A 12-volt car battery shows different voltages depending on the engine’s state:
- Engine off: When the engine is off, a fully charged battery usually measures 12.6 volts. If the battery reads under 12.4V, it needs recharging to prevent sulfation. A voltage below 12.2V indicates it’s roughly 50% discharged.
- Engine starting: During engine cranking, the voltage might drop, but it should stay above 10 V. A drop below this level signals a weak battery.
- Engine running: The voltage will rise to around 13.5V to 14.7V based on the vehicle model and alternator.
How to test your car battery voltage:
- Allow the car to sit for 15–30 minutes to get an accurate resting voltage.
- Attach the voltmeter’s positive and negative leads to the battery terminals.
- Read the voltage on the display.
You can also get your car batteries tested during routine car maintenance services, like oil changes.
Not sure how to test a battery?
Reach out to professionals like AutoNation Mobile Service. We can come to your location to inspect and replace your car battery.
If you’re interested in learning more about what each voltage stage means, the testing processes, and signs of a bad battery, keep scrolling.
This Article Contains:
- What Should Your Car Battery Voltage Be?
- How to Test Car Battery Voltage with a Tester
- How to Check the Car Battery Without a Tester
- What Are the Signs of a Bad Battery?
- Why Does a Car Battery Lose Voltage?
- How to Prevent Battery-Related Issues: 7 Best Practices
- 4 FAQs about Car Battery Voltage
What Should Your Car Battery Voltage Be?
A standard automotive battery operates on 12 volts of power.
A fully charged car battery should read about 12.6 volts when the engine is off — this is known as its resting voltage. The battery reaches this total through six internal cells, each producing roughly 2.1 volts.
When the engine runs, the alternator supplies power and recharges the battery. In this case, the battery’s voltage should fall between 13.5V and 14.5V. Anything significantly above this range can be excessive voltage.
But what do these voltage readings actually mean?
Measuring the resting voltage can indicate the battery’s state of charge or how much battery charge capacity remains.
In general, for a 12-volt battery, the state of charge is:
- 90% at 12.5 volts
- 80% at 12.42 volts
- 70% at 12.32 volts
- 50% at 12.06 volts
- 20% at 11.58 volts
- 10% at 11.31 volts
- Considered fully discharged at 10.5 minimum voltage
Next, let’s look at a simple test you can perform to check your battery’s voltage.
How to Test Car Battery Voltage with a Tester
To check your car’s battery condition, you’ll need a battery tester, like a voltmeter or multimeter.
The multimeter has two probes for measuring the battery voltage and load: red and black. The red probe is for contact on the positive terminal, and the black probe is for the negative terminal.
Here’s a quick rundown of the steps to measure the battery voltage:
- Turn off the ignition
- Set up the tester to measure DC volts
- Touch the probes to each battery terminal
- Check the voltage with the engine off
- Do a crank cycle test
- Measure voltage with the engine on
It’s best to let a professional handle it for better safety, accuracy, and diagnosis. But if you plan to test your battery yourself, take the following precautions.
Safety Measures
Here are a couple of pointers for safety and accuracy:
- Check the terminals for corrosion, as it can increase internal resistance and decrease the voltage reading. Be sure to clean off any corrosion before starting the test.
- Inspect the battery for leakage, bloating, or damage — don’t work on the battery if the terminals are damaged. Some batteries, such as an AGM battery, come sealed, which minimizes the risk of leaks.
- Avoid smoking around batteries, as sparks can ignite an explosion.
- Wear safety glasses and gloves to protect yourself from battery acid.
NOTE: For a more accurate reading, it’s best to test your vehicle’s electrical system about 12 hours after turning it off. This gives the surface charge time to dissipate, preventing false high readings.
Now, let’s check out the battery test steps in detail.
1. Turn Off the Ignition
Make sure your vehicle’s ignition is off. Then, turn on the headlights for 2 minutes to help remove the surface charge (excess charge).
Turn off the headlights before testing the battery capacity.
2. Set Up the Tester to Measure DC Volts
Set your voltmeter or multimeter setting to test direct current (DC) voltage.
If there’s a DC voltage range, set the voltage range higher than the voltage you’re testing. Since it’s a 12-volt battery, set the multimeter to 20V.
3. Touch the Probes to Each Battery Terminal
Locate the positive terminal (+) and negative terminal (-) on the battery. If a plastic cap covers a terminal, simply lift it off (there’s no need to disconnect the battery cables).
Your battery tester will likely have a red (+) and black (-) probe:
- First, touch the red probe (+) to the positive battery terminal
- Then, touch the black probe (-) to the negative battery terminal
NOTE: If the meter displays a negative number, it means the probes are connected to the wrong battery terminals. To fix it, just swap the probes.
4. Check Voltage with the Engine Off
Healthy car batteries should have a resting voltage of 12.6V.
If the battery’s voltage is less than 12.4 volts, it may simply be a case of low voltage. That’s because some electrical components might have drained it, or the alternator may be struggling to charge it.
If the reading is low, it’ll need recharging. A trickle battery charger can charge it slowly and safely, though it can take a couple of days to reach full capacity.
After recharging, test the battery again to see if it holds the charge.
NOTE: An AGM battery may display a higher voltage, which is around 52V at 100% charge and 42V at 0%. However, these values can vary by manufacturer and battery model. So, always refer to your manufacturer’s datasheet for exact details.
5. Do a Crank Cycle Test
The crank cycle test measures the battery’s ability to deliver voltage to the starter motor.
Have a friend start the car. As the engine cranks, the voltage drops quickly and then rises again, but it should not drop below 9.6V. If it does, the battery doesn’t have enough turnover strength, and you’ll likely need a new battery.
6. Measure Voltage with the Engine On
When the engine is on, your vehicle idles while the alternator actively charges the battery. You should see a voltage reading between 13.5V and 14.5V.
Anything notably higher or lower could signal a problem with the alternator or battery.
But what if you don’t have a voltmeter or multimeter?
How to Check the Car Battery Without a Tester
While you can’t accurately measure voltage without a tester, you can still gauge the car’s battery condition.
Here’s what to do:
- With the engine off, turn on the headlights — they will be your test indicator
- Get a friend to start the car or use a remote starter
- Watch the headlights as the engine cranks
If electronics, like the headlights, dim during cranking, there might not be enough battery charge. However, if the lights remain bright and the engine still won’t start, the issue is likely with the starter motor, not the battery.
Knowing when to test your car battery isn’t just about waiting for a no-start surprise — it’s about catching drama before it happens.
What Are the Signs of a Bad Battery?
Here’s a series of red flags you should look out for:
- Slow engine crank or no start: If the engine turns over sluggishly or won’t start at all, the battery may be too weak to deliver enough power to the starter motor.
- Electrical issues: Dimming headlights, flickering lights, slow power windows, or weak stereo sound often indicate a fading battery. In severe cases, electrical accessories may stop working entirely.
- Rough idling or stalling: Low or inconsistent voltage from a weak battery can cause unstable engine performance, leading to rough idling or unexpected stalling.
- Temperature sensitivity: If your car struggles to start on cold mornings but starts fine later in the day, the battery may be losing its ability to hold a charge in low temperatures.
- There’s an odd smell: A sulfur-like or rotten-egg scent indicates a chemical reaction from leaking acid or internal overheating in a lead-acid battery.
- A misshapen battery: A bloated, cracked, and leaking battery case usually indicates overheating or internal failure.
Next, let’s explore what causes these annoying things to show up.
Why Does a Car Battery Lose Voltage?
Here’s what might be behind a bad battery:
- A battery that’s too old and can no longer hold a charge.
- Parasitic drains, such as lit headlights or bad wiring.
- Insufficient battery recharge. Frequent short trips prevent the charging system from fully recharging the battery, which can gradually weaken it over time.
- Cold and high temperatures tend to limit the battery’s charge-holding capacity.
That said, a bit of regular care can keep these troublemakers out of your drive.
How to Prevent Battery-Related Issues: 7 Best Practices
Use these simple tips to maximize your car battery’s lifespan:
- Always ensure you get the right battery for your car.
- Keep the battery clean to prevent corrosion buildup that can shorten its lifespan.
- For flooded batteries — check electrolyte levels and ensure the fluid covers the lead plates in all six cells.
- Recharge the battery every three months with the right charger to maintain peak performance.
- Test your battery voltage periodically, especially before long trips or during seasonal changes.
- Try not to use electronics when idling, as it puts unnecessary strain on your vehicle’s electrical system.
- Avoid frequent short trips to allow the battery to recharge fully.
Got more questions?
Don’t worry, we’ll address them next.
4 FAQs about Car Battery Voltage
Here are the answers to some battery-related questions:
1. What’s the Difference between a Voltmeter and a Multimeter?
A voltmeter is a simple instrument that measures the electrical voltage or the difference in electric power between two points in a circuit or battery.
On the other hand, a multimeter can measure voltage, current, and resistance, combining the functions of multiple instruments in one. It’s sometimes called a volt-ohm milliammeter (VOM).
It tests whether a car battery has low or excessive voltage. You can use a digital or an analog multimeter.
2. How Often Should You Test Your Car Battery Voltage?
Check your battery voltage and charging system at least twice a year to monitor its health. Regular testing helps you spot early signs of trouble and know when it’s time for a professional check or replacement.
3. When Should You Use a Car Battery Charger?
If your voltmeter reading drops below 12.4 volts, connect the battery to a charger.
A reading under 12.2 volts means the battery is partially discharged and should be recharged soon.
4. What’s a Car Battery Load Test?
This test measures the voltage a fully charged battery can hold while powering a load. Car battery load tests are a more accurate indicator of battery health than voltage measurement.
How’s it done?
During this test, a fully charged battery is loaded with half its cold cranking amp (CCA) rating at 70°F or more. A cold cranking amp refers to the battery’s ability to start in cold temperatures.
A good battery can maintain 9.6V for 15 seconds with this load. If the load tester dips below 9.6V during the load test, it might be time for a battery replacement.
Ensure Smooth Starts with AutoNation Mobile Service
As the electrical components within batteries age, they lose their ability to hold a charge and eventually lose power. Measuring your car battery voltage is an easy way to check your battery’s condition.
And if it turns out you have a weak battery, give AutoNation Mobile Service a call. We’ll replace it with a new battery directly from your driveway. We also offer upfront costs, easy bookings, and a 12-month, 12,000-mile repair warranty.
Pick up your phone and give us a call to resolve your battery issues today.