Car Battery Basics

Fully charged voltage

Most car batteries are made of six cells, each consisting of 2.1 volts, 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 known 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 is not fully charged or has a fault.

At 12.3V the battery is 75% charged.

At 12.1V the battery is 50% charged.

At 11.7V the battery is 25% charged.

At 10.0V the battery is 0% charged.

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. During starting a healthy car battery voltage should be 10V or higher.

Voltage when the car is running

The voltage, when the engine is running, should be in the range of 13.5V - 14.7V. 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.

How Long Do Car Batteries Last?

Wet cell car batteries generally last four to five years. The length can be extended or shortened based on temperatures where the battery is exposed to extreme heat or cold and where the climate systems put excess strain on the battery causing more frequent charging.

VRLA car batteries can last between five and ten years depending on plate thickness and materials used.  This also contributes to their higher cost. Temperature extremes may affect the life of these batteries more so than traditional wet cell batteries.

Test a Car Battery with a Multimeter

01) Before being tested the car battery will need to rest for a minimum of 1 hour after it has been driven. Preferably it should be left over night, this allows to test the “resting voltage” to obtain an accurate measurement.

02) Gain access to the battery and expose the battery terminals. On some makes the battery will have a cover over it that needs to be removed.

03) Multimeter comes with a red (positive) and black (negative) lead. Some multimeters used for other testing may have dragon clips or alligator clips. For testing a car battery, the leads are best if they are of the point end probe type.

04) Set the dial on the meter to 20 Volts DC. This will allow accurate measurement in a range of 0-20 DC Volts, more than enough to capture the batteries capacity within that range.

05) Using the red probe, touch the end of the probe to the positive terminal of the battery. This terminal is normally red and marked with a + sign. Use the black probe to touch the probe end to the negative terminal. This terminal is normally black and marked with a – sign.

06) Turn on the headlights so the system has a bit of a load.

07) If the battery at “resting voltage” reads 12.6V, the battery is fully charged.

08) A reading less than 12.1V, it is identified as a bad battery.