Check a car battery with a volt meter whenever a car has problems starting or keeping electrical systems functioning. Most electrical shortfalls in a car motor can be traced to the battery or the alternator. Volt meters give numeric readings that show the power output from batteries. The readings from a car battery under a few different circumstances will show how well the battery is working.
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Set the volt meter for "DC" readings. Use the 12- or 20-volt selection if the meter has voltage scale settings.
Check the battery for ability to hold a charge. Test the charge of battery after the car has not run for at least eight hours. Leave the car motor off. Car batteries are clearly marked for positive (+) and negative (-) terminals. Put the red lead of the volt meter on the positive terminal of the car battery. Place the black lead on the negative terminal. The volt meter will show 12.5 to 12.9 for a battery that is holding its charge.
Check the battery while it is receiving a charge. Start the car. Connect the volt meter to the battery, red probe to positive terminal and black to negative. The reading will be between 13.5 and 14.5 volts if the battery is receiving the charge from the alternator. A lower reading may indicate a faulty alternator.
Check the battery with a strain on its charge. Disable the car's ignition or injection by removing the fuse that controls one of these elements. Connect the volt meter to the battery. Ask a helper to turn the key for about 15 seconds. Watch for a reading of 9 to 10 volts during the first 15 seconds. A reading lower than 9 volts means the battery is not good.
Tips and warnings
- Make sure the volt meter is rated for testing 12-volt car batteries. Meters made for small batteries will get overloaded by the voltage from a car battery.
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