If you have started to notice some issues with your vehicle and suspect your battery might be at fault, how do you diagnose it? Signs of a failing car battery can include trouble with the electronics, dimmed headlights and, of course, being unable to start your car.
If you want to check whether the issue is down to your car battery, there are a couple of tests you can run to help diagnose this. The most important one is simply to check if your car battery is fully charged. You can also read about which type of marine battery is best for boats.
How to Measure a 12 Volt Battery Voltage at Rest
To measure the voltage on your car battery you will need to use a tool called a multimeter. These are relatively inexpensive and can measure voltages, resistance and current on electrical items.
First, you’ll need to find your car battery, these are often in the engine compartment of your vehicle, but on some models, they can be found in the boot.
To connect the multimeter to your car battery you’ll need to first disconnect the terminals from the battery and connect the wires from the multimeter. To do this, remove the positive terminal cover first and connect the positive lead from your multimeter to the battery. Once this is done, you can remove the negative terminal cover and connect the negative lead from your multimeter to the battery.
After you have your multimeter connected you’ll need to turn it on and check the reading.
What Voltage Should My 12v Battery Show?
A fully charged 12 volt battery should read at 12.6 volts on the multimeter. If the reading is lower than this, you’ll want to charge your battery with either a battery charger or by taking your car for a drive.
If your voltage is lower than 12.6 volts by a small margin, this can have a big impact on the car’s performance. For example, at 12.4 volts, your battery will be at roughly 75% charge. At 12.2 volts it will be at 50% charge and at 12 volts it will be at roughly 25% charge.
If your battery is reading around 25% charge, this is a very low reading which will usually result in your car not starting.
What Voltage Should My 12v Battery Show When the Car Is Running?
It’s important to be aware that once a car is running, the voltage you will see from the battery will be higher than when at rest. This will typically read somewhere between 13.7 volts to 14.7 volts depending on the battery.
This is why you should check the battery reading while the car is at rest for the most accurate 12 volt battery reading.
You will also notice that when you initially start the vehicle, the voltage will drop below its initial reading. This is called a cranking test and is a good test to measure the effectiveness of your car battery. A healthy battery should maintain between 9 volts and 10 volts when trying to crank the engine. If your multimeter reads numbers lower than this, you might have a weak battery that could need replacing.