How many volts should be coming out of a car battery?

In most passenger cars, a healthy 12-volt battery should read about 12.6 volts when the engine is off, and the charging system should bring the voltage up to roughly 13.8 to 14.4 volts when the engine is running. During a hard start, the voltage can dip to around 9–10 volts briefly; readings outside these ranges may indicate a weak battery, a faulty alternator, or loose/corroded connections.

Baseline: what to expect when the engine is off

Here are typical resting voltages for a healthy 12V lead-acid battery, which reflect its state of charge in normal conditions (values are approximate and can vary slightly by battery type and age).

• Fully charged: about 12.6 to 12.8 volts


• Approximately 75% charged: around 12.4–12.6 volts


• Around 50% charged: around 12.2 volts


• Around 25% charged: around 12.0 volts


• Discharged or near empty: below about 12.0 volts

Concluding: Resting voltage gives a good snapshot of charge level. If the reading is significantly below 12.6 V after a proper rest, charging or a battery condition check is warranted.

While the engine is running: the alternator’s charging voltage

When the engine is running, the alternator charges the battery. The voltage at the battery terminals should stay in a narrow, higher range to maintain and restore charge.

• Normal charging voltage: typically 13.8–14.4 volts


• Cold weather adjustments: may rise toward 14.0–14.6 volts to compensate for slowed chemical reactions


• High-end or specialized systems: some setups may briefly approach 14.8 volts under certain conditions

Concluding: If the running voltage is consistently outside the 13.8–14.4V band (or noticeably above 14.8V), it could indicate a faulty voltage regulator, alternator, or battery condition that needs diagnostic attention.

During starting or heavy electrical load

Cranking draws a surge of current from the battery. Voltage tends to sag briefly, which is normal, but extreme or prolonged drops signal problems.

• During cranking: voltage may dip to about 9–11 volts at the battery terminals


• If voltage stays below ~10 volts for more than a second or two, the battery may be too weak or there could be poor connections


• After the engine starts, voltage should recover quickly to the normal charging range

Concluding: Short, temporary dips are expected, but persistent low readings suggest battery, wiring, or alternator issues that should be checked promptly.

Special cases: 24V systems and battery technologies

Some vehicles use a 24-volt electrical system (common in heavy-duty trucks and certain military or industrial applications) or specialized battery types (AGM, EFB) that behave a bit differently from flooded lead-acid batteries. Here’s what to know.

• 24V systems: total charging voltage is typically about 27–28 volts for the two-battery string, which means each battery ~13.5–14.0 volts


• AGM and EFB batteries: resting voltage for a healthy battery is often around 12.7–13.0 volts; charging voltage remains near 14.1–14.7 volts depending on system controls and temperature compensation


• Temperature compensation: charging voltage can increase in cold weather to ensure proper charging and avoid sulfation

Concluding: For 24V systems or nonstandard battery types, consult the vehicle’s manual or a qualified technician for exact targets. The core principle remains: measure with engine off and with engine running to verify proper charging behavior.

How to test your car battery voltage at home

Using a digital multimeter is a simple way to verify battery health. Here are the practical steps and what the readings mean.

• With the engine off, let the battery rest for at least an hour, then measure across the terminals (DC setting, 20V scale)


• Expected resting reading: about 12.6 volts for a healthy 12V battery


• With the engine running, measure again to check charging: expected 13.8–14.4 volts


• If readings are consistently low or high outside these ranges, recharge or have the battery/charging system tested by a professional

Concluding: Regular at-home voltage checks can catch issues early and help determine whether you need a recharge, a battery replacement, or a charging-system service.

Summary

For a typical 12V car battery, expect about 12.6 volts when the engine is off and roughly 13.8–14.4 volts when the engine is running and the alternator is charging. A brief dip to 9–10 volts during starting is normal for a healthy battery, but persistent deviations point to possible battery, alternator, or wiring problems. In vehicles with 24V systems or advanced battery types (AGM/EFB), voltages follow the same principles but per-battery or per-string targets differ; always consult the owner’s manual or a qualified technician for exact specifications.

Is 11.7 volts a dead battery?

Yes, 11.7 volts is considered a deeply discharged or weak battery that is unlikely to start a car and may need replacement. While a fully charged 12-volt battery should read around 12.6 to 12.8 volts, a 11.7-volt reading suggests it's only about 25% charged. A reading below 11.8 volts indicates the battery is deeply discharged and may have a damaged cell or be unable to hold a charge. 
What you should do

• Recharge the battery: A jump start or a trickle charge may provide enough power to start the car, allowing the alternator to recharge it. However, if the battery was fully drained, it may not be able to recover. 
• Check the charging system: If the battery doesn't charge properly, there could be an issue with the alternator, which should be checked. 
• Load test the battery: The best way to confirm the battery's health is to have it load tested, as a battery at 11.7 volts could indicate a failed cell or a battery that is simply too old to hold a charge. 
• Consider replacement: If the battery repeatedly drops to this level or fails a load test, it is time to replace it. 

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

A bad alternator is often indicated by flickering lights, slow-moving electrical accessories, odd sounds like whining or grinding, and a burning smell. In contrast, a bad battery is primarily associated with a slow engine crank, frequent need for a jump-start, clicking sounds when trying to start, and corrosion on the terminals. 
You can watch this video to learn how to identify the sound of a failing alternator: 29sGPVlogYouTube · Sep 19, 2015
Bad alternator

• Lights: Dim or flickering headlights and dashboard lights. 
• Electrical issues: Slow power windows, malfunctioning radio, or other electrical accessories. 
• Sounds: Whining, grinding, or squealing noises from the engine bay. 
• Smells: A burning rubber or wire smell. 
• Stalling: The engine stalls while you are driving because it's not getting enough power to run. 
• Warning light: The battery or charging system warning light may come on. 

Bad battery

• Starting: Slow or sluggish engine crank. 
• Clicking sounds: You may hear a clicking sound when you turn the key. 
• Corrosion: You may see a powdery, blue-green substance around the battery terminals. 
• Swollen case: The battery case appears swollen or bloated. 
• Frequent jump-starts: The car needs to be jump-started often. 

How to differentiate with a jump start

• If you jump-start the car and it runs fine until the next time you turn it off, you likely have a bad alternator because it wasn't recharging the battery. 
• If the car dies again immediately after you remove the jumper cables or stalls shortly after, it's more likely a bad battery. 

What voltage is too low for a car battery?

A car battery is too low when its voltage is consistently below 12.4 volts at rest; a fully discharged "flat" battery is at or below 12.0 volts, and a reading of 11.8 volts or lower often indicates a dead battery that cannot start the car. A battery should read about 12.8 volts when fully charged and is considered healthy between 12.4 and 12.8 volts.
 
Resting car battery voltage

• 12.8V and above: Fully charged
• 12.5V: Healthy, but monitor for drops
• 12.1V - 12.4V: Partially discharged and needs recharging
• 12.0V or below: Fully discharged ("flat")
• 11.8V and below: Likely a dead battery that cannot start the car 

When the engine is running

• A healthy car battery should have a voltage between 13.4 and 14.8 volts when the engine is running, as the alternator charges the battery. 
• If the voltage is consistently below 14V with the engine running, it could indicate a problem with the alternator, not just the battery. 

What to do

• If your battery's resting voltage is below 12.4 volts, you should recharge it with a battery charger. 
• If the battery repeatedly drops below 12.4 volts, have it tested at an auto parts store, as it may be old and nearing the end of its lifespan. 
• If the battery voltage is still low even after charging or the voltage is low while the car is running, have the charging system (alternator) checked. 

What voltage shows a bad car battery?

A bad car battery can have resting voltage below 12.612.612.6V (fully charged), drop below 101010V when cranking, or show an unusual voltage reading when the engine is running (either too low to be charged by the alternator or too high, damaging electronics). A healthy battery typically rests at 12.6−12.812.6 minus 12.812.6−12.8V, drops no more than two volts when starting, and reads 13.8−14.513.8 minus 14.513.8−14.5V with the engine running. 
You can watch this video to learn how to test your car battery with a multimeter: 32sLowe's Home ImprovementYouTube · Nov 1, 2023
Resting voltage (engine off) 

• Healthy: 12.612.612.6V to 12.812.812.8V
• Bad/Weak: Below 12.412.412.4V
• Very bad: Below 12.212.212.2V 

Voltage during cranking (engine off) 

• Healthy: Should not drop below 101010V
• Bad: Drops below 9.59.59.5V 

Voltage when the engine is running 

• Healthy: 13.813.813.8V to 14.514.514.5V 
• Bad (Alternator issue): The voltage is too low, meaning the alternator is not charging the battery properly. If it's below 12.412.412.4V while running, the alternator is not working correctly. 
• Bad (Overcharging): Anything above 161616V can damage the car's electronics, indicating an alternator or voltage regulator issue. 

Other signs of a bad battery

• Dimming headlights or flickering interior lights
• Accessories like power windows or seats are slow to operate
• "Check Engine" or "Charging" lights are illuminated on the dashboard
• Difficulty starting the car
• Unusual noises like grinding or whining when the car is starting