It is common for a car battery to show a voltage of around 15 volts while the engine is running.
It is due to the alternator in charge of charging the battery creating a higher voltage than the battery’s normal voltage.

Why is my car battery showing 15 volts

The best answer is, a rectifier transforms the alternating current (AC) generated by the alternator when the engine operates into direct current (DC). The rectifier produces a higher voltage than the car battery’s nominal voltage, typically between 13.5 and 15 volts. This higher voltage is necessary to properly charge the battery and maintain the electrical system’s voltage.

Why Is My Car Battery Showing 15 Volts
Why Is My Car Battery Showing 15 Volts

If your car’s battery’s voltage consistently shows 15 volts or higher, it could be a sign that the battery is being overcharged.
This could happen if the rectifier has to be repaired or if the alternator isn’t working properly. A battery can suffer harm and finally fail if it is overcharged.
It is important to have your car’s electrical system checked by a mechanic if you suspect that the battery is being overcharged.

The alternator or rectifier could be overcharging the battery for some causes, including:

Alternator failure:
The alternator is in charge of generating the electrical current that charges the battery. A malfunctioning alternator could produce too much electricity, overcharging the battery.
An engine’s crankshaft is attached to a belt that powers the alternator. The alternator may break down if the belt is frayed or broken.
The alternator’s brushes, which transfer the electrical current to the rotor, can also wear out over time and cause the alternator to malfunction.

Faulty rectifier:
The rectifier is a component that converts the alternator’s alternating current (AC) into direct current (DC), which is what the battery uses.
If the rectifier is faulty, it can produce too much voltage, causing the battery to be overcharged.
Normally, diodes make up the rectifier, allowing only current flow in one direction. These diodes can cause the rectifier to malfunction if one or more of them fail.

Wiring issues:
It has some problem with the wiring in the electrical system that is causing the battery to be overcharged.
Which connects the alternator to the battery, and the rest of the electrical system can become damaged or corrupted over time.
This can cause the alternator to produce too much voltage, resulting in the battery being overcharged.
Overcharging a battery can lead to damage and eventual failure, so fixing these problems as soon as possible is crucial. A mechanic can detect the issue and make the necessary repairs.
If you are experiencing issues with your car battery, there are a few other things you can check to try and determine the cause:

Examine the battery cables:
Check the battery wires to ensure they are securely connected to the battery terminals. The battery may be unable to charge correctly due to loose or damaged cables.
To check the battery cables, remove any dirt or debris covering the battery. Next, use a tool to unscrew the nuts that hold the cables to the battery terminals.
Look closely for any evident damage, such as fraying or corrosion, on the cables. Any broken cables must be replaced if you discover them.

Check the connections between the battery terminals:
Check to see if the battery terminal connections are corrosion-free and spotless. Corroded connections may make it difficult for the battery to charge correctly. Remove any dirt or debris from the battery terminal connectors before cleaning them.

Is a car battery able to handle 15 volts?

The voltage of a fully charged automobile battery should be around 12.6 volts. A voltage of 15 volts is relatively high and could indicate a problem with the battery or the charging system.
The alternator or voltage regulator is not functioning correctly and overcharges the battery.
The battery and other elements of the charging mechanism may sustain damage as a result. It is recommended to have the battery and charging system checked by a mechanic if the voltage is consistently high.

Are 15 volts too high for an alternator?

It is generally not normal for the alternator in a vehicle to produce a voltage of 15 volts. The typical range for the output voltage of an alternator is between 13.5 and 14.5 volts.
If the alternator is producing a voltage of 15 volts or higher, it could cause damage to the battery and other electrical components in the vehicle.

If you see excessive voltage levels, it’s crucial to get the alternator inspected by a mechanic.
The cause of the high voltage could be a malfunctioning alternator or a problem with the charging system in the vehicle.

Is my auto battery overcharged?

There are various indicators that your automobile battery is overcharging:

Dimming headlights:
If the headlights on your vehicle appear dim or are not as bright as usual, it could be a sign of an overcharging battery.

Swelling battery:
An overcharging battery may become swollen or balloon-like in appearance.

Excessive heat:
An overcharged battery may generate excess heat, which could cause the battery to become hot to the touch.

Shortened battery life:
An overcharged battery may last as short as a properly charged battery.

Electrical issues:
An overcharging battery may cause issues with other electrical parts in your cars, such as the alternator or starter.

Does high voltage affect battery life?

High voltage can affect battery life, depending on the type of battery and how it is used.
Some types of batteries, such as lead-acid batteries, can be damaged by overcharging, which can occur if the voltage applied to the battery is too high. This can lead to shortened battery life and reduced capacity.
On the other hand, some batteries, such as lithium-ion batteries, are designed to operate at a specific voltage range and may not be damaged by high voltage.
However, using a battery at a higher voltage than designed can affect its performance and lifespan, although this may vary depending on the specific battery and how it is used.
It is generally best to use batteries within the recommended voltage range for optimal performance and longevity.


In conclusion, it is normal for a car’s battery to show a voltage of around 12.6 to 12.8 volts when it is not being used. If the battery is showing a voltage of 15 volts, it could be an indication that there is a problem with the charging system in the vehicle.

Potential causes of high battery voltage include a faulty alternator, a wiring issue, a failed voltage regulator, or other issues. It is important to have the charging system checked by a mechanic to determine the cause of the high battery voltage and address any problems that may exist in order to prevent damage to the battery and other components in the vehicle.

Sustained exposure to high voltage can affect the lifespan of a car battery, so it is important to ensure that the battery is not being exposed to excessively high voltage levels.

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