How to test a car battery with a digital multimeter
A car that won't start is a source of panic for most drivers, but feeling helpless is optional. Diagnosis of the problem begins with a basic battery test that is within reach of all vehicle owners and can be safely done at home.
An inexpensive digital multimeter can identify or eliminate the battery as the cause of your inoperative vehicle. Before making that call to the auto shop and inviting expensive bills, use a digital multimeter test to test your vehicle's battery.
classic impala headlights image by Jorge Moro from Fotolia.com
Turn the headlights on for three minutes to remove residual charge. Switch off the headlights before proceeding. It is not necessary to disconnect the battery after removing the residual surface charge.
Clean the battery terminals with a rag soaked in a solution of 1 tbsp baking soda and 2 cups water. If the positive terminal is covered by a protective plastic cover, lift the cover before cleaning.
Remove residual dirt or visible spills on the battery casing, using the solution and rag.
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Select the DC voltage scale on the digital multimeter, and set the range to 20 V.
Insert the positive (red) test lead of your multimeter into the terminal marked with V (voltage), and the negative (black) test lead into the terminal marked COM (common).
Turn on your multimeter, and clip the free end of the red test lead to the positive terminal of your battery, and the black test lead to the negative terminal. If the test leads end in probes and not clips, hold the tip of the probe against the battery terminal and maintain contact while taking the measurement. The voltage will appear on the multimeter screen.
- A fully charged battery should produce a voltage reading of a minimum of 12.6 V. If the reading is between 12.4 and 12.6 V, your battery should be charged for 12 hours and the voltage measured again. A reading of less than 12.4 V may indicate a defective battery that you may need to replace.
- If the battery terminal posts have significant corrosion, disconnect your battery and use a wire brush to remove the corroded material before cleaning the posts with the baking soda solution. A corroded or dirty terminal may result in an inaccurate voltage measurement.
- Remove metal jewellery before battery testing to prevent burns, electrical shock or damage to the jewellery.
- Wear protective eye glasses or a face shield, because batteries contain acid that may cause facial burns in the event of an explosion.
- Performing the battery test while the positive (red) test lead is inserted into the multimeter terminal marked A or ma (amps) will result in a false reading and damage to the multimeter.