How to test a 12-volt car battery using a voltage instrument

Written by andy pasquesi
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Your car's battery doesn't operate in a vacuum. The power demanded from its dozens of electrical components changes on a frequent basis. Therefore, properly testing a 12-volt battery requires that you simulate the two major energy states of the car's electrical system. As long as you follow all the safety precautions, the actual technique for testing with a voltage instrument is easy to master.

Skill level:
Moderately Easy

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Things you need

  • Safety goggles
  • Work gloves
  • Multimeter (analogue or digital)

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Instructions

  1. 1

    Turn off the car's engine and any electrical appliances, such as headlights, fans, cabin lights, the radio and cigarette lighters. Let the car sit for nine hours.

  2. 2

    Put on the safety goggles and work gloves.

  3. 3

    Release the bonnet of the car and safety prop it open.

  4. 4

    Turn the multimeter's dial to the symbol for "DC Voltage," which consists of a capital "V" underneath two parallel, horizontal lines. The top line will be solid but the bottom line will be dotted.

  5. 5

    Touch the metal tip of the multimeter's black probe to the metal on the battery's negative (-) terminal. Hold it there during Step 6.

  6. 6

    Touch the metal tip of the multimeter's red probe to the metal on the battery's positive (+) terminal.

  7. 7

    Note the measured voltage on the multimeter's readout. For a properly functioning battery, expect a reading between 12.5 and 12.8 volts.

  8. 8

    Have an assistant turn the key in the ignition to start the car, letting the car idle in park. With the car idling, keep both probe tips in contact with the battery.

  9. 9

    Note the measured voltage on the multimeter's readout as the engine idles. For a properly functioning battery, expect a reading between 13.6 and 14.3 volts.

  10. 10

    Turn the car off and lower the bonnet back into place.

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