When your car doesn’t start, often the battery is the culprit. Unfortunately, most vehicle owners do not check their battery until it fails. As preventative maintenance, the suggested best practice is to conduct a car battery voltage test regularly – at least twice per year – using a multimeter.
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When the probes touch the terminals while the car is off and the battery is resting, the multimeter display should show a reading of 12.2 to 12.6 volts (full charge). This voltage range means the battery is in good condition for starting the vehicle. If the measured reading is less than 12.2 volts, the battery’s resting voltage is weak, which means it most likely needs to be charged or replaced.
With the motor running, the multimeter rating should stay in the 14 to 14.5 volt range. Dropping below 14 means either the battery is weak and unreliable for sustained vehicle operation or the alternator is failing. The alternator’s job is to generate energy to feed the electrical system and charge the battery while the vehicle is running.
Alternative tests:
Voltage drop under load. My multimeter can be set to record minimum voltage. While you crank the engine, the battery shouldn't drop below 11 volts. If it drops below 10 volts, I'd definitely recommend replacing the battery soon.
Ripple or AC voltage: with the engine running, measure AC voltage across the battery terminals. You should see less than 0.1V AC, more and the diodes in your alternator are bad.
Parasitic Drain: Ensure that the lights are off, the doors are shut, and the vehicle is completely powered down. Disconnect the negative battery cable. Connect your multimeter such that one probe is on the negative battery cable, and the other probe is on the negative battery terminal. Select Amps or Milliamps on multimeter. Your multimeter is now completing the circuit, if you turn anything on, it will likely pop the 5~10 amp internal fuse in the multimeter. There should be 10~50 Milliamps on most vehicles, keeping the clock, radio memory, security system, and keyless entry systems alive. Anything over 50 Milliamps indicates a circuit not powering down, which will drain the battery overnight. You can pull one fuse at a time to identify which circuit is drawing the excess power.
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