Lesson 2/Learning Event 1
TESTING AUTOMOTIVE BATTERIES
Through use and age, the capacity of a battery decreases. This is due to some of the active
material dropping off the plates and the build-up of a hard coating of sulfate on the plates. Because
of this, testing the gravity of the battery with a hydrometer may not always reveal the true
condition of the battery. The electrolyte may be strong enough, but the battery capacity may be
reduced to a point where the available current supply is too small. This condition can be detected
by making a high-rate discharge test of the battery.
Do not perform a high-rate discharge test when the specific gravity of the electrolyte is 1.225 or less
or if the variation of specific gravity between cells is more than 25 points. After charging, the
battery should be allowed to sit for several hours before a high-rate discharge test.
High-rate discharge testing instruments vary a great deal. Always perform a high-rate discharge test
according to instructions for the test instrument you are using. In use, the test instrument draws a
high current from the battery and the battery voltage is read on a voltmeter during the discharge.
The correct amount of voltage will vary depending on the test instrument used.
If a high-rate discharge tester is not available, batteries can still be tested if they are installed in the
vehicle. This is done by using an ordinary voltmeter to measure the battery voltage while cranking
the engine. The starter motor serves as the high-current load. Refer to -20 technical manual for
the vehicle for the exact procedures and correct voltage readings.
CONNECTING BATTERIES IN SERIES AND SERIES-PARALLEL
As you know, tactical vehicles have 24-volt electrical systems, twice the voltage supplied by each 12-
volt battery used. In addition, some of the vehicles demand more current for cranking than the
100 ampere-hour battery can supply. To satisfy the increased voltage and current requirements,
more than one battery is used in each vehicle. To satisfy the specific requirements of different
vehicles, the batteries are connected in series or series-parallel.