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This article will examine why your car battery might smell like rotten eggs,  whether it is dangerous, and what you can do to remedy the situation. 

Further on, we’ll examine other symptoms of a bad battery that, taken together, will indicate to you with more certainty whether the battery is bad. We will also examine what other car components produce a rotten egg smell.

Briefly, though, before we go into detail.  

Car batteries can smell like rotten eggs. This smell happens when the battery overheats and sulfuric acid (H2SO4) is converted into Hydrogen Sulfide (H2S); this is due to the battery not holding a charge given to it by a battery charger or the alternator. 

A Rotten Egg Smell Under The Hood

When a  car battery goes bad, it can emit Hydrogen Sulfide. Anything sulfur-related is often described as smelling like rotten eggs.

To add to the problem, Hydrogen Sulfide is colorless, making it hard to detect.

What’s concerning about the gas is it’s very poisonous when breathed in.

Add to that the gas is flammable too. Hydrogen Sulfide gas doesn’t easily disperse either.

Even a few breaths of it can cause confusion and a loss of taste and smell. This makes it potentially deadly if the main sense – smell – that would warn you of this danger is now not working correctly.

Even providing ventilation may not be enough to remove a pool of gas. As it’s heavier than air, it may still sit below a window level and not be dispersed if indoors. Always best to open a garage door if you suspect it’s present.

In liquid form, sulfuric acid is created when the battery goes bad. This can burn the skin and, if got in the eyes, can cause blindness.

What Should You Do If Your Battery Smells Like Rotten Eggs?

  • Open the garage door or the windows straight away.
  • Leave the garage and if you feel unwell, seek medical attention. 
  • Once you’re confident, the smell has gone, get your battery replaced as soon as possible.

There is no point in delaying changing your car battery as the situation will not improve, and you are putting yourself at further risk by not changing it.

There is one other occasion when you may smell rotten eggs coming from your car battery, which is when you are charging it.

Can Overcharging A Car Battery Cause It To Smell Like Rotten Eggs?

Yes, this can happen when there isn’t enough water covering the plates inside the battery cells. It could also be that the battery is just old, and the plates can no longer store the chemical energy required to charge your battery.   

During this heating process, the sulfuric acid within your battery turns into a gas form of hydrogen sulfide, which is the egg smell you could be smelling if you are charging your battery.

The battery can swell in extreme circumstances. 

Rotten egg smell battery being overcharges

Old-style battery chargers couldn’t be adjusted and always gave the same charge to a battery regardless of what condition the battery was in. Modern battery chargers can identify how much charge is inside a battery and adjust the charge to ensure it does not overheat. 

Other Signs of A Failing Battery

Battery Warning Light Stays On

When you turn the key to the first position, your battery light should illuminate with the other lights on the cluster. However, once you turn the key a second time and start the car, the battery light should go off after a few seconds. 

If it fails, the ECU – the car’s computer – lets you know that the battery isn’t charged adequately.

When you look at this symptom added to the rotten egg smell, you can be confident that your battery is bad and needs to be replaced as soon as possible.

Bloated Battery Case

A bloated battery is a sure sign that it is bad and needs replacing. Sometimes batteries can be repaired, but it needs to be changed once it gets to this stage. 

The chemical reaction taking place within the battery is causing this to happen.  

The battery cannot store the charge given to it by the alternator or a battery charger because the plates within the positive and negative terminals are already corroded or don’t have enough water and sulfuric acid within them to hold the charge.

Lights Very Dim When Engine Off

Quite an obvious one, but ensure your engine is switched off when you do this check. If you don’t, you may see the alternator working correctly and keeping the lights nice and bright.  

When the car is switched off, the alternator stops working, and the battery takes over. If you see dim lights or lights that dim very quickly, this is your sign that the battery may need replacing. 

It could be the alternator; however, if you smell rotten eggs, this is another sign that the battery is bad.

Difficulty In Starting

This is the most obvious symptom of a bad battery for most people. Are you turning the key to your car in the morning and either getting a click or the car cranking very slowly? 

If this happens with a bad egg smell, it points to the battery being bad and not an alternator issue.  

Can A New Battery Smell Like Rotten Eggs?

All Modern car batteries are sold fully charged, so if you have been charging your car battery, it could be that you are overcharging it. 

If you are not recharging the battery and have just had it fitted and get a rotten egg smell, take it back to where you bought it and ask them to check it out or replace it.

New car batteries normally have a warranty for at least 12 months and many for 24 months, so even if your battery is not brand new, within this period, a car shop or battery manufacturer should still replace a battery for you free of charge.

Many visitors also read this article: How Long Does A New Car Battery Last? 5 Reasons It Dies Early

Other Car Parts That Can Smell Like Rotten Eggs

If you have no other bad battery symptoms listed above, it may be worth considering other vehicle parts that can smell like rotten eggs.

Catalytic Converter

Catalytic converters are part of the exhaust system and often produce a rotten egg smell. It’s quite normal for them. 

Despite what many others say, a slight eggy smell does not mean you have to replace your catalytic converter. 

This smell can be more apparent when a person drives short distances and never puts the pedal to the metal. 

Many visitors also read this article: New Catalytic Converter Smell – Dangers, Causes and Fixes

Driving more aggressively once in a while will blow out the particles that can start to clog your catalytic converter. The sulfur egg smell you may notice is the remanence of the combustion process in your engine that the exhaust is trying to remove from your car.

Old Transmission Fluid

If you maintain your car well, this is unlikely to cause the smell. Like any fluid, whether it be coolant or oil, the additives within the fluid can separate away from the oil itself over time. 

Transmission fluid can leak and let air into the system, it also goes black when it’s old.

It can also thicken up. 

It can smell like rotten eggs too.

To check, gently open your transmission fluid reservoir. If it looks black and thick and you get an egg smell, you have found the issue. Transmission fluid is cheap and is a job most DIY mechanics can do themselves. 

Fuel System Problems

Gasoline and diesel, by nature, contain hydrogen sulfide. It is an essential part of their makeup that helps fire the fuel inside the engine to get the car going.

When there is a breakdown in the engine’s efficiency to burn fuel, an egg smell can emanate from the front of the car.

Two main components of the fuel system can cause the smell.

Fuel Rail Sensors

When the fuel rail sensor goes bad, they may misinterpret the pressure and give an incorrect reading to the car’s computer, the ECU. This may cause the car’s engine to receive more fuel than it can burn efficiently. 

An inefficient engine will leave partly combusted gases that smell like rotten eggs coming from the engine or further down in the exhaust.

If your car is less efficient and shows signs of jerkiness and lack of acceleration, these are other common symptoms of bad fuel rail sensors.

Dirty Fuel Filter

The fuel filter sits within the fuel tank itself or outside the fuel lines. Its job is to catch impurities and imperfections in the fuel from the engine. When these become broken, they can let impurities through into the engine and stop the efficient combustion of the fuel and oxygen mix. 

 A bad fuel rail sensor is more likely to fault the fuel system than a dirty fuel filter.

Air Con Blocked

An egg smell coming from the aircon is also common. In spring, when the aircon might be switched on for the first time in months, it’s not unusual for the smell of rotting leaves and pine needles to come rushing through the vents. This is more commonly described as a musty smell though.

In Conclusion

A rotten egg smell indicates your battery needs replacing. The sulfur rotten egg odor indicates a leak in your battery. One of the most obvious symptoms that a car battery is failing is leaking battery acid. Hydrogen sulfide gas, which has an odor similar to sewage water, or rotten eggs,  is produced while a lead battery is deteriorating.

However, If you smell it when you’re charging the battery, it may be that too much charge is being put into it, and the plates can’t cope. If you stop charging right away and adjust the charge, you might be able to save it.

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