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Short Summary
Main recommendation: test a car battery with a multimeter by first running the headlights for about two minutes to remove surface charge, then checking resting voltage (about 12.6 volts is good), and finally checking voltage while starting the engine, where it should not drop below 10 volts. A battery that drops below 10 volts under cranking is failing, and one that drops below 5 volts needs replacing.
Key Points
- A battery light on the dash, slow or no cranking, clicking without starting, weak battery or lights, and weak headlights can point to a bad battery or a bad alternator.
- Resting voltage alone does not prove a battery is good, because a battery can hold a charge yet lack the cold cranking amps to start the car.
- Cold cranking amps (CCA) measure how many amps a battery delivers at zero degrees Fahrenheit for 30 seconds without dropping below 7.2 volts.
- Batteries are typically warrantied for about four years, but the speaker says only about 30% of batteries sold today reach that mark.
- Dirty or corroded terminals can cause problems and should be cleaned before testing.
Details Worth Noting
- Concrete instructions:
- Turn the headlights on for about two minutes to remove surface charge, then turn them off.
- Set the multimeter to 20 volts (or a setting above 15 volts) before connecting.
- Connect red to positive and black to negative (ground). A resting reading of about 12.6 volts is good; the speaker's good battery read 12.85 volts.
- Start the car with the meter connected. Voltage should stay at or above 10 volts; below 10 means the battery is starting to fail, and below 5 means replace it. Once running, a healthy charging system should read around 14 volts.
- Clean corroded terminals by loosening the clamps, sanding the posts and the inside of the clamps until clean, reattaching, and tightening with a ratchet. Wear goggles and gloves because the corrosion is corrosive.
- After cleaning, spray the terminals with WD-40 to help prevent rust and corrosion from returning.
- For batteries with removable caps, keep fluid about a quarter inch above the plates, and top up only with distilled or bottled water.
- The speaker's bad battery, six years old, had been charged overnight to 12.6 volts but read about 2 volts with terminals disconnected the next day, showing it could no longer hold a charge.
- A major parts store may test your battery for free with better equipment.
- The alternator test is covered in a separate video referenced by the speaker.
Bottom Line
A simple multimeter check, done correctly with a warmed-up resting reading and a cranking test, can reveal a failing battery before it strands you.
