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, Your Mechanic 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.