Signs of a bad car battery vs. bad alternator

On the Road 4 min read

The battery gives your vehicle the juice the engine needs to crank daily. When the battery starts to go bad, you can easily replace it. On the other hand, a bad alternator fails to recharge your battery as the car runs. If the alternator goes bad, it’ll cost more to repair than a dead battery. The signs of a bad alternator versus a bad battery are similar, making it difficult to diagnose the exact problem. Knowing the different warning signs of each going bad can help.

Signs you have a bad battery

Almost everyone has had to jump-start a car battery or seen someone do it. Suppose you try to start your car in the morning, and it doesn’t turn on. Maybe you left an interior light on all night, causing your battery to die, but it can also mean your battery is going bad.

According to Matthew Hart, owner of, signs your battery is going bad include:

  • The car is difficult to start.
  • Once started, the car doesn’t hold a charge for very long.
  • Dim or flickering taillights and headlights.
  • Dim interior and dashboard lights.

What causes a battery to go bad?

Factors that contribute to a car battery going bad include:

  • Extreme temperatures
  • Irregular use
  • Poor maintenance

Leaving the lights on when the car isn’t running or a faulty component causing a ‘parasitic draw’ can deplete the battery. A parasitic draw occurs when you inadvertently leave a light on or anything that draws power in your car, slowly draining the battery.

AutoZone reports that typically, the average car battery life is between three and five years and some manufacturers recommend a replacement schedule of five years. It can reduce lifespan if you have connected additional accessories to a car battery, like an internal inverter. You can also do things to extend battery life, such as driving it regularly and occasionally long distances, ensuring all the lights are off when you don’t need them, and protecting your vehicle from extreme temperature fluctuations. If you notice issues with your car, look at the battery itself. Patrick McCann from states, “There may be corrosion around the points on the battery.” This corrosion can be a sign of degradation, but the battery may also need cleaning. Your vehicle might also have a warning light indicator on your dashboard if your battery is beginning to give out.

Signs you have a bad alternator

Your car’s battery holds a charge, but the alternator works to recharge it while the vehicle is on. That’s why you can drive all day without a problem but leaving the lights on overnight will drain the battery. If the alternator begins to fail, the battery will no longer recharge while the engine is running.

Here are a few ways to tell if your alternator is bad:

  • One of the most obvious signs is if the car dies after a jump start. The alternator should provide enough current to keep the vehicle running even with a bad battery. If it doesn’t, then your alternator needs to be examined.
  • “If your alternator is going bad, you might see the voltage meter on your dashboard start to drop, or your car might start to vibrate while you’re driving,” According to Matthew Hart. The vibrations are typically caused by misfires within the spark plug caused by a lack of current from the alternator.
  • Another way to know if you have a bad alternator is by checking the battery voltage after charging it up. “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,” McCann 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.”

What causes an alternator to go bad?

The alternator’s job involves considerable wear and tear. Factors such as heat, vibration, and electrical spikes can eventually cause the alternator to fail.

According to Repair Smith, your car’s alternator may fail because of:

  • Age and use-related wear
  • Engine oil of power steering fluid leaking onto the alternator
  • Salt and water intrusion, especially if it’s located at the bottom of the engine

Can you jump-start a car with a bad alternator?

Yes. You can jump-start your car with a bad alternator. However, according to AAA, your vehicle will likely stall once the battery runs out of enough power to keep it running.

How long will a car run with a bad alternator or battery?

It’s impossible to say how long a vehicle will run with a bad alternator or battery. With a bad alternator, the car might run off the battery quickly. But once the battery is depleted, the car will shut down. When you’ve got a bad battery, it could briefly hold a charge from the alternator, but JD Power recommends if you have jump-started your car, you should let it run for at least 30 minutes to give it sufficient time to charge the battery.

How to fix a bad battery or alternator

If you have a bad battery, you can purchase a new one from almost any auto parts store. Many stores will replace the battery as part of the purchase cost. You can also replace it yourself if you know how to do it. Do your research to find the best battery for your needs. An alternator may cost more to repair than to replace the battery. The price you pay will depend on the make and model of your car and the parts and labor costs.

If your battery or alternator goes out, take your vehicle to a mechanic and have it inspected. While the signs of a bad car battery versus an alternator can be obvious, they can also point to other, more subtle mechanical problems.

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