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Why Car Battery Smells Like Rotten Eggs? Is It Dangerous?


If you are into automobiles and their components, you must have experienced different odors from each auto part. But no odor can compete with the stench when the car battery smells like rotten eggs.

Basically, this irritating smell erupts when you overcharge a lead-acid battery. The smell of rotten eggs is due to the chemical compound hydrogen sulfide (H₂S). It’s a flammable gas that might be dangerous for your eyesight and respiratory system if inhaled in greater concentration.

Overcharging of the car battery is one reason for that pungent smell. Besides, there are other reasons as well with solutions that we’ll cover in this automobile guide.

What Makes My Car Battery Smell Like Rotten Eggs?

Generally, that happens when the battery of your car is overly charged. During the charging process, a battery converts electrical energy into chemical energy. But when the concentration of the chemicals exceeds the limit, you start getting a strange odor similar to rotten eggs or garlic.


Besides, overcharging of the car’s battery breaks down an important chemical Sulfuric Acid (H₂SO4). As a result, you get the smell of rotten eggs from your car.

When we talk about a car’s battery being overcharged, there are two meanings.

  • Over-Sized Charger
  • Equalization Mode

Over-Sized Charger

If you are using an oversized charger to charge your car’s battery, you might get the smell of rotten eggs.

We are talking about the difference in the current or amperes going into your car’s battery. Suppose your car has a 12-volts battery that can store up to 600 amps of current. Now, the charger you are using to give juice to your battery is bigger than the actual car’s battery.

Technically, you should choose a charger that’s no bigger than 20% of the battery’s total amperage.

Now, the overcharging will cause disruption in the flow of current inside the car’s battery. If you don’t take notice, you will see the car battery smoking and smells like rotten eggs.

Therefore, it’s important to choose the charger that’s 20% less than the battery’s current capacity.

Equalization Mode

In equalization mode, the voltage flux becomes too great that the chemical concentration at the bottom becomes greater than at the top. This phenomenon is known as stratification.

Moreover, the equalization mode gives an additional charge to the car’s battery which also becomes the reason for battery overcharge. Thus, you might again get the smell of rotten eggs from your car’s battery.

The above two factors are clear reasons why your car’s battery gives off such a pungent smell. However, there are secondary reasons as well.

Other Reasons for Rotten Egg Smell in Car

Defective Charger

The charger may follow the 20% amperage rule, but you are still getting that smell. Why is that so?

The first thing that pops up in mind is the charger might be defective.

If you frequently read and watch automobile battery reviews, you must have heard that never to use a defective charger to charge your car’s battery.

Here are the reasons:

  1. A faulty charger can destroy your car’s battery life because it cannot provide consistent current to your car’s battery.
  2. The flow of current fluctuates from the charger, which again negatively affects the performance of the car’s battery.
  3. A defective charger slowly charges the car’s battery, increasing the average charging time.

The above-mentioned reasons also make the car’s battery release different gases, including hydrogen sulfide. The battery might also heat up, which is quite dangerous if you don’t take preventive measures.

car battery checking

Old Charger

The term “Old Charger” refers to the charger’s technology to charge a battery.

Old battery chargers don’t have a microprocessor controller. Therefore, you never know when the current flowing from the charger to the battery has exceeded the threshold.

If the charger keeps on charging the battery, you will start getting smoke and the smell of rotten eggs.

Now, what does a microprocessor controller or a microcontroller do?

You can set a timer while charging the car’s battery using a microcontroller battery charger. That timer might give you two of the following options:

  • Set charging based on charge stages.
  • Set charging based on time.

Since we are talking about automobile battery chargers, most microcontroller battery manufacturers provide the first option.

You can also check out the 4 amp automatic microprocessor controlled battery charger/maintainer for your car.

The charge stages are divided into three parts:

  • Bulk
  • Absorption
  • Float

Bulk Charge Stage

The first or the Bulk Stage is responsible for about 80% of the overall recharging. In this stage, the current constantly flows (if the charger only gives constant current). As a result, the voltage increases.

Moreover, if you use a properly-sized battery charger, it will only provide as much current as accepted by the battery’s capacity, i.e., 20% of the car’s battery capacity.

Absorption Charge State

The Absorption Charge stage deals with the remaining 20% to fully charge the car’s battery.

In the absorption stage, the charger holds the voltage (typically between 14.1-14.8 VDC, which depends on the charger’s already set points). It decreases the current until the car’s battery is fully charged.

Moreover, you might also read Absorption as the Equalization stage.

So, if the charger doesn’t hold up the voltage, your car’s battery might get overcharged, which will give the smell of rotten eggs.

Float Charge State

The final or the Float Stage while charging your car’s battery reduces the voltage between 13.0-13.8 VDC. Moreover, this voltage limit is kept constant as the battery is almost fully charged.

In the float stage, the charger reduces the current flow from 20% to less than 1% of the battery’s total current capacity.

In this mode, you can maintain a fully charged battery.

Short-Circuit Inside the Battery

This reason is critical and might cause a little explosion. Moreover, you don’t need to be driving or charging the battery. The short circuit can happen inside the battery due to old wiring, chemical leakage, rusting, or severe high or low temperature.

The short-circuit happens to the lead plates inside an automobile battery. So even if the battery is just sitting there without any external force acting upon it, you might start getting the smell of rotten eggs.

Therefore, it’s important to take quick notice once you get that odor from your car.

Why the Smell of Rotten Eggs from Battery?ReasonsSolutions
Over-sized battery chargerThe value of the flowing current is greater than the overall capacity of the battery’s amperage.Use a properly-sized charger having a current flow of 20% of the actual battery’s amperage.
Equalization ModeThe voltage goes beyond the charging threshold and the battery heats up.Use a microcontroller charger to limit the voltage flux.
Faulty ChargerThe charging of the battery becomes unstoppable or fluctuating, destroying the car battery life.Change the charger at once.
Old ChargerCharging doesn’t stop after 100%, affecting the battery’s temperature and performance.Use a microcontroller automobile battery charger to set charge stages.
Short-CircuitInternal short-circuit due to rusting, severe temperature, or overcharging.Test the performance of your car battery’s cells and replace the battery if needed.

As you have seen, the reasons for the rotten egg smell in your car are obvious. Therefore, let’s move onto the solutions.

How to Fix Rotten Egg Smell in Car

Where is the Smell Coming From?

First of all, locate the source from where the smell is coming. Sometimes if you have left your car ideally for too long, the battery might get cracked due to severe temperature.

Moreover, the cracking of the battery might also allow the sulphuric acid to seep into the metallic components. After that, the reaction between the metal and the chemical gives rise to the smell of rotten eggs.

Therefore, you should bring the cracked battery to the nearest automobile service. They will tell you whether the damage is permanent and you must buy a new battery.

In a similar case, when you leave your car ideally for months, always check the connections of your car’s battery before giving it the ignition.

Even though you might not smell any pungent odor, follow this method as a safety precaution.


Get Rid of Rotten Egg Smell from Your Car’s Battery

An automobile battery contains sulfuric acid, making the whole battery work properly. Therefore, the concentration of sulfuric acid must be balanced.

Moreover, the concentration must be higher at the top portion of the battery than at the bottom.

Now, if you are getting the smell of rotten eggs from your car, it’s to get rid of that mess.

  • First, take an old toothbrush and dip it in baking soda or vinegar (liquid form.) That chemically reacts with hydrogen sulfate and kills the pungent odor.
  • Now, clean off the corroded spots from where the smell is coming from. You might have to focus on the terminal side of the car’s battery. That’s usually where the acid gets in touch with the metallic components.
  • After thoroughly cleaning the battery, rinse the battery with fresh water.
  • Wait till the battery is dried.
  • Use petroleum jelly to coat each part of the battery before reconnecting the wires. This will make sure that each circuit is intact for a smooth flow of current.

Once you are ready, give the ignition and see if there is any kind of smell left in your car.

Now, that smell might also come from auto-parts other than the car’s batteries. Therefore, let’s find out more about that issue.

Clean the Catalytic Converter

The catalytic converter is a useful component in your vehicle. It converts the toxins and other harmful substances into safer byproducts. The common catalytic converter’s byproducts are”

  • Water
  • Carbon Dioxide

If the catalytic converter gets faulty, you might receive strange smells in your car. Moreover, one of the smells might be the smell of rotten eggs.

Now, to identify whether the converter is faulty, take your car to the nearest maintenance shop.

Usually, the converter gets full of carbon deposits if you don’t maintain it over time. When you start the engine, those deposits burn up and produce hydrogen sulfide gas. That gas travels from the car’s exhaust system, and you get to smell the odor of the rotten eggs.

Unfortunately, if the catalytic converter is faulty, you have to replace that with a new one.

Check out the inlet/outlet universal catalytic converter with O₂ port.

Transmission Fluid Leakage

If you haven’t brought your car to the service center for a year or two, you might get the smell of rotten eggs due to the transmission fluid.

Your car’s internals age with time. If you don’t regularly visit the car maintenance, the transmission fluid line might get a leakage.

Now, this leakage momentarily increases your car’s temperature. If you are driving on a neat, straight road, the temperature might go high for a short period of time.

However, if you drive on inclined terrain, the transmission fluid runs faster than in normal conditions. The RPMs shoot up, and the car’s temperature increases. That’s one of the reasons why your car shows abrupt change in the temperature reading.


Also, the metallic components are running at their maximum speed while driving on steep hills. Their temperature also heats up due to intense activity.

Thus, the transmission fluid breaks down due to heat into sulfur compounds, including hydrogen sulfate and gives the smell of rotten eggs.

The leakage must be patched permanently to fix the leaking of the transmission fluid. This solution is a helpful one. However, that patch might open up again if you forcibly accelerate your vehicle on inclined terrains.


The sulfur smell from car battery comes when the battery’s sulfuric acid gets in touch with the metallic component. If you are experiencing that, your car’s battery might be leaking. Moreover, the leakage may happen due to intense weather conditions and the battery’s internal short-circuit.

You should take quick action and fix the battery problem in both cases.

Although the smell of rotten eggs is not that dangerous, it can become lethal if taken in higher concentrations.

Therefore, always be wary of your car battery’s performance and incoming smell, and never miss the regular vehicle examination to avoid any bigger problems.

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