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How can I tell if my battery is good or bad?

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.
Battery Testing
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.
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