There are few things more frustrating than discovering that your car won’t start because the battery is dead. A mechanic can measure the voltage in the battery, in order to determine whether it needs to be replaced. There are two ways to do this.
Open circuit voltage test
This car battery test determines how fully charged your battery is. This test is done twelve hours after shutting off your car, so that any surface charge can dissipate. This means a more accurate reading. The test is done with the engine off. Then your mechanic may either use a multimeter/voltmeter, or a dedicated battery tester. The dedicated tester is simply connected to the battery terminals and the voltage can be read. With the multimeter, the device is configured to measure voltage up to 20 volts. Then the leads are connected to the battery terminals, positive to positive and negative to negative to get the voltage.
A reading of 12.65-12.77 volts means your battery has a full charge. 12.45-12.54 volts means you have a 75% charge, 12.24-12.29 is 50% charged, and 11.99-12.06 volts is 25% charged. 11.75-11.89 volts means your battery is dead.
Electrolyte gravity voltage test
This is the method used for non-sealed lead acid batteries. With this method, a hydrometer measures battery voltage. A specific gravity of 1.269 means that the battery is fully charged. A reading of 1.229 indicates a 75% charge, 1.194 is a 50% charge, and 1.159 is 25% charged. 1.124 means you have a dead battery.
Performance and capacity load test
This type of car battery test measures the ability of the battery to produce current. Before you do this test, YourMechanic will test the car battery to be sure that it has at least a 75% charge. Then, the voltage of the battery is measured under a load similar to what it takes to start your car. Then, he or she will use a dedicated load tester – it’s a large resistor, with a resistance much like your car’s starter, and a meter that shows the output voltage – to apply a load equivalent to ½ of the cold cranking ampere rating of the battery for approximately 15 seconds. You should expect a reading no lower than 9.1 volts.
Have your car battery tested regularly. You should have your car battery tested at least once a year, preferably when the cold weather begins.
This article originally appeared on YourMechanic.com as How to Check the Voltage of a Car Battery and was authored by Valerie Johnston.