How to Check the Battery Voltage With a DMM

Digital multimeter

Several terms are associated with checking a lead-acid storage battery. "Surface charge" refers to the charge that builds up on the outer surface of the battery's positive plate or "anode." The surface charge provides a false indication of the battery's condition and must be discharged before testing the battery with a digital multimeter (DMM). "Open circuit" voltage is present at the battery's terminals when there's no load being applied to the battery. The open circuit voltage, when measured with a DMM, is a good indication of the battery's actual condition. A battery's voltage taken under load conditions is the best indication of its condition and serviceability.

Things You'll Need

• Digital multimeter
• Safety goggles or face shield
• Baking soda
• Small plastic container
• Paint brush
• Wire brush
• Shop rags

Instructions

1. Mix the baking soda with a little water in the plastic container to form a paste. Use this paste to clean the corrosion from the battery cables and battery terminal posts. A solid, clean connection is required to get an accurate voltage reading.

2. Wearing eye protection, brush the paste onto the battery posts. Let it set for 15 minutes, then rinse off with water. Dry the posts using shop rags.

3. Remove any remaining corrosion with the wire brush. Be careful to not get any of this corrosion on your clothes--lead-acid batteries use an electrolyte of diluted sulfuric acid, which will burn holes in your clothing.

4. Set the function switch on your DMM to the DC Voltage function. If your DMM isn't an auto ranging meter, set the range switch to the 20-volt range.

5. Discharge the battery's surface charge by turning the vehicle's headlights on for a minute or two.

6. Press the tips of the test probes firmly into the battery's posts. A reading of 12.7 volts indicates a fully charged battery. A reading of 12.6 volts indicates a 90 percent charge. If the battery indicates 12.4 volts or less, charge the battery, then retest the battery using the DMM.

7. Check the battery's voltage under load. While holding the test probes in place, have a helper crank the engine over. A good battery shouldn't drop below 12.4 volts during the load test.

References

• Euro MotoElectrics: Battery Diagnosis (PDF)
• Autoshop 101: Battery Service (PDF)

Resources

• Fluke Corp.: Testing Electrical Systems with a Digital Multimeter (PDF)