Why Is My Car Battery Showing 15 Volts?

What if something you see makes you think a lot about it? Most car batteries are 12 volts, with a normal voltage range for the engine of 12.2-12.6 volts. Similarly, when the engine runs, the voltage remains between 13.7 and 14.7 volts. But it will be worrisome if you see the battery shows different volts.

So, why is my car battery showing 15 volts? If it appears, it may happen due to overcharging, defective alternators, incorrect charger settings, extreme heat, and loose connections. Thus, high voltage can lead to various problems, including electrical system damage, reduced battery life, and even battery failure.

In this discussion, I will explain the reasons elaboratively with their troubleshooting. I will also add preventive tips and further relevant information. So, do not freak out yet, after getting the wrong readings of 15 volts.

Normal Car Battery Voltage: What You Must Know

For sure, a car battery is essential to provide the necessary power to start the engine. It sends a high current to the starter motor. After that, the current turns the engine over. This component can also power all the electrical systems in your car. For instance, you can turn on the lights, listen to the radio, and control the computer system.

It is important to know the normal car battery voltage. You may also call it Resting voltage. Well, these measurements are for a healthy battery. All these figures may vary slightly for your battery’s age, temperature, and state of charge.

“Normal” refers to the voltage range for a car battery when you keep the engine off. I have found its value remains within 12.2-12.6 volts, yet some say up to 12.8 volts (1,2,3,4). Again, once you turn on the engine, the volt reading will be higher. It will be within 13.7-14.7 volts (3,5,6).

Suppose you are driving, and battery’s charge drops to 75%. Then, the voltage can decrease to around 12.4 volts. In the same way, if the charge goes down to 25%, the voltage will measure approximately 12 volts (3, 5). It may indicate that you need to recharge the battery now.

Remember, an unhealthy battery or incorrect readings of battery voltage can make things terrible. If it is too low, it may not work to turn on the engine. If the battery’s voltage is too high, it could damage the starter motor. As I am discussing the 15 volts issue, it is too high for your car’s battery. It is not good for a 12-volt battery.

Causes of Car Battery Showing 15 Volts

A 15-volt reading is not always a warning but needs to be investigated. After all, it may indicate some mishaps you need to consider keeping the battery healthy. Likewise, high voltage (like 15 volts) in car batteries can be caused by several factors.

Overcharging the Battery

Overcharging is a common issue where the battery’s voltage exceeds the recommended levels. It leads to excessive heat and potential damage to the battery and the vehicle’s electrical system (7, 8). It can reduce battery life and sometimes cause battery failure.

Faulty Alternator

It works like a powerhouse to the battery, generating electricity and regulating voltage. Also, an alternator charges the battery while the engine is running. But what about a defective alternator? It can send the charging system into overdrive and push the voltage beyond the healthy range (9).

Loose Connections

Are you sure that all the battery connections are secured properly? This is because loose connections can also cause high voltage in car batteries. It can disrupt the regular flow of electricity. As a result, there will be issues like improper charging or battery discharging (1, 8).

Faulty Voltage Regulator

Every car has a voltage regulator. It ensures that the battery is supplied with the right amount of voltage. But when it has issues, it may cause overvoltage (10).

Battery Charger & Its Settings

Have you used a battery charger recently? Then, you may need to double-check its settings. An incorrect voltage setting or the wrong charger can lead to overcharging (11). Also, you will get a high voltage reading and your car will not move smoothly.

Extreme Temperatures

Is it summer or winter? Both timings can impact the battery voltage. For instance, extreme heat can cause water evaporation within the battery. It will cause a higher voltage reading. Alternatively, severe cold can slow down chemical reactions within the battery. It can make the battery appear undercharged.

Other Factors

Other reasons for high voltage (15-volts) are heavy accessory power when you drive short distances and internal short in alternator windings.

Sometimes “parasitic drain” be a minor reason for the problem. It means that accessories like interior lights or aftermarket gadgets may still use the battery power of a parked vehicle. It puts stress on the battery. Hence, you may see the spike of voltage readings during charging.

How to Diagnose a Car Battery Showing 15 Volts

If you want to diagnose the issue, you must put on gloves and safety gear first. It will protect your body from electric shocks. Then, you can use a multimeter or battery tester to check the battery’s condition.

If you choose a multimeter, you can measure electrical properties, including voltage. To use it, connect the multimeter to the battery terminals. In this case, you should remember that sensor color is important. The reading you get will tell you the voltage of your car battery.

Battery testers are also a good way to check the health of car batteries. You can use these devices more easily than multimeters. Also, they are better at giving you more accurate results.

Troubleshooting Steps:

If you get 15-volts readings on meter, you can follow these steps to troubleshoot the problem.

  • The first thing I suggest is to note the circumstances under which the 15-volt reading occurred. Did it happen just after starting, or has it been persistent? 
  • Then, you can open the car hood and give your battery and surrounding wiring a thorough once-over. At this point, you should look for any loose connections, corrosion on terminals, or damaged wires. Sometimes, a simple cleaning or tightening can solve the problem.
  • Now, measure the voltage at the battery terminals with the engine off and on. Compare the readings to the healthy ranges mentioned earlier. Note, if the voltage stays above 14.5 volts even after the engine warms up, there may be a deeper issue.
  • Replace the faulty parts, secure the connections, and try to maintain the proper settings for battery charger.

You can seek professional advice if all the above steps do not work. They may perform more advanced diagnostics and fix the underlying problem.

Preventive Measures: Keep Your Battery Voltage Normal

You can prevent the car battery from showing 15 volts by following ways. They will prevent the components from further damage.

  • You should carefully handle the battery and avoid bringing metal into contact. Metal conducts electricity and causes electrocution (12).
  • Always try to clean your battery terminals with a wire brush and corrosion remover. This is because dirt and grime can hinder proper connection and affect voltage.
  • For lead-acid batteries, periodically check and top up the distilled water level. In this way, you can ensure proper electrolyte levels. Dehydration can cause battery overcharging and voltage spikes.
  • Suppose you won’t be using your car for an extended period. Then, consider disconnecting the negative terminal or investing in a battery maintainer. It will prevent the parasitic drain I have talked about earlier.
  • Avoid using the wrong charger for your battery. Instead, choose the charger that matches the battery’s type and voltage requirements.
  • Do you charge your battery frequently without maintaining charging cycles? Avoid unnecessary deep discharges and frequent charging cycles. It will prevent fluctuations in voltage.
  • Try to check the battery voltage regularly. Early detection of any abnormal readings allows for timely solutions before problems arise.
  • If your car has dimming headlights, slow engine cranks, or ex unusual electrical behavior, then consider whether there are battery or charging issues.


As I said, a car battery showing 15 volts is not normal. It may suggest different issues going on with the battery. Throughout the blog, I have explained why you see misreading on the dashboard. If there are parts issues, replace them soon.

You should also maintain battery health with regular maintenance. You may call the mechanic if things get worse or tough to solve.

I hope you got the answer to “Why is my car battery showing 15 volts?”

Thanks for reading!

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