Check a battery using a voltmeter by assessing the static voltage, testing the voltage under full load and checking the charging system voltage. While there are other tests of a battery, these three are a quick way to determine the health of the battery and the charging system.
- Assess the static voltage
Turn off the engine and all accessories. Open the hood of the car. Set the meter to the appropriate voltage range. Touch the red lead of the meter to the positive battery terminal and the black lead to the negative terminal. The reading should be between 11.5 and 12 volts. Charge a battery that tests below 11.5 volts, and test it again.
- Test under a full load
Have a helper turn the key to engage the starter while you read the voltage. Keep the leads in the same position as before while the car cranks. Batteries that drop below 10 volts during this operation are getting weak and require a replacement.
- Check the charging system
Once the vehicle starts and the alternator is charging the battery, take a final voltage reading. Charging voltage should range between 12.5 and 13.8 volts. If the reading is over 14 volts, the voltage regulator is likely bad and requires replacement. Continuing to operate the car in this condition overcharges the battery and ruins it quickly.