioning properly and requires inspection.
Maintaining the proper charging voltage helps prevent deep discharges and extends battery life.
Red flags and action steps
If you notice unusual voltage readings, take these steps to diagnose and address the issue.
- Engine cranks slowly or fails to start, and lights are dim when starting.
- Voltage readings are consistently outside the normal ranges (resting below ~12.0 V or running above ~15 V).
- Battery terminals are corroded, loose, or visibly damaged, or the battery case is swollen.
- The vehicle requires frequent jump-starts or a battery replacement within a short period.
Addressing these signs promptly can prevent a sudden breakdown and protect the vehicle’s electrical system.
Battery replacement vs recharge
In many cases a battery can be recharged and may test as healthy, especially if it has a good voltage after charging and passes a load test. However, a battery that repeatedly shows low resting voltage, fails to hold a charge, or fails a load test is usually at the end of its useful life and should be replaced. A faulty charging system can also force premature battery failure, in which case both the battery and the alternator or regulator may need service.
Summary
A car battery shown with bad voltage typically signals either an undercharged or overcharged condition. Resting voltage should be about 12.6–12.8 V for a healthy battery, while charging voltage with the engine running should be roughly 13.8–14.8 V. Readings outside these ranges, voltage drops under load, or signs like slow starts, dim lights, or corroded terminals require testing and likely maintenance or replacement of the battery or charging system. Regular checks help prevent unexpected failures on the road.
How to tell if a car battery is bad voltage?
And the positive probe to the positive terminal. The resting voltage of the battery should be no lower than 12.6. Vol. Now have someone start the car while you keep the probes on the 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
At what voltage should I replace my car battery?
You should consider replacing your car battery when its voltage drops to 12.2 volts or lower with the engine off, especially if it's consistently below 12.6 volts and doesn't charge well. A healthy battery should read at least 12.6 volts when at rest. Another key indicator is if the voltage drops below 9.6 volts while the starter is cranking, as this signals it is failing to deliver enough power.
This video demonstrates how to check your car battery's voltage and what the readings mean: 58sChrisFixYouTube · May 6, 2021
Checking your battery voltage
- Engine off: A healthy, fully charged battery should read between 12.4 and 12.6 volts. If it's below 12.2 volts, it may be failing or needs to be charged.
- Engine running: With the engine on, a healthy charging system should show a voltage between 13.7 and 14.7 volts, indicating the alternator is properly charging the battery.
Signs you need to replace your battery
- The voltage is consistently below 12.6 volts when the engine is off.
- The voltage drops below 9.6 volts when the engine is cranking.
- Your car is slow to start or struggles to start.
- Your car's electric components, like headlights, have low power.
- The battery warning light on your dashboard is on.
- The battery is more than five years old.
You can watch this video to learn about the signs that indicate your car battery needs to be replaced: 1mHonest Mechanic ColoradoYouTube · Dec 26, 2023
Is 12.2 volts a dead battery?
No, 12.2 volts is not necessarily a dead battery, but it is low and indicates the battery is partially charged (around 50–60%) and may struggle to start your car. A fully charged battery should read about 12.6 volts or higher when the engine is off. You should recharge the battery to prevent damage and shorten its lifespan.
You can watch this video to learn about checking your car battery's voltage: 39sTorqueCarsYouTube · Oct 31, 2023
Understanding your battery's voltage
- Fully charged: A healthy, fully charged battery will show a resting voltage of about 12.6 volts or higher.
- Partially charged: A reading of 12.2 volts suggests the battery is around 50-60% charged and should be recharged. Running it at this low level for extended periods can reduce its lifespan.
- Discharged or failing: A reading below 12.0 volts indicates a discharged or failing battery, and a reading of 10.5 volts is considered fully discharged.
What to do next
- Recharge the battery: Drive your car for about 30 minutes, or use a battery charger, to bring the voltage back up.
- Monitor the voltage: After charging, test the battery again. If the voltage is still low, or if it fails to hold a charge, the battery may need to be replaced.
