What Causes an Alternator To Overcharge?

The alternator, sometimes called a generator, generates electricity to charge the battery and power all the electric auxiliaries while driving. Without a functioning or with an underperforming alternator, the car will drain the battery after no more than a couple of miles of driving. But just like the alternator can undercharge the battery, it can also overcharge it, which can cause significantly more damage.

The most likely causes of alternator overcharging are a faulty voltage regulator, which is directly responsible for the alternator’s voltage output at any given moment. However, it could also be damaged alternator wiring, corroded battery ground, or an old and damaged battery. 

why is my alternator overcharging
Alternator Overcharging Voltage: 15 to 19 volts

Alternator Overcharging Meaning

The alternator overcharging means the alternator sends more voltage to the battery than it needs and more than it’s designed to handle. When the engine is running, it’s drawing current to power the engine and whichever electric auxiliary you turn on, be it lights, the AC, or seat warmers.

To compensate for that extra voltage draw and to charge the battery, the alternator should be making a steady 14 to 14.7 volts instead of 12 volts which is what the car’s electrical system is. The alternator also has systems in place that keep the voltage constant regardless of engine speed or the voltage draw.

However, if that system is faulty or something is sending the wrong signal, the alternator will start sending more voltage than the battery or the electric system needs. That can be anywhere from 15 to 19 volts. And that’s what’s called an alternator overcharging problem.

What Causes an Alternator To Overcharge

A couple of things can cause the alternator to overcharge, but in the vast majority of cases, it’s a faulty voltage regulator. Still, it’s worth checking the wires and paying attention to how long ago you got a new battery. That said, here is what causes alternator overcharging.

Voltage Regulator

The voltage regulator does just what the name implies; it regulates the voltage being produced by the alternator. In other words, it’s an alternator controller. The voltage regulator gets a signal from the battery telling it how much voltage it’s picking up. Then the voltage regulator either increases or decreases the alternator output to maintain a charged battery.

However, if the voltage regulator fails, the alternator will start working at maximum output, which is significantly more than the battery can handle and enough to cause damage to other electrical components.

Damaged Wiring

We already mentioned how the voltage regulator gets a signal from the battery, telling it how much voltage it needs. Well, if the signal wire is broken,  corroded, or damaged,  that signal will be incorrect or nonexistent. That leads to the same outcome as if the voltage regulator was faulty.

Furthermore, you also want to check the battery ground wire for corrosion and damage. If the battery ground isn’t good, the voltage signal will drop. In turn, the voltage regulator increases the voltage output to abnormally high levels. 

Old Battery

First of all, we should mention that a standard lead-acid car battery lasts between three and four years before it starts to show performance issues. So, if that’s the amount of time that has passed since you have had a new one, the battery should be your main focus.

Now, when the battery gets old or damaged, it can’t hold voltage anymore. Meaning no matter how much you charge the battery, the voltage can’t pass 12 or even 11. Now, that 11 or 12-volt signal is being translated to the voltage regulator, which amps up the alternator output in an effort to charge the battery. But since the battery can’t hold a charge, that results in a permanent overcharging condition until you replace it.

Alternator Overcharging Symtpoms

In case your car has a battery voltage gauge, then that’s your most apparent sign the alternator is overcharging because the reading will be too high. But since most cars don’t have such a gauge, you may notice the lights are brighter than usual and that the light bulbs last much shorter than usual.

Furthermore, you may notice a strong sulfuric smell in the cabin (a smell of rotten eggs) which is a sign that the battery is overcharged to the point of leaking acid. But before that happens, the battery will overheat after a short drive, and its case will bulge on all sides.

Also, you may notice a check engine light, battery warning light, and a whole slew of different electrical gizmos, such as infotainment issues, for example, shutting down randomly. Also, some simpler electrical components like power windows may stop working. 

And lastly, you might hear a high pitch buzzing when you start the engine, which increases and decreases with engine speed. The buzzing will also be the most noticeable when the radio is on. 

Alternator Overcharging Symptoms – List

  • Battery voltage gauge readout is high
  • Lights brighter than usual
  • Light bulbs burn out too often
  • A sulfuric smell in the cabin while driving
  • Battery overheating
  • Battery bulging
  • Check Engine Light
  • Battery warning light
  • Infotainment system issues
  • Random electrical problems
  • High-pitch buzzing when driving

How To Fix an Overcharging Alternator

To fix an overcharging alternator, you first need to diagnose what’s causing it to overcharge in the first place. The first course of action is to inspect all the wires, starting with the battery ground strap and then the voltage regulator connector. If you find that the battery ground is corroded or damaged, clean it with a steel wire brush or replace it. The same goes for the voltage regulator connector. 

But if all wires look good, we would advise you to take your car to a local AutoZone store, where they will test both the alternator and the battery for free. After that, either replace the battery or the voltage regulator. Replacing the voltage regulator is often easy enough where all you need to do is remove two bolts and don’t even have to remove the alternator.

But in some cases, it’s cheaper to replace the entire alternator because the labor cost of replacing the voltage regulator outweighs the cost of a new alternator. That’s because, in some alternators, the voltage regulator is integrated, so the whole unit needs to be disassembled. But it’s best to consult your local mechanic on that matter to see what your best course of action is.

alternator output voltage
Alternator Output Voltage: 13.5 to 14.5 volts

How Much Does It Cost To Fix an Overcharging Alternator

We already mentioned all the alternator overcharging causes, so we will divide this into three parts,  one for the battery, one for the voltage regulator, and one for the wiring. But keep in mind that the costs to fix each issue can vary widely between different car models. Also, most of the cost estimates are coming from yourmechanic.com, repairsmith.com, and autozone.com. 

Cost To Fix a Voltage Regulator

A brand-new voltage regulator for virtually any car that’s older than ten years costs between $20 and $50. Unfortunately, the labor cost is much more difficult to estimate and can be anywhere from $50 to $150, which makes the average cost of replacing the voltage regulator $135 all in. 

However, if the voltage regulator is integrated, the only alternator overcharging battery fix is to replace the entire unit. That can be anywhere from $250 for a small hatchback to $600 for a full-size truck and even up to $1,000 for a newer premium European brand. But on a positive note, that’s with parts and labor included.

Cost To Replace a Car Battery

Depending on the size and production year of your vehicle, a new battery can cost anywhere between $70 and $250. To be more specific,  smaller cars need smaller batteries, so the price will be lower. On the other hand, vehicles with a start/stop system and hybrids need AGM or EFB batteries which are significantly more expensive than conventional lead acid ones.

Still, some stores like NAPA will take your old battery and give you a discount on a new one, plus they will replace it for free in all circumstances.

Cost To Fix Battery and Voltage Regulator Wiring

Fixing the wiring is the cheapest job on this list, and you can easily do it at home. If your battery ground is damaged, you can buy a new one for under ten bucks and replace it in a couple of minutes. On the other hand, if it’s corroded, buy an electrical contact cleaner spray and steel wire brush to clean it, both of which will cost no more than a couple of bucks.

Next, if the voltage regulator connector is corroded, use a small wire tube brush and, again,  some contact cleaner spray to clean it. However,  if the wires are broken, it’s best to take the car to a professional to fix it because soldering is necessary. But even that shouldn’t be more than $50 to $70.

What Happens If You Don’t Fix An Overcharging Alternator

The potential cost of not fixing an overcharging alternator is far greater than it is to fix it, regardless of what’s causing it. First of all, an overcharging alternator will kill the battery in a matter of a couple of weeks or sooner, depending on how much you drive. 

But more worryingly, all that excess voltage is also going to all the car’s electrical systems, which is a huge problem in modern cars with ECUs and multiple control modules. Given enough time, the overcharging alternator will start destroying those modules resulting in thousands of dollars worth of damage. Not to mention, the car leaves you stranded at least once before that happens.

alternator low voltage
Alternator Low Voltage: 12.5 volts or less


Q: Is 15 volts too high for an alternator?

Yes, 15 volts is too high for an alternator. Although there may be some cases where 15 volts is the upper limit, anything over 14.5 volts is generally overcharging, and anything above 14.7 volts is a cause for concern.

Q: Can you drive with an overcharging alternator?

Technically, yes, you can drive with an overcharging alternator. However, by driving a car in such a condition, you are risking battery damage, as well as frying all or most of the vehicle electronics and control modules.

Q: Can a bad battery cause overcharging?

Yes, a bad battery can cause overcharging. A bad battery that doesn’t hold charge will send a low voltage signal to the voltage regulator and thus increase the alternator output to abnormal levels, in other words, cause it to overcharge.

Q: Are 14.7 volts too high for an alternator?

No, 14.7 volts isn’t too high in most cases, but in all cases, it is just slightly over the upper limit. If you see 14.7 volts on your multimeter, it’s a good idea to have the charging system and the battery inspected.

Q: Can bad ground cause an alternator to overcharge?

Yes, the bad ground can cause an alternator to overcharge. A bad ground will lower the battery voltage signal going to the voltage regulator, which results in a needlessly high alternator output.

Q: Can an overcharging alternator drain the battery?

No, an overcharging alternator cannot drain the battery unless it causes some damage to it, after which it can’t hold a charge anymore, but that’s another matter. That said, a bad alternator diode can drain the battery even with the engine off.

Q: What voltage is too high for an alternator?

As a rule of thumb, anything over 14.5 volts is too high for an alternator. However, different cars have different charging system setups and requirements, so in some cases, even 15 volts can be normal. That said, unless you know what your specific alternator should be making, you should have the charging system inspected if it’s anything above 14.5v.

Final Words

At the end of the day, if your alternator is overcharging, it’s more than likely caused by a faulty voltage regulator. However, if it’s been a couple of years since you last replaced the battery, it’s a good idea to test it before you do anything else. And before you replace the voltage regulator, inspect all the alternator wiring and the batter ground strap for any damage, corrosion, or missing insulation.

Ibro Cehic

Ever since I was bitten by the automotive bug during early childhood I was obsessed with cars. My first driving experience came when I was ten and I already started tinkering with cars and motorcycles at thirteen. So, right from the beginning, I knew my life would revolve around cars, even if I wasn’t sure how that would happen. And today, thanks to my second passion, writing, I get to share my love for automobiles with other enthusiasts through my articles.

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