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An alternator’s job is to produce power that operates your car’s electrical functions. It does so by transforming the engine’s rotational energy into an electrical charge. A magnetized rotor assembly spins within a set of copper coils. When a small amount of power from the battery is introduced, it forms an electromagnet that now generates many times the amount of power. It’s then directed through fused circuits to your car’s electrical systems and a portion of it goes back to the battery.

Alternators typically last for many years and over 100,000 miles before they need to be changed, but what happens if you’re frequently dealing with a dead battery? It could be that the alternator isn’t performing its functions which include charging the battery or, worse yet, it could be draining the battery. 

A thorough diagnosis can identify if the alternator is at fault for your battery drain, or you can install a good new or remanufactured alternator to see if it clears up the issue. This information can help you identify what can go wrong with the alternator and how to deal with it.

How A Bad Alternator Can Drain a Battery

Contrary to its purpose, an alternator can drain a battery. It’s rare, but it’s possible. A draw on the battery only occurs if there’s a problem with either a component in the alternator itself or connections to it. 

Alternators contain a rectifier that converts the current from an alternating current (AC) that the alternator generates to a direct current (DC) that your car’s electronics need. The rectifier uses diodes to allow the current to travel only one direction along the circuit. A bad diode in the rectifier can allow current to pass the wrong direction through the circuit, even when the engine is no longer running. It’s like the tap on a water jug that has a constant trickle, eventually depleting the resource it holds inside. It’s called a parasitic draw or drain.

As well as a parasitic draw, a bad alternator can prevent the battery from charging. It may not be directly running down the battery, but an alternator that produces insufficient energy might direct all of its production to devices inside your car, and the battery’s stored energy might be summoned to top up the power those systems need. This can also be due to bad diodes but can also be from a failed voltage regulator, worn brushes, or corroded connections inside the alternator, not to mention bad connections or broken wires externally. 

How to Perform a Parasitic Drain Test

A parasitic drain is frustrating to deal with since it can be intermittent and often leaves you with a dead battery when you least expect it. In some cases, it can be severe enough to deplete your fully charged battery overnight. Although you can test any circuit for a parasitic draw, this is how it works for a draw on an alternator, and all you need is a digital multimeter.

  • Fully charge the battery. For accurate results and enough time to finish testing, a full battery is recommended.
  • Turn off powered devices. Remove the key from the ignition, accessories you have plugged in, turn off your dome light, and wait at least 30 minutes for computer modules to go to “sleep”. 
  • Test circuits for power where there shouldn’t be any. Place the multimeter into the DC milliamps setting and connect its black cable to the battery’s negative post. Probe the negative battery cable with the red lead on the multimeter set to the highest limit, then slowly turning it down until it detects amperage. If you have much more than 50 milliamps of draw, there’s a parasitic load in your car. 
  • Next, remove the alternator fuse and recheck for draw. If the parasitic draw is gone, the alternator is to blame.
  • Perform an alternator diode test. To confirm your suspicion, set your multimeter to AC Volts (ACV) and reconnect the battery cable. With the engine running, touch the red lead to the positive battery post and the black lead to the negative post. If there’s more than 0.5 ACV, the alternator likely has one or more bad diodes.

Can an Alternator Overcharge a Battery?

On the other end of the spectrum, it’s also possible for an alternator to overcharge the battery. This happens when the alternator’s output is more than the typical 13.5 volts to 14.5 volts for an extended period of time. It can happen that voltage spikes to 16 volts for a moment but it isn’t healthy for that to be sustained.

Overcharging is usually due to a problem with the voltage regulator inside the alternator. It constantly switches on and off to maintain voltage output in the “sweet spot”. If the voltage regulator sticks or doesn’t receive a signal to turn off, the output could be sustained over its safe range.

It’s unsafe to maintain an overcharging condition, perhaps more so than a parasitic draw. Too much charging can cause the battery to heat up and the fluid inside to boil. Too much voltage can blow fuses or cause damage to electrical components and wires too. 

How to Deal with a Bad Alternator

Whether you have a battery drain caused by the alternator or it’s overcharging, immediate attention is necessary. If you’re unsure how to accurately diagnose the condition or you want a second opinion, a licensed mechanic is a resource you should seek out. 

Alternators aren’t complicated to replace. After disconnecting the battery, remove the serpentine belt and disconnect the wiring on the back of the case. Unbolt the alternator from the mounting location. From here, you can choose to either open it up and rebuild it or replace the alternator with a remanufactured or new part. 

For help finding the right part for your car or advice to determine the root cause of the problem, AutoZone associates are here for you. Your local store can make sure you find the right parts for the job according to your year, make, model, and engine size and offer Trustworthy Advice to get it fixed right.

Advice, how-to guides, and car care information featured on AutoZone.com and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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