Alternator Overcharging: Should You Be Worried Due to This?

Alternator overcharging can be a mere indication of a much bigger problem. There are only two solutions to this problem: repairing or replacing the alternator. 

Comprehensive Guide to Fix Alternator Overcharging

We will discuss this in detail below, including the cost of replacing the alternator. We will also tell you whether you should continue driving with a defective alternator, plus other crucial information you’ll be glad to discover. 

How Can You Fix Alternator Overcharging the Easiest Way?

To fix an alternator overcharging problem, you must remove and have the alternator repaired by replacing the built-in voltage regulator (VR). That’s because an alternator overcharge will happen if the VR is faulty. However, installing a new alternator may be a more feasible alternative. 

The alternator is one of the essential components of a vehicle. It’s responsible for charging your car battery and powering all its electronics. Like any other part of the vehicle, the alternator is a wear-and-tear item. Its voltage regulator can break anytime without warning, resulting in the alternator overcharging your battery.

The best solution to this problem depends on whether the VR is built-in inside the alternator or is mounted separately in the car engine compartment. It’s simple to fix the overcharging problem using the five steps below if the regulator is outside the car’s alternator. 

But first, you must have the following tools:

  • A wrench
  • A new voltage regulator
  • Screwdriver
  • Battery tester or multimeter

 – Disconnect the Battery

You will typically find the battery under the wood, directly behind the car’s grill on either the left or right side. Using the wrench, remove the two cables connected to the battery terminals.

Detach the positive cable (has a + sign) by loosening the cable’s nut. Turn the nut anti-clockwise. Repeat the process for the negative terminal. 

 – Locate the Voltage Regulator

Vintage or classic vehicles mostly have their voltage regulators in a black plastic box mounted to the back of the alternator.

The exact location may vary, depending on your make and model. Please consult the repair manual for more specific details. 

 – Disconnect the Old Regulator

Unplug each cable from the old defective regulator and plug it into the new regulator. You might have up to four cables attached with screws.

So, loosen the screws using a screwdriver. Next, fit the cables into their corresponding screws on the new regulator. Tighten the screws.  

– Fix the Battery Cables

Replace the cables to the battery and tighten the nuts with the wrench. Ensure you match each cable to its corresponding terminal, negative to negative and positive to positive.

– Test the Battery

You must test the battery to ensure that the alternator overcharging issue is resolved. The problem should disappear if the new voltage regulator is working correctly. Check out how to test alternator overcharge in our next section below.

Modern car alternators have built-in voltage regulators. To fix such a regulator, you would need to open the alternator. That requires advanced knowledge of electrical systems, a task that may be far beyond the skills of a DIYer. Instead, you can take your car to a local repair shop for assistance. 

Test the Battery to Ensure Overcharging Issue Is Resolved

Although it’s possible to refurbish an overcharging alternator at a lower price, doing so may be unreasonably costly in the long run. It can cost you a lot of time, and a repaired alternator will likely develop another mechanical issue requiring another repair. That may be why the practice of repairing alternators is gradually becoming a thing of the past. 

You may temporarily fix the problem by manually cutting the positive wire that supplies power to the alternator’s field windings and then putting a switch in between to interrupt the overcharging. Unfortunately, that’s also a shortcut that can cost you big time! 

The most feasible solution for a bad alternator is to get a new one. Even in most garages, some technicians will directly swap the whole alternator with a new one.

How to Know the Cost of an Alternator Replacement?

To know how much it costs to replace an alternator, you must identify your car’s make, model, and manufacture year. The price to replace the component varies, depending on these factors. On average, expect to pay about $200, the part and labor inclusive.

Here’s a breakdown of alternator replacement costs. 

Buying a new car alternator alone can cost as high as $200. The lowest price for an alternator is about $100, and the highest is $500. Replacing the component can be challenging for many as it requires extensive automotive knowledge. That means you should prepare to hire a professional. 

Depending on location, labor costs vary; expect to pay about $100 to $200. In addition, you must set up to $50 aside if the serpentine belt needs replacement. The overall cost of replacing an alternator runs from $250 to $750. 

How Does an Alternator Charge a Car Battery?

An alternator charges a car battery when the car engine runs, and spins the alternator, generating alternating current. A stator converts this current to a direct current, which the vehicle needs to power its electronics. A regulator regulates the voltage amount the alternator produces with the varying engine speed. 

Let’s a breakdown of how the alternator works.

The alternator works by using a rotor (electromagnet) that spins inside a fixed part with copper wiring referred to as the stator. The wiring on this stator is put through an alternating current (AC) when the rotor spins inside. 

Alternator Charges a Car Battery when The Car Engine Runs

The vehicle engine spins the alternator’s rotor through a shaft connected to its accessory belt via a pulley. That means the alternator consistently creates a current as long as the engine runs. Since the alternator produces AC but the vehicle needs direct current (DC), a component referred to as a rectifier comes in play. This part converts the AC into the DC that the vehicle can use.

The car alternator also has a regulating system that regulates the amount of current the electromagnet receives and the magnet’s strength. That’s because the engine speed is subject to change. When the engine spins faster, the alternator will generate a higher voltage than needed. Without regulation, the alternator can damage the vehicle’s electronic system, including the battery. 

How Can You Identify an Alternator Overcharging?

To identify an alternator overcharging, you must watch for the following signs: irregularly high voltage readings, shortened bulb life, and check engine light. Other common symptoms of an overcharging alternator include frequent fuse blowing and battery abnormalities, such as swelling and cracks due to gas buildup. 

Here’s a detailed version:

  • Irregularly high voltage readings

One common sign of a bad alternator is the irregularly high reading on the car’s instrument cluster voltage gauge. A reading of 15 volts and above is abnormal and should be a concern.

Your vehicle may display a warning message when the voltage readings get unreasonably high. 

  • Shortened bulb life

Sometimes, you can know that something is wrong when the vehicle’s headlight, tail light, and fog light bulbs need repetitive replacement. In this case, you will notice that bulbs blow more frequently. It means that the bulbs are receiving too much voltage than necessary. 

  •  Battery abnormalities

You might notice that your battery case suddenly swells or is hot to the touch. An under-hood inspection may also reveal issues like cracks in the battery case due to gas buildup and loss of electrolyte fluid inside.

That may also be accompanied by a sulfur-like smell coming from the battery. The gas buildup could even cause the battery to burst. 

  •  Check the engine light

Although the red battery warning light might come on, manufacturers usually link it to a low electrical output.

Since an alternator overcharging generates too high voltage, the battery light will probably not come on. Instead, you’ll be more likely to see the check engine light come on

  •  Frequent fuse blowing

Another common symptom of an alternator overcharge is frequent fuse blowing. The fuse is only set to allow a certain voltage limit to pass through. Past this limit, it will most likely blow.

A blown fuse can cause many problems, including the alternator’s inability to charge the battery. An OBD2 scanner can help you accurately diagnose and confirm that the alternator is behind a power issue.

How To Diagnose an Alternator Overcharging?

To diagnose an alternator overcharging, you must have a multimeter or a battery tester. Manufacturers usually set the car batteries to operate between 12.6 and 14.2 volts. An alternator is overcharging if the battery voltage reads 15 volts or higher. 

Diagnose an Alternator Overcharging

Here are the steps to check for alternator overcharge using a multimeter:

  1. Set your multimeter to 20V DC.
  2. Park the car, apply the parking brake and turn on the engine. 
  3. Touch the multimeter probes to the corresponding terminals of the battery, positive to positive and negative to negative. 
  4. As mentioned, you should see readings on the multimeter of about 14 volts. Acceptable readings vary from 13.5V to 14.5V. 
  5. If you get a consistent reading of 15V or higher, the alternator is overcharging! 

You can also use a good-quality battery tester to identify an overcharging alternator. Here’s how to go about it:

  1. Park your car and open the hood. 
  2. Attach the tester’s clips to the battery terminals. Ensure you match the terminals correctly. 
  3. Follow the directions to test the battery to know its condition and how many volts it holds. 
  4. Next, follow the instructions to check the alternator. The tester will help you know the alternator’s output and take action accordingly.  


1. Can You Drive With an Overcharging Alternator?

Driving with an overcharging alternator may lead to costly consequences in the long run. For example, the overcharge will damage the battery if you don’t take action quickly. Sustained operating of a car above its intended operating voltage can greatly affect the car’s electronic system and, consequently, your driveability. 

So, we don’t recommend continuing to drive with an overcharging alternator. The longer you drive your vehicle in this state, the higher the possibility of costlier damages. It would be best if you had the cause of your vehicle’s overcharging problem diagnosed and repaired as soon as possible.  

2. What Really Causes an Alternator Overcharging?

The cause of an alternator overcharge is mostly a defective voltage regulator. As its name suggests, this component regulates the alternator’s voltage output and prevents the generation of excess voltage. 

Your alternator operates under a manufacturer-specific voltage regulator. As mentioned, the regulator is mostly incorporated into the car alternator itself. The component, in most cases, ensures that the alternator delivers a consistent charge rate between 12.7 and 14.5. Failing the regulator means that there’s no voltage control.

Although the alternator can fail in a way that prevents sufficient voltage production, voltage regulator failure mostly causes an overcharging problem. Check out how to diagnose such a condition below. 


After reading our comprehensive guide, you now know the steps to fix an alternator overcharging.

Here is a summary of what we have covered:

  • An alternator overcharging is mostly a result of a defective voltage regulator.
  • The only way to fix an alternator overcharge issue is to replace the faulty regulator or buy a new alternator altogether.
  • Buying a new alternator is more feasible than repairing the old one.
  • An alternator replacement by a professional’s costs $250 on average.
  • Continuing to drive with an alternator overcharging can have serious consequences.
  • An alternator is overcharging if the battery voltage reads 15V or more!

You have all the information you need to resolve your alternator overcharging issue. So, gather the tools to get started or get the assistance of an expert. 

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