Your car’s alternator is a critical component of your car’s electrical system. It takes a small portion of the mechanical energy of the car’s engine rotation and turns it into electricity that is stored for use in the battery.

Most of the time, a properly functioning alternator keeps the car’s lead-acid battery at around 12.6 Volts. Though a malfunctioning alternator can overcharge the battery, which can lead to disastrous consequences for your car’s electrical system and your bank account.

An overcharging alternator is usually caused by a fault with the voltage regulator. In some cases, it can be a short or bypass, causing the alternators windings to be powered independently of the regulator. In more modern systems with computer control, it can also be a fault with the sensor circuit sending the wrong battery voltage signal to the regulator.

Symptoms of an overcharging alternator often include a battery warning light and/or check engine light coming on in the dash, blown-out fuses and bulbs, and an overheating battery.

If you’ve been seeing the battery needle pegged on your dashboard, then it’s time to pull over and find out why your alternator is overcharging. This is one of those times when procrastination pays a price!

Fortunately, it’s a fairly easy problem to diagnose and fix yourself. All you’ll need are general tools and a multimeter. Read on to learn more about alternator overcharging, as well as its causes and a comprehensive how-to guide on how to diagnose and fix an overcharging alternator.

What’s The Job Of An Alternator?

An alternator is essentially a generator. It draws power from the engine via a drivebelt that’s connected to the crankshaft pulley, and converts mechanical energy from the spinning motion of the engine into electrical energy. The main job of the alternator is to charge the battery, so it can start the car again and again. After the engine is running, the alternator also takes over the job of supplying electricity to all the car’s systems.

What Does It Mean When Your Alternator is “Overcharging”?

What Does It Mean When Your Alternator is “Overcharging”

The alternator is overcharging when it supplies the battery with more voltage than it needs to charge correctly. Most alternators are set to supply the battery with a bit more than 14 volts, which is enough to charge it without damaging other electrical components. But, if there’s a fault with the alternator’s voltage regulator, it might be charging the battery at a higher voltage, which may result in damage to the battery and to other electrical circuits in your car. 

What Are the Consequences of Alternator Overcharging

The biggest consequence of alternator overcharging is that you can damage your car’s 12 Volt lead-acid battery. Not only can overcharging damage the internal electrolyte balance in the battery’s cells, but it can even cause the battery to swell and boil over.

At that point, you’re dealing with battery acid in your engine bay. Not to mention a battery that is going to fail to perform in a matter of moments!

Even if the battery doesn’t immediately suffer catastrophic failure, overcharging can still affect the car’s electrical system. Delicate electrical components can burn out, and there’s even a chance of some wiring harnesses short-circuiting in some models.

What Causes An Alternator To Overcharge?

A lot of alternator faults affect alternator performance to the point that it can no longer properly charge the battery. The battery light comes on, and your car slowly dies as the battery can no longer sustain its role in the internal combustion process.

Though the alternator can develop a fault that causes it to overcharge the car’s battery. This is usually related to a problem with the internal voltage regulator.

As the name implies, the voltage regulator’s primary job is to meter out the voltage produced by the alternator to make sure that the battery doesn’t go over 12.6 Volts or thereabout. When the voltage regulator breaks down, alternator overcharging can occur with reckless abandon.

This is especially likely to happen if you’re driving fast at high RPMs. As the engine is turning rapidly which can put it in a state where it’s truly operating beyond peak performance.

A few issues can cause an alternator to overcharge. Here they are:

1: Fault with the voltage regulator

The regulator works by controlling the current flow to the alternator’s rotor on and off. Older systems used electromechanical devices, while modern ones use semiconductors to achieve the same result.

Fault with the voltage regulator

The voltage output of the alternator varies with its rotor speed, which is connected to the engine’s rpm via the drive belt.

The voltage regulator ensures that the output from the alternator is somewhat constant regardless of engine speed, and keeps it at the optimal value for charging the battery quickly without frying your car’s circuits.

If the regulator is faulty, it won’t be able to keep the alternator output in check – causing the battery to overcharge.

2: Electrical short bypassing the regulator

The rotating part of the alternator becomes an electromagnet when current flows through it, creating a magnetic field that generates an alternating current in the stator’s windings. Once that is converted to DC, it’s what feeds the battery and the electrical circuits in the car.

An electrical short bypassing the regulator can cause the alternator’s rotor to be powered independently of the regulator, causing a potential overcharging condition.

3: Problem with the battery voltage sensing circuit

In modern cars, the alternator’s output is controlled by computer. A circuit monitors the battery’s voltage, telling the regulator when it needs to supply the alternator with current.

If there’s a fault in this circuit, it might tell the regulator that the battery voltage is low when it is actually high, resulting in an overcharging situation.

4: Wrong battery or alternator

Wrong battery or alternator

The battery may also be overcharged if you fit a battery that is too small or an alternator that is too big for your vehicle. Capacity of the battery and the alternator may vary depending on engine, trim level, or equipment.

Make sure to always check the manual for the correct part numbers and get the right replacement parts.

5 Signs That Tell You Your Alternator Is Overcharging

At first, it can be hard to tell if your alternator is overcharging. Sometimes the voltage regulator that’s at fault, fails slowly over time. The natural state of the battery, how drained it is and the car’s performance can also make it hard to spot alternator overcharging in the early days. Though it’s a good bet that you have an alternator problem if you notice:

1: The Alternator Has a High Voltage Reading

A normally functioning alternator will give a slightly higher voltage reading when you start the engine.

It’s when you still see this higher-than-normal voltage output from the alternator once the engine’s been running for a few minutes that you should start to suspect that the alternator is overcharging. By this point, the car’s ECU should have told the voltage regulator to perform normally.

2: The Battery Is Hot and/or Bulging

One of the other telltale signs that an alternator is overcharging shows up in the car’s battery itself. A lot of newer vehicles will have some type of battery monitoring system that actively works to regulate the proper charge of the battery to prevent damage from older charging. Though these systems aren’t always 100% effective for preventing problems from an overcharging alternator.

Most of the time the battery monitoring system of the ECU will throw a red battery warning light on the dash. Though a completely failed voltage regulator may still cause the alternator to overcharge the battery.

With a lot of older vehicles and cars that don’t have an onboard ECU battery monitoring system, an overcharging alternator can continue unchecked until the battery fails.

When this happens, the heat produced inside the overcharged battery can start to boil the electrolyte battery acid. This can cause swelling and cracks in the battery case. If this overcharging condition persists it can also lead to a catastrophic buildup of gases, and the battery case will rupture releasing the gasses and a fair amount of battery acid.

If you’ve noticed the early signs of the alternator overcharging on the dashboard or elsewhere, and a check of the battery reveals excess heat, bulging, or cracks, then you are likely staring at a dangerous situation. You can also bet that you’ll need to completely replace the battery as part of the final repair!

The cost of a new car battery ranges from $100 to $350 depending on the size and quality of the replacement battery.

3: The Red Battery Light and/or Check Engine Light Comes On

ECU will throw a red battery warning light on the dash

As I mentioned earlier a lot of newer vehicles have active battery monitoring systems, and the ECU will throw the battery light or the check engine light when it detects a gross voltage deviation. Though you shouldn’t rest easy, as it could still allow the alternator to continue overcharging the battery.

On a lot of older vehicles from 2008 or earlier, there isn’t an active battery monitoring system that specifically monitors for overcharging. Your most likely indicator would be the needle on the battery performance dial going higher than the three-quarter mark and possibly even pegging out.

4: Fuses, Relays, and Bulbs are Burning Out

 burn out car fuses

A lot of the sensitive electronics in your car have fuses or sometimes relays that are protecting them from power surges. It’s possible for the excess electricity from an alternator overcharging to burn out these fuses and relays as they essentially sacrifice themselves to protect the vulnerable components they’re connected to.

At the same time, the current surges of an alternator overcharging can also tax the filament inside headlights, turn signals, and sometimes even brake lights. In a lot of older vehicles, this is sometimes the first sign of an alternator overcharging.

5: The Smell of Burning or Melting Wires

The heat produced by an alternator overcharging can often exceed the protective coating of internal wires in the car’s engine bay. It can also start damaging the wiring in and around the alternator itself. This is usually something that happens in conjunction with changes in the car’s battery.

How to Diagnose an Overcharging Alternator

If you’ve been noticing the telltale symptoms that your car’s alternator is overcharging, then the next step to confirm diagnosis calls for a multimeter or voltmeter.

Successfully diagnosing an overcharging alternator is a very simple process which only requires a multimeter and 5 minutes of your time. All you need to do is check the battery voltage while the engine is running and with all electrical consumables (lights, radio, etc.) turned off. Any reading above 15.0 volts is too high and indicates the alternator is overcharging.

How to Test an Alternator

For reference, the “normal” range of alternator voltage is usually between 13.5 and 14.5 volts. If you get a reading between those numbers, it means your alternator is healthy and supplying the battery with exactly the juice it needs. A lower value means the alternator is undercharging.

You can further confirm this by having someone else rev the engine up to 2,500 RPMs with the multimeter still connected. In an alternator with a properly functioning voltage regulator, the volt reading on the meter should stay relatively consistent. If it starts ramping up dramatically at 2,500 RPMs, then the alternator is overcharging and the voltage regulator is either dead or dying.

If your car pops a check engine light, and you have an OBD II code reader, you can hook it up and check the codes. If the ECU has thrown the P2504 – Charging System Voltage High code, it’s another salient sign of an alternator overcharging.P2504 is typically triggered in a newer vehicle with a battery monitoring system when the ECU detects a high-voltage incident somewhere in the charging system.

Since the voltage regulator in the alternator is supposed to help keep this from happening, it’s a good bet that the alternator is fully or at least partly to blame. Though this might not be a standalone alternator overcharging problem. P2504 can also be thrown if there are short, circuited wires and/or the battery terminals are severely corroded. So, you’ll want to make sure to eliminate those possibilities before turning your focus on the alternator itself.

How Can You Fix An Alternator That Is Overcharging?

Unfortunately, there’s no easy way to fix an overcharging alternator. Repairing the voltage regulator would require advanced electrical expertise which is beyond the scope of normal mechanical DIY. If you have an external voltage regulator, you might get away with replacing just this part, however it is usually a built-in component, which means the only solution is to replace the entire alternator.

Some companies still offer an alternator rebuilding service, but the way that generally works is you give them your old alternator for them to fix and they give you a refurbished alternator for a discounted price, often with a warranty. Whether you prefer to go that route or buy a brand-new alternator from an auto store is up to you.

Here’s the complete guide on how to remove and replace the alternator. Please note these are general guidelines and the actual procedure might vary for your car. Whenever possible, check your car’s service manual for specific instructions for your vehicle before starting the job. It will advise you if there are any particular steps you need to take or any special tools you’ll need to complete the job.

  • Open and secure the hood. Locate the alternator. It’s going to be in front of the engine, connected to the engine crankshaft pulley via a drive belt.
  • Take note of your radio code, if you have one. You’ll need it after the next step.
  • Remove the battery’s negative terminal. Secure it safely, so there’s no chance of it accidentally touching the battery until you are done with the job.
  • Remove any obstacles in the way, so you can access the alternator and the drive belt clearly.
  • Locate the drive belt tensioner. Take note of how the belt is routed around the various pulleys, idlers, and the tensioner. Relieve tension in the tensioner and slip the drive belt from the crankshaft pulley, the alternator pulley and all other pulleys that may be present. This step sometimes requires a specialized tool. Your service manual will advise you on how to proceed.
  • Remove any electrical connectors going into the alternator.
  • With the drive belt off, locate and remove the bolts that hold the alternator onto its bracket. This will normally be two to four bolts.
  • Remove the alternator.

Once you have your old alternator in hand, you may look around it for a plate or check your car’s parts catalogue to obtain the part number you’ll need for your replacement. Some cars might have different alternators depending on engine number, trim level, or equipment. Make sure you get the right one for your vehicle, as that will avoid further headaches down the road.

The refitting of the new alternator is basically the reverse of removal, but you need to take extra care on some of the steps:

  • Make sure to reconnect all the alternator’s electrical plugs you pulled out during removal.
  • Tighten the alternator bolts in a crisscross pattern and use a torque wrench to apply the correct tightening torque, if your manual calls for it.
  • When re-routing the drive belt, release tension in the drive belt tensioner and feed the belt carefully around the various pulleys in the same pattern it was before you removed it. Refer back to your notes or the manual if you need help. Slowly release the tensioner to re-apply the right tension to the belt. Make sure that the belt is tight and correctly seated in all pulleys.
  • Take the time to clean any corrosion in the battery leads with a wire brush, and apply some anti-corrosion fluid to make sure the electrical connection remains strong.
  • Re-insert your radio code if applicable.
  • Double-check that everything is tight under the hood and that you didn’t leave any tools or screws behind. Start the engine and check the battery voltage with a multimeter. It should now show a normal value in the 13.5 to 14.5-volt range.

If the voltage output reads consistently and is under 15 Volts, you can rest easy knowing that you fixed your alternator overcharging problem.

How much Dose It Cost ?

Technically you can rebuild the alternator, which will involve replacing the faulty voltage regulator. However, the time and cost of this are equal to or even greater than the cost of completely replacing the alternator.

This is something a reasonably handy person can do with some basic tools and a Saturday afternoon to sacrifice.

The cost of a replacement alternator ranges from $125 to $250 depending on the size and model.

The cost of having a mechanic replace your alternator ranges from $225 to $400.

Replacing your own alternator usually takes around 3 hours with cost savings of $150 that would otherwise go into a mechanic’s pocket. So, replacing your own alternator is like paying yourself around $50 an hour, give or take the price of the knuckles you are destined to scrape.

Frequently Asked Questions

How Can I Tell If an Alternator Overcharging Damaged My Car’s Battery?

If you see any overt signs of swelling or cracking in the battery case, or the top seals boiled over with battery acid, it’s safe to assume that you will also need to replace your car battery. If the battery is fully charged and reading between 12.6 and 13.2 Volts you likely escaped battery damage caused by the alternator overcharging.

How Long Can I Drive When My Alternator’s Overcharging?

The longer you drive with your alternator overcharging the greater the risk is of damaging your batteries, and sensitive electronics. If you just happened to notice the problem when a check engine with a battery light comes on, you might be able to limp home five miles or so at low RPMs. Though the smart money is to pull the car over and have it towed. A tow truck is usually only $75 to $100, which is much cheaper than adding a battery and other electronic repairs to the cost of fixing the overcharging alternator.

Where Can I Get My Alternator Tested For Free?

If you don’t have a multimeter, most auto parts stores in the United States will test an alternator for free. Though these bench tests typically require you to take the alternator out of the engine bay.


Most of the time when an alternator is overcharging it’s because the internal voltage regulator is dead or dying. Early signs of this often start with burned-out headlamps and fuses with a battery meter that reads slightly higher than usual. As the voltage regulator continues to degrade, you’ll likely get a check engine or battery warning light on the dash with the battery meter pegging.

You can test if the alternator is overcharging with a code reader that throws a P2504 code. You can also hook a multimeter up to the alternator, have someone hold the revs at 2,500, and watch for the volt reading to go up.

Since the voltage regulator is an integrated component, it’s usually wiser to simply replace an alternator that’s overcharging than it is to pay a mechanic to rebuild it. You can usually replace the alternator yourself at a savings of $150, and a cost of three hours eaten out of your Saturday afternoon.

Jason Farrell

Written By

Jason Farrell

Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.

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