How much voltage needed to start car

In this article we discuss the car battery voltage in a ‘traditional’ combustion engine vehicle, none of that new-fangled space age electric car stuff for us (at least not for this guide!).

What voltage should a car battery be?

Image: Car Battery Voltage Chart

The above chart gives an indication of what the voltage should be at different states of charge and operating conditions. This is intended as a guide only as the voltage reading will be effected by the battery age, technology and temperature.

A fully charged car battery voltage should be 12.6v or above, this is with the engine off. To get a true voltage reading of the cars battery it is important to measure the voltage after the car has been sat for a period with the engine off, preferably overnight, this provides you with the “car battery open circuit voltage” or also know as the “resting voltage”. Measuring the voltage shortly after the engine has been running can give you a higher, misleading, reading.

If the voltage is below 12.6v it indicates that the battery isnt fully charged or has a fault. The voltage doesn’t have to differ much from 12.6v for it to indicate a discharged battery. At 12.2v the battery is in effect 50% discharged.

Voltage when the car is starting

When you start your car, the battery provides the necessary power to the starter motor to turn over the engine. During starting the batteries voltage will decrease for a short time, before going up to the running voltage (see below).

During starting a healthy car battery voltage should be 10v or higher. As mentioned above cold weather needs to be considered as this will decrease the voltage and increase the amount of power to turn over a cold engine.

Voltage when the car is running

The voltage, when the engine is running, should be in the range of 13.5-14.7 volts. When your car is running the cars alternator is charging the battery, this is why you should see a higher voltage. If you’re readings are significantly higher or lower than these then it would indicate a fault with the battery and/or the alternator.

Battery charge and water level

Wet cell car batteries lose water during use and charging, these batteries require regular maintenance. The water level will need periodically checking. For further advice see our guide on filling a car battery with water.

Your car battery is made up of 6 2.1v cells connected in series, combined making a nominal 12.6v. However what voltage your battery should be at any given time is dependent on:

Temperature: The ability of a fully charged, fully functioning car battery to deliver the high current needed to the starter motor is reduced by approximately 40% at freezing point. This is why a weak battery is often discovered in winter when you are about to set off to work on a cold frosty morning!

Age: As your battery ages and is used, its ability to hold charge will diminish, with a resulting decrease in the voltage.

State of charge: The voltage of your battery will change as it is charged and discharged. Understanding at what point it is in a charge cycle is important when determining what the voltage should be.

If the cars engine is running or not: The batteries voltage will be higher when the engine is running.

How to measure car battery voltage.

You can check a car battery voltage with a multimeter or buy a car battery monitor. Both are relatively cheap and easy to use. Check out our guide on checking a car battery with a multimeter.

Image: Car Battery Maintenance

Why the voltage of a car battery is important.

Knowing your car batteries voltage is important in understanding the condition of the battery and the likelihood that it could fail.

Your car battery provides your vehicles electrical needs when the engine isn’t running. And importantly it provides the necessary “spark” to get the engine turning over when starting your car. When you start your car the battery provides the high energy (current) needed to the starter motor, which in turn turns over the engine. Once the engine is running the power for your cars electrical systems is provided by the alternator.

The car battery also stabilizes the circuit voltage of the car when the engine is running, protecting the electrical systems from voltage spikes.

Without a fully functioning battery you run the risk of being stranded and unable to start your car.

We advise that you check your car battery a couple of times each year, and in particular towards the end of autumn before winter sets in. It is worth keeping a noted of the voltage readings so you can compare them overtime.