Alternator Smoking When I Hook Up Battery [5 Easy Fixes]

Alternators fail from time to time but seeing smoke coming out of them is a rare sight. When it does happen, it usually occurs when you connect the battery to your vehicle. So, are you wondering – why is my alternator smoking when I hook up battery?

There are many potential reasons behind it. You’d have to diagnose it, find the underlying issue, and take corrective actions to fix it. Here are the main reasons behind it:

  • Installing The Battery Backwards
  • Faulty Alternator
  • Bad Alternator Grounding
  • Shorted Battery
  • Corroded Cables
alternator smoking when I hook up battery

Keep reading this guide about smoking alternators and batteries to learn all about these issues and their fixes in detail. So, let’s dive right in!

Alternator Smoking When I Hook Up Battery [5 Easy Fixes]

Let’s look at the main reasons why you get a smoking alternator when you connect it to the battery.

car alternator smoking

Note: You can read our guide on why hooked battery up backwards now car won’t start.

1. Installing The Battery Backwards

The number one reason behind smoke coming out of an alternator is installing the battery backward. It’s important to connect the battery properly and maintain the polarity between the positive and negative terminals to operate the vehicle smoothly.

installing the car battery backwards

It may seem like a rookie mistake but trust us – even people with decent knowledge of cars make this mistake. That’s because sometimes the positive and negative markings on the cables aren’t correct.

So, how do you figure this out? Simple – you need to follow the cables of the battery instead of the colors to know which one goes to the negative terminal and which goes to the positive. The shorter cable that connects to a bolt is the ground cable and it needs to be connected with the negative terminal.

Automatically, the other cable would be the positive cable. Remember that doing this thing can also make your battery smoke. So, if you see both of them smoking, you know what’s up.


How you can fix this issue depends on the severity of the problem. It only takes a few milliseconds for the alternator diodes to get fried when you install the battery incorrectly.

Some of the wires starting from the alternator to the starter would also be fried. So, inspect how much damage is done before coming up with a solution. At the very least, you’d have to replace the alternator and the damaged wiring.

You’d also have to replace the battery if you see smoke coming out of it. The smoke is a strong indication that the battery is also toast. If any fuses were blown, you’d also have to replace them. In all fairness, the solution varies from car to car depending on the level of damage.

2. Faulty Alternator

There are tons of issues with the alternator itself that can make smoke come out of it. Here’s a quick look at all these issues:

i. Bad Alternator Bearings

The alternator bearings are one of the most crucial components of the alternator. It spins more than a thousand times in a minute when you drive your vehicle to make sure that the alternator works correctly.

bad car alternator bearings

As your alternator ages, the lubrication of the bearings starts to wear out. When it gets too bad, the friction between the bearings rubbing together can cause smoke in the alternator. Sometimes, you’d also hear a squeaking noise as these bearings start to die out.

ii. Faulty Alternator Diodes

The rectifier in the alternator has diodes. The main job of the diodes is is to convert AC (Alternating Current) to DC (Direct Current) so that the battery and the electrical components of the car can use the energy generated by the alternator.

faulty car alternator diodes

The image above should help you understand what the diodes look like. All of them are located in the black housing and you’ll find it when you remove the cover of your alternator.

When the diodes go bad, the alternator starts sending AC to the electrical system and the battery instead of the DC. But the alternator isn’t designed to do that and it starts to get overheated. When the matter gets too worse, smoke comes out of the alternator.

There are some signs that can help you identify if you have malfunctioning alternator diodes. They make a whining sound when you press the accelerator. Your car also runs rough because of a damaged alternator diode. If you notice these signs as well, you’ve found the culprit that’s causing this issue.

iii. Overheated Alternator

It shouldn’t come as a surprise at all that a smoking alternator is the result of an overheating alternator. There are two situations that commonly lead to the alternator getting too hot.

overheated car alternator

The first one happens more often during the winter season. The vents of the alternator get filled with dirt, debris, leaves, or even snow. All of these things will block the airflow in the alternator and make it hot.

Secondly, modern alternators are equipped with cooling fans inside the alternator. It keeps the alternator cool by circulating the air sucked in through the vents to the other parts of the alternator.

The fan won’t work when the vents get blocked due to the build-up of dirt. That’s because it would have no air to circulate in the first place. But even if the vents are clean and clear of any dirt, the fan can still malfunction, make the alternator overheated, and smoke may come out of it.

iv. Wet Alternator

If your car has been submerged in water over a long period and you’ve noticed that your alternator is smoking, it may have gotten wet. Though alternators are water resistant, they’re not waterproof.

That means they would normally be fine when you drive through water but not if they’re underwater for a long time.

wet car alternator

The water gets inside the alternator this way. The temperature inside the alternator rises when you drive your car and it starts boiling the water. So, what seems like smoke to you is steam in reality.

Though it’s not a common issue, it’s still worth mentioning as this is a complete guide.


As you’ve seen, the alternator can malfunction due to a wide number of reasons. So, the fix of the alternator also depends on the issue. The one solution that will surely work here is to replace the alternator altogether.

But if you can get by just replacing the diodes or the bearings or the cooling fan, there’s no need to replace the entire alternator. You can just replace the faulty component and you’re good to go.

If the vents are dirty, you can just clean them to get your alternator to work properly again. You can disconnect the alternator and remove the water from it if it’s filled with water.

The right thing to do here is to get a mechanic to take a look and determine what has gone bad. You can just replace the faulty part that way and won’t have to pay for replacing the alternator.

Having said that, there’s no point in replacing just the bad part if you have an alternator that’s too old. If your alternator has been driven around for more than 80,000 miles, then replacing it is a good idea as other parts of it may also start to wear out soon.

3. Bad Alternator Grounding

Even if nothing is wrong with the alternator itself, bad alternator grounding can make the alternator smoke hot. Though the alternator produces the right voltage, the circuit doesn’t get completed properly because of this issue.

bad car alternator grounding

The alternators which have the voltage regulator outside might get too hot due to this issue and the diodes can get fried as well. Generally, bad alternator grounding will also blow fuses.


You need to inspect and fix the grounding issue. Make sure that you don’t have bad ground from the engine to the frame. While you’re at it, check the wire from the battery to the alternator as well and ensure that everything is good.

If anything is fried, it needs replacement. Regardless of what you do, you’d most likely have to replace the alternator as well as it has most likely short-circuited, gone bad, and started smoking.

4. Shorted Battery

Apart from connecting the battery backward, a battery that’s already shorted or dead can also lead to a smoking alternator. That’s why it’s also common for the alternator and the battery to go bad together.

dead car battery

If you use a battery that’s dead or cannot hold a charge anymore, it’s going to be an issue as well. The battery needs to be able to handle the voltage generated by the alternator.

You can easily diagnose if you have a shorted battery by performing a load test on it. Connect the battery to a multimeter and see how much voltage you get from it. If it’s around 12 volts, it means the battery is fine. But if the reading is significantly apart from the 12 volts mark, you have a short battery.

Another sign you can be on the lookout for to know if you have a bad battery is to see how long it takes to be charged. A battery that loses charge quickly also requires less time to get charged.

So, calculate how long it takes to get charged. If it’s less than what’s mentioned in your owner’s manual, then you have a faulty battery beyond any doubt.


If all the signs point to a bad battery, you have no other choice but to replace it with a new one. Make sure you use the right type of battery for your vehicle. As your alternator has also gone bad by now, you’d have to replace it as well.

Even if the entire alternator hasn’t gone bad, you’d have to replace the faulty components with it. Once you do that, you should have your car running back to normal without any smoke coming from the alternator or the battery.

5. Loose Or Corroded Cables

When any of the alternator or the battery cables get loose, it creates resistance in the flow of electricity. The alternator gets extremely hot if the battery cables are loose and it can rarely lead to the alternator smoking as well.

loose or corroded cables

Sometimes, the alternator cable will get loose and it would touch something that it’s not supposed to. That can also lead to the smoking issue. The good news it’s easy to check these issues.

A quick visual inspection will reveal if the cables are in good condition or if they’ve worn out. If they appear green or black, it means they’re corroded. Also, check physically that the cables are connected nicely and tightly.


If the cables are not connected right, all you need to do is to tighten the loose connections. If the cables are corroded, you need to replace them. Doing these things won’t undo the damage to the alternator and you’d have to replace it as well if you want a permanent solution.

What Causes A Smoking Car Battery

A smoking car battery is extremely dangerous as it’s one of the major reasons behind burnt cars. If you come across smoke coming out of your car battery, you need to know what causes them and how you can fix it.

smoking car battery

Before diving deep into this issue, remember that the fumes produced by the battery are toxic to humans. So, get away from the vehicle and wait for the smoke to go away before you attempt to solve the problem. With that said, here are the most common reasons behind it:

i). Overheating

The number one cause of smoking car batteries is overheating. That’s why the number of car batteries producing smoke increases during the summer season. You are more likely to face this issue if you live in a warmer area.

overheating car batteries

It happens because the electrolytes (the liquid part) of the battery start evaporating when it’s exposed to extreme heat. That’s why it’s never a good idea to charge the battery during high temperatures as there’s the risk of the battery being too overheated and it may explode in severe cases.

There are many things you can do to counter this issue. The first and most obvious thing you can do is to keep your car in the shade as much as you can. It won’t always be possible when you drive your vehicle. But when you keep it parked, try to find a good spot.

The next thing you can do is to cover the battery with insulated materials if your car has the space for it. But if it’s not there, the most effective solution for you would be to get a battery that’s designed to handle higher temperatures. They have better heat dissipation and they’ll work better in such conditions.

ii). Overcharging

As we’re talking about lead-acid batteries here, remember that it’s natural for the battery to produce a little gas when it’s working. But when the battery gets overcharged, it produces too much gas and the end result is smoke coming out of it. The main reason behind it is using battery chargers.

It’s never a good idea to use battery chargers unless you have excellent knowledge of them. Most chargers keep on charging the battery even if the battery is fully charged as there’s no mechanism for it to turn off. When the battery has too much charge, it releases pressure by producing smoke.

overcharging car battery

The simplest solution here is to avoid using battery charges altogether. But if you must use them, then invest in good quality battery chargers that regulate amperage and voltage. They should have the ability to stop charging the battery when the battery is fully charged.

iii). Dead Battery

Like all things, car batteries aren’t meant to last forever. If you have a really old battery and you see smoke coming out of it, it’s highly likely that it has reached the end of its lifespan. No matter what you do at that point, that battery is beyond repair and you’d have to replace it.

car old battery

You may be perplexed by how a dead battery could have started your car. But it utilized the last of its capacity to start the car. Though jumping the battery could be possible if you have an emergency to get somewhere, you need to replace the battery as soon as you can.

iv). Malfunctioning Alternator

Yes, a bad alternator and a bad battery go hand in hand. So, it shouldn’t come as a surprise that a faulty alternator can lead to battery smoking. You already know how to deal with a bad alternator as we explained what to do in the first section about smoking alternators.

malfunctioning car alternator

v). Internal Short Circuiting

It’s not common for modern batteries to have an internal short circuit but it can happen in a few cases. Start by checking the battery terminals and making sure they’re connected properly. Loose battery connections are the most common reason behind the internal short-circuiting of batteries.

The positive and negative plates in the battery may also collide and that can lead to the smoking issue. You may have to diagnose the cause of the short circuit and you’d end up replacing the battery in most cases.


Is it normal for a new alternator to smoke?

There is a little lubricant applied on alternators so that they don’t rust when stored in car shops. But it’s not normal for them to smoke. Double-check that you have connected it properly and the cables are tight.

Why is my alternator smoking when the car is off?

The alternator may have already been smoking when you were driving your vehicle but you have noticed it only after putting the car to a stop. It’s most likely due to an overheated alternator bearing.

Can a broken fuse in a car also lead to a smoking alternator?

Yes, a blown fuse can cause smoke in the alternator. Inspect the fuses and see if that’s the case. If a blown fuse caused the issue, then you’re lucky as you’d just have to replace the fuse instead of the alternator.

Can a bad trunk switch lead to a smoking alternator?

In theory, a fault within any electrical components in your car can cause a smoking alternator. So, it’s possible. That’s why it’s important that you have an experienced mechanic diagnose the issue if you haven’t found the culprit.


Why is my alternator smoking when I hook up battery? Hopefully, you know the answer to that question after completing this guide. Smoking alternators can be a huge cause of concern but if you diagnose it with patience, you’ll soon find the culprit and be able to solve the issue.

If you are unable to determine what’s causing the alternator to smoke, you have no other choice but to contact a mechanic. Let us know what caused your car’s alternator to smoke by leaving a comment below.

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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