Alternator Overcharging: Symptoms, Causes, And How To Fix

Is your alternator overcharging for some reason? Is it caused by a bad battery? Here are the main symptoms, the possible causes and how to fix it.

Do you think your alternator’s overcharging your battery? This can happen. It creates problems with fuses blowing and batteries struggling.

The topic of automotive electronics (which this title fits well within) can be highly complex. That means you’ll probably need help from a professional to fix it.

How much will it cost to fix an overcharging alternator? Well, that depends on the problem, but you should usually expect to pay around $800 to repair it.

This guide will explain what an alternator does, how it overcharges a battery, and how to fix it.

Let’s get started.

Table of ContentsShow

What Is An Alternator?

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In very simplistic terms, the alternator is responsible for charging the car’s battery.

In real life, it simultaneously charges the battery and runs some vehicle appliances. It all depends on the instantaneous output of:

  • the battery
  • the alternator
  • the electrical draw of the car’s system

The current outputs/draw of these components are continuously varying. This can make charging the battery quite difficult – should it be sending 1A or 30A, for example?

If the charging current is too little, the battery won’t recharge while you’re driving. This might lead to a battery light on your dashboard, or, sometimes, you won’t notice anything until you turn the car off. It then won’t start the next time you come to it.

If the charging current is too high, the battery will overcharge. And that’s the subject of this article. Scroll down to learn more about what causes an alternator to overcharge.

How Does An Alternator Work?

When a battery ‘creates’ electricity, it draws ions from the acid solution onto the cell plates. This creates a charge imbalance, driving the flow of electrons – in other words, inducing a current.

An alternator’s function is to reverse this chemical reaction. Its charge sends the ions back into the solution. This frees them up to react again, meaning the battery can go again and again.

Here’s an oversimplified model of how an alternator works:

  1. The crankshaft – the focal point of an engine – turns due to the standard ICE process.
  2. A pulley connects to it. To this pulley, a belt is attached. That’s the belt you can see when you open the hood. This ‘serpentine drive belt’ spins several other pulleys. One of these is the alternator.
  3. The belt spins an electromagnetic rotor within a stator housing.
    1. The electromagnetic rotor receives charge from the battery as the car starts. Once the engine’s running under its own steam, the alternator directly supplies this current. In a way, it powers itself.
    2. The stator has three sets of interlocking wiring grounded to the same point.
  4. As the electromagnetic field spins across the stator wiring, it induces a current in each of the three wire coils. As a result, you end up with a three-phase alternating current (AC).
  5. Car circuits use direct current (DC). The AC passes through a rectifier to convert it to DC.
  6. Finally, a car’s circuit runs at about 14 V when the engine’s on. If left unchecked, the alternator would produce far too much potential – way more than 14 V. A regulator is the final piece to the puzzle. It’s a circuit board monitoring how much current leaves the alternator. Depending on this, it adjusts the input current to the rotor’s electromagnet accordingly.
  7. As a result, the output voltage – what’s going into the car’s electrical system – can be constantly changed.
  8. This voltage is responsible for charging the battery, hence its name: the charging voltage.
    1. Depending on the vehicle’s overall electrical load, it also runs certain appliances.

Alternator Overcharging Symptoms

Malfunctioning Electronics

car headlights

If the alternator is overcharging, excess current and voltage flow into the system.

This puts too much ‘pressure’ on the appliances. You’ll start to notice bulbs and fuses blowing due to this energy increase.

You’ll notice flickering in the lights that don’t all-out break. That’s visible evidence of the extra potential passing through them.


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If you’ve just replaced your battery, you might notice the new one dying not long after. This could also be caused by a number of other things, such as parasitic drains.

Some batteries could swell, crack, and leak their acid solutions. This will only happen in unvented batteries. These days, most are vented models, so you should be okay.

When an alternator overcharges the battery, the acid and water solution could boil. Because of this, it won’t function so effectively at producing current. It becomes a sort of choke point in the car’s overall circuit.

Check Engine Light

How To Reset A Check Engine Light

Your car’s battery light might come on. Might. You’re more likely to see the Check Engine Light.

Manufacturers often link the red battery warning light to a low electrical output. An overcharging alternator results in a voltage that’s too high, so the battery light might not come on.

An OBD II diagnosis will probably reveal a P2504 code: ‘Charging System Voltage High.

How To Check If Your Alternator Is Overcharging

You can check for an overcharging alternator using standard equipment available over the counter.

Specifically, you’ll need either a multimeter or a multi-function battery tester.

Use A Multimeter To Check For An Overcharging Alternator

How Long Does It Take To Charge A Car Battery While Driving
  1. Set the multimeter to 20 V DC.
  2. Switch the engine on with the car in Park, and the parking brake applied.
  3. Touch the red multimeter probe to the battery’s positive (+) red terminal. Touch the black probe on the negative (-) battery terminal.
  4. Under most normal running conditions, you should be seeing about 14 Volts. It might vary between 13.5 V and 14.5 V. These readings are perfectly acceptable and mean your alternator isn’t overcharging.
  5. If you’re getting a consistent reading of 15 Volts or higher, most would consider the alternator to be overcharging.
    1. If the potential only temporarily flickers above 14.5/15 Volts, it’s probably fine.
  6. Turn the multimeter off and switch off your engine.

Use A Battery Tester To Check For An Overcharging Alternator

Battery Capacity Tester For Car

Several battery testers have built-in features to test alternators.

Ensure you buy a good quality battery tester with alternator testing enabled.

  1. Leave the car switched off. Ensure it’s in Park with the parking brake turned on. Open the hood.
  2. Attach the clips to the battery terminals. Remember, red (+) connects to red (+); black (-) connects to black (-).
  3. Follow the instructions to start the battery testing process. This should let you know the battery’s condition and how many Volts it holds.
  4. After the battery check, follow the instructions to start an alternator check. Wait until it tells you before you turn the engine on.
    1. Some battery testers will also measure the effectiveness of the starter motor.
  5. The battery tester will let you know the alternator’s output. If it’s too high, it’ll tell you.
  6. Disconnect the battery tester, turn off the engine, and close the hood.

Causes: Why Is My Alternator Overcharging?

Here are a couple of the most likely reasons for your alternator overcharging.

Of course, it could be a complex, in-depth problem. It’s recommended to take tricky electrical issues straight to an automotive electrician.

Regulator Making Alternator Overcharge

The regulator is actually a circuit of its own. It’s essentially a sensor keeping a constant eye on the alternator.

Any issue with the regulator could mean the alternator’s charging output is too high. (Equally, a problem could lead to low outputs, too.)

Suppose the regulator incorrectly identifies the output voltage as being lower than it actually is. In that case, it’ll increase what it sends to the rotor. That leads to more output voltage and, thus, overcharging.

Issues with the regulator could encompass internal circuit board problems. There could also be a corrosion complication, producing a faulty ground connection.

Either way, the alternator will need to be removed from the car for an in-depth diagnosis if you want the regulator fixed. In most cases, replacing the entire unit will work out cheaper.



The ECU and the regulator are basically the same in modern cars. You could definitely consider the ECU the primary component responsible for voltage regulation.

The problem could lie in a malfunctioning ECU. It only takes a slight surge or water intrusion to do some severe damage.

You might need some ECU reprogramming or even a replacement. Expect this to be very expensive and well into the thousands of dollars.

How Do You Fix An Overcharged Alternator, And How Much Is It?

Mechanic using tablet to fix car at the repair garage

The steps for fixing an overcharged battery are pretty simple. Take your car to a mechanic or automotive electrician.

Don’t work on this yourself unless you’re comfortable with electrical systems. They can be dangerous and even fatal, not to mention damaging to your car, if you make a mistake.

In almost every case, you’ll need a new alternator and probably a new battery.

As a vague average, these two jobs will total around $800. It could be $250 higher or lower.

You could ask a mechanic to repair your alternator rather than replace it. This would be more environmentally friendly and very good of you.

But it’ll most likely cost you much more. That’s because it takes much longer to disassemble, repair, and rebuild something than to switch it out for a new one.

In more extreme (but thankfully rarer) circumstances, you might need a reprogrammed ECU. That’s alongside a new alternator and battery, unfortunately.

If you need ECU work alongside part replacements, the total costs might reach $2,000.

Alternator Overcharging: Conclusion

Mechanic Using Digital Multimeter

A multimeter or battery tester will confirm whether your alternator is overcharging.

If it is, take your car along to a shop and get it fixed immediately. The longer you leave it, the worse the damage will get and the more it’ll cost.

In most situations, you’ll pay around the $800 mark to fix an overcharging alternator.

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Benjamin Kitchen

Ben is an IMI-qualified light vehicle technician from England with experience in a fast-fit garage. He aims to help drivers worldwide with common automotive problems. You’ll often find him working with his 1.2 Vauxhall Corsa. It may have a tiny engine, but in eight years it's never once let him down!