Can a Bad Alternator Ruin a Battery? (Prevent Damage)

The alternator is an important component in the car, responsible for generating electricity which the battery stores.

It powers up the vehicle and charges it as you run the engine or drive it. When you start the car, the alternator manages the battery’s voltage and supplies power to keep it charged by converting mechanical energy into electrical energy like a dynamo. But can it ruin the battery if it goes bad?

Yes, a bad alternator can potentially ruin an otherwise healthy battery. It can drain the battery completely within half an hour, overcharge it, damage other electronics, or cause intermittent charging. In either case, it will ruin the battery and must be repaired or replaced.

In addition to confirming that a bad alternator can ruin a battery, I’ve also covered how it impacts the battery, the signs of a bad alternator to look out for, the causes of a bad alternator, and how to diagnose the situation.

A Bad Alternator

A bad or faulty alternator can completely drain a battery within 30 minutes, leaving you with a dead battery and perhaps stranded somewhere.

It can disrupt other components connected to it, including the battery. A weak alternator will be unable to charge the battery adequately, and a damaged one will probably not be able to charge it. Consequently, you can only continue driving the car as far as the battery’s remaining charge will allow.

When the battery’s voltage drops below approximately 12.5 volts, it will no longer be charging, and the car will eventually halt.

Impact on the Battery’s Charge

A bad alternator can cause either one of the following to the car’s battery: (1) It can undercharge it, (2) It can overcharge it, or (3) It can cause intermittent charging.

The alternator will not adequately charge the battery if it’s not operating efficiently. It can damage the battery if it happens often. Similarly, a malfunction can make the alternator work harder and overcharge the battery. This can not only damage the battery but also damage the car’s electronic components and accessories.

Intermittent charging is also harmful to the car’s battery. When the alternator operates normally, it ensures a steady power supply while the car runs. But if this power supply is intermittent, the engine will keep shifting for its source of power, which can dry up and damage the car if it happens frequently.

Signs of a Bad Alternator

Knowing that a bad alternator can ruin your car’s battery, you’ll want to know the signs of a bad alternator.

Here are the main signs to look for before confirming a bad alternator.

Sign 1: Battery Indicator Light

The battery indicator light on the dashboard could indicate a problem anywhere in the battery charging system, but one cause may be a bad alternator.

Sign 2: Headlight Issues

A headlight that flickers have diminished brightness or suddenly darkens means it’s not getting sufficient power. It could be due to a weak battery, but its low charge could, in turn, be due to a faulty alternator.

Sign 3: Abnormal Functioning of Accessories

Like the headlight issue, if the car’s accessories also behave abnormally, it’s another sign of a possibly faulty alternator. It might not be charging the battery adequately to operate the accessories. You should turn off unnecessary accessories anyway when there’s a fault with the electrical system.

Sign 4: Stalled Engine

A frequently stalling engine could be due to a bad alternator.

Sign 5: Strange Sound

The alternator’s rotating shaft comprises copper coils, brushes, and rectifiers. You may hear a strange sound if they are displaced or stop working properly. This originates from the strange sound, usually like an unpleasant grinding noise.

Sign 6: Pungent Smell

If you notice a pungent smell like burning rubber, it’s probably the belt.

The belt is subject to continuous friction, tension, and heat. It wears over time and can eventually loosen or give off a pungent smell when worn out too much. You might even notice sparks as it rubs against the shaft rotating at high speed.

Causes of a Bad Alternator

To repair a bad or faulty alternator, you’ll need first to identify the cause of the problem.

The most common causes are a tight belt and accumulation of dust.

Cause 1: Loose or Tight Belt

As mentioned, the belt can wear out from constant use (exposure to friction, tension, and heat).

The alternator will be unable to do its job properly if the belt is loose or too tight. If it’s too tight, it could even damage the bearings. Signs of a loose belt are undercharging or intermittent charging of the battery.

Check all the wires and connections to the alternator and ignition switch if the belt is tight. They should be tight and uncorroded. [Bonner Corporation, 1980] No wire should be broken or have its insulation missing.

Cause 2: Dust and Oil

Living in a dusty area is neither good for your health nor your car.

Dust can accumulate on the alternator. Oil particles can also gather on it. Both contaminants can hinder its optimum functioning and make it operate less efficiently.

Cause 3: Worn-Out Brushes

If this is a cause, you can usually replace the brushes, but you must open the alternator. See under fixing an alternator.

Diagnosing the Alternator

You should diagnose the problem before attempting to fix the alternator.

Here’s what to do.

Step 1: Open the Hood and Start the Car

Open your car’s hood and start, i.e., run the engine.

Step 2: Disconnect from the Negative Terminal

Disconnect the cable from the battery’s negative terminal.

If the engine stops when disconnecting the black wire, the alternator might not be supplying enough electricity.

a hand disconnecting negative terminal on the car's battery
Video | FIX MY CAR

Step 3: Check the Voltage

Check the voltage to confirm this further.

Turn the engine off and use a voltmeter or multimeter to check the voltage. It should be at least 12.5 volts but not less than 12.2 volts. A normal reading would be around 13.5 volts. It should neither be too high (above 14.5 volts) nor too less. If it’s outside this range, the alternator may be faulty.

man checking car battery voltage using a multimeter
Video | ChrisFix

Fixing a Bad Alternator

It would be best to fix a bad alternator quickly to prevent further damage to your car’s battery and other electrical components and accessories.

If an auto electrician can fix the alternator, you can keep using it but be on the lookout for signs of the alternator weakening or malfunctioning. If not, it’s best to replace it. Your battery and other electronics are worth more than sticking with a bad alternator.


An alternator connected to a car’s engine.

Bonnier Corporation. Popular Science, Jun 1980, Vol. 216, No. 6. Bonnier Corporation. 1980.

Video References

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About Alex Robertson

c3c9d43f1f0d14c4b73cb686f2c81c4e?s=90&d=mm&r=gCertifications: B.M.E.
Education: University Of Denver - Mechanical Engineering
Lives In: Denver Colorado

Hi, I’m Alex! I’m a co-founder, content strategist, and writer and a close friend of our co-owner, Sam Orlovsky. I received my Bachelor of Mechanical Engineering (B.M.E.) degree from Denver, where we studied together. My passion for technical and creative writing has led me to help Sam with this project.

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