How to Test a 12-Volt Car Battery

Written by arthur barnhouse
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More than any other component on your car, the battery is the most likely to fail. This means you stand a greater chance of being stranded due to a battery with an insufficient charge than with a flat tire or blown engine. Therefore, it’s imperative to ensure that your battery has a complete charge and that it’s working correctly. Thankfully, performing these tests does not demand superior mechanical aptitude. All that’s required is a little time and the proper instruments.

Skill level:
Moderately Easy

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Things you need

  • Digital voltmeter
  • Hydrometer (for serviceable batteries only)
  • Battery charger (if necessary)

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    Testing a 12-volt Battery with a Digital Voltmeter

  1. 1

    Place the 12-volt battery on your work bench or flat surface. If you plan on testing the battery while it remains hooked up to a vehicle, you will need to ensure that you turn off all power switches on the car or truck and remove any shields covering the battery terminals.

  2. 2

    Connect the voltmeter to the battery terminals or posts. In order to do this properly, you must first connect the positive clip or probe on the voltmeter to the positive terminal and do the same with the negative clip or probe. You will want to make sure you securely attach the voltmeter, as a loose connection will not give a proper reading.

  3. 3

    Look at the digital readout on your voltmeter. A 12-volt battery that is adequately charged will display a reading between 12.5 and 12.8 volts. If you note a reading of less than 12.4, your car battery will need to be recharged. Proceed to Step 4 if your battery shows a reading of 12.4 or less.

  4. 4

    Recharge the battery using a battery charger. After the battery has been fully charged, let it rest for a 24-hour period. Then retest the battery using your voltmeter. If the battery is in working condition, the voltmeter should show a good reading. If, however, the battery tests low once again, you may need to purchase a new battery or recharge and retest to verify your findings.

    Testing a 12-volt Battery with a Hydrometer

  1. 1

    Use a hydrometer only if you have a conventional, unsealed battery. You can immediately discern whether you have this type of battery by checking for removable caps on the top of the battery. If you do not have the capacity to refill the battery by removing these caps, you have a sealed battery. For a sealed battery, testing can be done with a voltmeter.

  2. 2

    Remove the refill caps from the battery and insert the tube of the hydrometer into one of the refill holes. Draw some electrolyte fluid up into the hydrometer and release it back into the refill hole. Do this for every refill hole on the battery. This ensures the electrolytes in each cell are adequately mixed.

  3. 3

    Draw the battery’s electrolyte fluid into the hydrometer. While each hydrometer’s gauge and measuring system will vary depending on the manufacturer, a battery with good electrolytes will cause the flotation device in the hydrometer to rise high. If the battery is bad, though, the flotation device will not rise nearly as high and may, in fact, sink. Read the manual for your specific hydrometer in order to understand its gauge or measuring system.

  4. 4

    Release the electrolytes back into the refill hole before continuing to the next cell. You should not notice more than a .05 difference between the readings for each individual cell or refill hole. If you get a bad reading, try recharging the battery, letting it rest for a day and then retesting. Another bad reading may indicate that you need a new battery.

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