The condition of the cell plates inside the battery determines whether or not a battery is still serviceable. Current is produced when sulfuric acid in the battery reacts with lead in the cell plates. As the battery discharges, sulfate accumulates on the plates and reduces the battery's ability to make current. The sulfate is returned to solution when the alternator recharges the battery by forcing current to flow in the opposite direction.
Over time, some of the sulfate becomes permanently attached to the plates. The sulfate forms a barrier that diminishes the battery's ability to produce and store electricity. This process can be accelerated if the battery is run down frequently or is allowed to remain in a discharged state for more than a few days. If the plates have become sulfated, therefore, the battery won't accept a charge and will have to be replaced.
Average battery life is only about four to five years under the best of circumstances -- and sometimes as short as two to three years in extremely hot climates such as Arizona and New Mexico. But the battery may become "sulfated" prematurely if it is chronically undercharged (charging problems or frequent short-trip driving), or if the water level inside the battery drops below the top of the cell plates as a result of hot weather or overcharging and allows the cell plates to dry out.
This is something you can't really do yourself, so you need to take your vehicle to a service facility that has the proper test equipment. The battery's condition can be determined one of two ways: with a carbon pile "load test" (that applies a calibrated load to the battery) or electronically with a special tester that measures the battery's internal resistance.
Equipment that uses a carbon pile for load testing requires the battery to be at least 75% charged. If the battery is less than 75% charged, a good battery may fail the test. So the state of charge must be checked first, and the battery recharged if it is low prior to testing. NOTE: The battery does NOT have to be fully charged prior to testing if an electronic tester that measures internal resistance is being used.
If load testing with a carbon pile, apply a load that is equal to half the battery's cold cranking amps (CCA) rating. A good battery should be able to supply half its CCA rating for fifteen seconds without dropping below 9.5 volts.