My Car Battery Smells Like Rotten Eggs! Why Is That?

If your car battery gets older then the battery likely tends to give off bad smell like rotten egg.

Have you ever had an experience where at times your car battery smells like rotten eggs? If you have, be warned. It is one of the signs that your car battery is in trouble. Let me give you the reasons why you get that unpleasant smell from your car battery.

Why does a battery emit such a disgusting smell like that of a rotten egg? Your lead-acid battery contains a solution made up of water and sulfuric acid called the electrolyte. Because of age and wear some of the water evaporates and upsets the solution causing it to boil and the battery overheats. The unpleasant smell comes from the fumes generated by an overheated battery or boiling electrolyte. A fetid odor like that of rotten eggs is one sure sign that you have a problem with your battery.

When the battery is overcharged it produces a gas called hydrogen sulfide. This is a flammable and toxic gas which is colorless but smells like rotten eggs. Hydrogen sulfide is naturally produced when organic matter in sewers and swamps breaks down. Hydrogen sulfide is also present in natural gas, volcanic gases and dirty well waters.

Since it is weightier than air, hydrogen sulfide concentrates at the base of a sealed container such as a car battery casing or in spaces that are not well ventilated. Such a concentration is a danger to health and could be lethal when inhaled for an extended period of time. It becomes dangerous since at first it is not so perceptible, and as time passes, the sense of smell is desensitized making potential victims oblivious of the danger.

3 Reasons Why A Car Have That Stinky Smell Of Rotten Eggs

The more recognizable reason why your car has that fetid odor of rotten eggs is, as already mentioned above, a probable problem with your car battery. In addition to this, there are also other reasons why you get that unpleasant smell, particularly in relation to the catalytic converter.

1. A crack in the catalytic converter

A catalytic converter is a component of the emission control system that modifies exhaust gasses to lessen harmful emissions. A damaged catalytic converter is unable to process the exhaust gasses, thus the rotten smell of eggs.

2. A faulty fuel flow sensor

A fuel flow sensor measures and controls the flow of fuel to the engine. When it is damaged it cannot function properly. As a consequence, too much fuel flows into the engine. The fuel is in excess of what the catalytic converter could deal with and process, thus the buildup of exhaust byproducts. When the byproducts accumulate, it flows to the tailpipe untreated and produces that stinky odor.

A faulty sensor can also cause the catalytic converter to overheat by allowing too many byproducts to flow into it. An overheated catalytic converter releases the unpleasant odor of a rotten egg.

3. A build-up of sulfur and hydrocarbons

At times, a buildup of sulfur and hydrocarbons can develop within the catalytic converter, When you rev the engine of your car, this deposit burns up and discharges a strong sulfur stench that is similar to the stench of rotten eggs.

If the smell goes away as you drive, nothing much is wrong with your car. But if the stench lingers or is present even during normal operations, it is time to have your emission control system checked.

Signs Of A Problematic Battery

Besides the stinky smell of rotten eggs, your battery also gives out other signs that it is not as healthy as it used to be. Check out these warning signs so you don’t get caught unaware.

1.Sluggish engine cranking

When you crank up the engine and it takes time to fire up or you need several attempts before the engine comes to life, the most probable culprit for it is your battery. Slow cranking could also be due to a damaged starter, or problem with the charging system. But when it happens early in the morning, no doubt, it’s your battery. Either your battery is weak or dying, or there is a parasitic draw that is draining your battery.

2. The appearance of the “Check Engine Light”

One of the reasons why the “Check Engine Light’ icon on your dashboard appears or lights up is when your battery is weak. Although it goes on to warn you of other troubles such as a fixable problem of a loose or missing fuel cap, it may also be calling your attention to more serious trouble such as a malfunctioning catalytic converter that can do a lot of damage to your car. Still, one primary reason why the warning goes on could be a weak battery.

3. Low electrolyte level

The average level of the electrolyte in the battery cells is around half an inch above the top of the battery plates. If upon checking, you observe that the electrolyte level is below the normal and the heads of the plate are exposed, you need to have the battery tested. A drop in the electrolyte level could damage the plates and cause the early death of your battery. Low electrolyte level is most often caused by overcharging.

4. A swollen or bloated battery case

A bloated or swollen battery casing is a reminder to have it already replaced. It could cause a leak of the fluids which is a danger to both persons and property, in addition to reducing the effectivity of the battery and shortening its life.

5. Old age

A three-year-old battery is old since batteries typically gets a life expectancy of three to five years. But factors such as extreme weather, bad driving habits, frequent short trips, overcharging or undercharging can cause your battery to die prematurely. Once your battery hits the three-year mark, prudence dictates that you have it checked and tested regularly.

If you like to learn more about the symptoms of a bad car battery, then read this article.

Effect Of A Bad Battery On Other Engine Parts

A weak or bad battery won’t affect the performance of the car or engine but it will undoubtedly have an adverse effect on the healthy parts of your car. Since the battery is not performing at its best, the healthy parts of the car compensate for the loss of efficiency and becomes subjected to more stress.

A defective or flawed battery which is not charging correctly can affect the alternator and voltage regulator. Since the demand for power from the alternator of a weak battery is enormous, it decreases the system voltage so that the voltage regulator has to compensate by delivering more power to the system through the rotor.

While on idle mode, the alternator is unable to generate the required current. As such, the system voltage decreases further and the voltage regulator delivers the maximum power via the rotor.

This situation of a minimum-speed, maximum-load is what wears down the voltage regulator and the alternator. The high demand for power because of a weak battery causes the rotor, slip rings and brushes of the alternator to overheat.

On the other hand, the minimum-speed limits the cooling produced by the built-in fan, aggravating the overheated condition and eventually wearing out the alternator and voltage regulator.

Other parts that are adversely affected by a weak battery are the charging system, starter solenoid or starter motor. These parts can be impaired because they are straining to draw more voltage to make up for the lack of power from the battery.

How To Tell If The Problem Is The Battery Or The Alternator

When your car refuses to start, the question that comes to mind is, is it the battery or the alternator? Fortunately, there are simple tests that will give you the answer to this question.

Start with the battery by checking the battery gauge in the dashboard. You don’t need to start the car to know if the battery is still providing a charge. If the light of the gauge is dim, something is going wrong with your battery.

Another test is to turn on the automatic window, lights, and windshield wipers then turn off the engine. See to it that all these accessories are off when you switch off the engine. Then make a second attempt of starting your car.

If the engine stays dead and won’t start, jump-start it. Run the motor for a little while, then turn it off again. That the engine starts with a jump-start and runs is a sign that the alternator is working and is charging the battery while the engine is running.

Turn off the switch and start again. If it still refuses to start, chances are your battery is the culprit It cannot hold a charge without the help of the alternator.

If your car starts, that means that your battery is working. Now, it’s time to check on the alternator. Look for the following signs which indicate a bad alternator:

● Headlights that shines bright when you are accelerating but dim when you slow down and idle on a stop sign of a traffic signal. Dimming is an indication that the alternator is not keeping the battery sufficiently charged.
● Interior lights that shine bright as you run the car but gradually dim. This is another sign that the alternator is faulty.
● A growling sound before the problem started is a precursor of a failing alternator.
● The smell of hot wires or burning rubber could be coming from an overheated alternator.

If you are interested in learning more about this topic, then read this article on bad alternator vs bad battery.


When a car battery smells like rotten eggs, the first thing that you have to consider is that it may be caused by a defective battery. Although other causes can bring about such fetid odor, it’s the battery that gets the blame first because it is precisely how a weak or dying battery smells.

Consider the foul odor as a warning signal and do what needs to be done to try to prolong the life of your battery. A weak battery is not exactly a dead battery, but it will get there soon if you don’t do anything.