How to Test If Your Car Battery or the Alternator Is Bad?

by Christian Killian
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Joe Belanger/iStock/Getty Images

Testing the alternator or battery in your car requires a digital multi-meter with a voltage setting on it. The battery in your car has a rating of 12 volts and should have at least that to start your car. If the voltage is low on the battery, the alternator needs to be checked to rule out a problem with the charging system before you replace the battery. You can test the system with a multi-meter to determine the output of the battery and the alternator, giving you a better idea of the condition of your battery and alternator.

Step 1

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Turn the dial on your multi-meter to the 20V setting. This will allow you to read the voltage from your battery and alternator.

Step 2

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Place the red test lead from the multi-meter on the positive post of the battery. Place the black test lead on the negative battery post. Note the reading on the meter, you should have at least 12 to 13 volts. If the reading is lower than 12 volts, check the alternator to determine if it is not fully charging the battery.

Step 3

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Start the car and place the red lead from the multi-meter on the “B” terminal on the back of the alternator. Place the black lead on a ground, do not place it on the alternator case. Note the reading on the meter. The reading should be around 13.5 volts and may be as high as 15 volts. If the reading is outside this range, remove the alternator for more testing.

Step 4

Test the battery again while the engine is running. If the voltage reading at the battery is higher than 13.5 volts, the alternator is trying to charge it and the battery needs to be tested further. Most auto parts stores can test the battery for you for free if you remove it from the car.

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