Considering the technology behind car batteries is over 140 years old, it is still an unwelcome surprise when a car will not crank over. This technology is so reliable, we have come to depend on it to work with little or no maintenance. It is also very likely to continue as the battery of choice for auto manufacturers for some time to come. The large amount of readily-available cranking power lead-acid batteries contain in such a compact size is ideal for automotive applications.
Test the battery voltage with a voltmeter. Set the meter to DC (direct current) and touch the probes to the positive and negative battery posts. Read the voltage. If it is low, below 11.0 volts, charge the battery for further testing. If it is above 11.0 volts, test the starting circuit.
Test the starting circuit. Have an assistant turn the ignition switch and try to start the car. If the battery voltage is above 11.0 volts and does not drop at all, repair the defective starting circuit. If the battery voltage drops very low, charge the battery for further testing.
Charge the battery and check the voltage. If it rises above 14.0 volts and goes as high as 17.0 volts, the battery is defective. If the battery begins to spew and sputter battery acid after a prolonged, slow charge, replace it. If the battery remains near 14.0 volts and does not spew acid, install it for a cranking test.
Ask an assistant to crank the car and watch the voltmeter. If it cranks slowly and the voltage drops very low, replace the battery. If the car cranks well and starts right away, note the voltage reading. It should rise to 14.6 volts as the alternator charges the battery. If the battery stays at 12.0 volts, repair the defective alternator circuit.
Test for the battery draining overnight, commonly known as a parasitic drain. Remove the negative battery cable. Use a test light, (available at any auto parts store) and attach the wire lead to the negative cable and touch the probe end to the battery negative post. Shade the light. There should be a very faint glow, almost invisible in sunlight. This is the normal current that is continually supplied to circuits such as the clock and computer memory. If there is a brighter glow, this is not normal. It means there is a short circuit in need of repair.
