How do I know if I need a new alternator or battery?

“If the [battery’s] voltage is low, replace with a new battery or charge the battery to the recommended 14.2 to 14.7 volts,” he says. “Then try to run the vehicle and see if the voltage after is outside of that range. If it is, that’s a sign of a bad alternator.”

How do I know if I need a new alternator?: If it stalls immediately, the alternator is most likely the offender. On the other hand, if the vehicle continues to run after about five minutes, the battery is likely at fault.
Does my alternator need to be charged?: The battery needs the alternator to stay charged, and the alternator needs the battery to start charging. Neither works well without the other. So if you have alternator or battery issues, get them resolved quickly to avoid any further troubles down the line. Fortunately, you have RepairSmith.
What does it mean if your alternator is bad?: When your vehicle runs fine at first but begins to fail over the course of minutes, you have a bad alternator. If your car’s battery isn’t holding a charge, you have a problem with your alternator or battery. If your battery can start your vehicle but dies as the car runs, you have a faulty alternator.
What is the output of my alternator?: What is the output of your alternator? Battery voltage measured at the terminals with the engine running should be about 13.7-14.7 volts. That is the output of the alternator at the battery and that range is needed to keep the battery properly-maintained.

Is 12.4 volts OK car battery?

12.5 volts – Your battery is at a healthy state of charge, but we’d recommend re-checking it within a few days to ensure the voltage hasn’t dropped any further. 12.1 – 12.4 volts – Your battery is partially discharged and should be recharged as soon as possible, using a suitable battery charger.

12.4 volts. Car battery. 12.5 volts. Battery. The voltage. 12.1 - 12.4 volts.

Is a battery voltage of 12.4 volts OK?: A decrease in this voltage can have a massive effect on the car. A battery voltage reading of 12.0 has lost 75% of its charge (25% charged). A battery voltage of 12.2 volts is about 50% charged, and 12.4 volts is about 75% charged. So a battery with 12.4-volt reading can be considered OK for a car. What Is Battery Voltage?
What is the voltage of a car battery?: The voltage of a car battery should be between 12.2 to 12.6 volts when the engine is turned off. A fully charged car battery voltage falls between 13.7 and 14.7 volts with the engine running. With the battery charge at 75%, the voltage can drop to 12.4 volts. At 25% charged, the voltage will measure around 12 volts.
What happens if your car battery voltage drops below 12 volts?: A low battery voltage can also cause other electrical problems in your vehicle. Most car batteries have a voltage of 12 volts. If your car’s battery voltage drops below 12 volts, it is considered low and needs to be recharged. You can use a voltmeter to check your car’s battery voltage.
What is a low voltage car battery?: Most car batteries have a voltage of 12 volts. However, some can have as low as 10 volts or even 8 volts. If your car battery has a voltage of 10 volts or less, it’s considered low and needs to be charged. If your car’s battery is running low on voltage, it may not be able to start the engine.