Car Battery Smells Like Rotten Eggs And Sulfur

If your car battery smells like rotten eggs and sulfur, then it means there’s definitely a problem. And if you don’t get the issue fixed, the problems will only get worse. In this article, not only will you find out why your car battery smells like sulfur, but you’ll also learn whether it’s safe to drive, how harmful the fumes are, and most importantly, how to fix it!

So keep reading to find out everything you need to know!

What Causes A Car Battery To Smell Like Rotten Eggs?

There are actually three reasons that your car battery is beginning to smell like rotten eggs or sulfur.

The Battery Is Not Getting Charged For Long Enough

The first reason is if the battery isn’t getting charged for long enough. When car batteries are working, they produce lead sulfate. However, as the battery starts to recharge, the lead sulfate will dissolve, and that’s the end of it.

However, when your battery isn’t getting charged enough, the residue doesn’t dissolve completely. Instead, it ends up sticking to the plates of your battery which then produces a bad smell. But it’s not just the smell. The more sulfate residue there is, the shorter your batteries lifespan and capacity will become

Your Battery Is Getting Overcharged

However, more commonly, the smell of rotten eggs and sulfur can also be caused by overcharging your battery. When your battery is overcharged, it will release more sulfate, which in turn reacts with the air causing the smell of rotten eggs as well.

The Battery Is Frozen

And lastly, while it’s not as likely. The rotten egg smell could be occurring because your battery has frozen. When the battery freezes, it may start to vent gas, which in turn causes the rotten egg smell to appear.

Of course, this is a much simpler issue to fix. You’ll need to make sure that the battery has fully defrosted. Once it’s fully defrosted, you can then begin to charge it with a battery charger. Just make sure that the battery is in good order first (no cracks or broken areas).

Battery charging cables transferring power to a dead battery.

But WHY Does Overcharging Cause The Smell To Occur?

Now you know the reasons why your battery might be smelling like rotten eggs, you may still be wondering why the smell occurs. Well, here’s a simple breakdown.

Batteries can only convert so much energy at a time, but the less charged the battery is, the more it will be able to convert. So a battery that is 10% charged will convert a lot faster than a battery that is 90% charged.

However, you also have to consider the amount of power that is going into the battery as well. When you’re charging your battery and too many amps are going in, then it results in the battery becoming overcharged.

As a good rule of thumb, you should keep the peak charging amps about 10% of less of the total amp hours of the battery being charged. However, you can even go lower than this if you want! Doing so will greatly reduce the chance of your battery becoming overcharged.

And when the overcharging occurs, more hydrogen sulfide is released, which of course, results in the rotten egg smell.

How Do You Get Rid Of The Rotten Egg Smell

If your battery smells like rotten eggs, clearly there’s an issue, and it’s up to you to fix the issue! Of course, the best thing you can do is take your vehicle to a mechanic to make sure you avoid violating any warranty terms. However, if you’re determined to do it by yourself, then you can try to desulfate the battery.

This can be a slow process, though, in some cases taking up to 3 weeks to work. And the cost of a new battery compared to the equipment you’d need to desulfate your current battery may make buying a new one more cost-productive.

However, if you want to desulfate your battery, here’s what you’ll need to do:

Gather Your Tools

The first thing, before anything else, is to gather all the tools you’re going to need to desulfate your battery successfully. The tools you need include:

  • A voltage meter
  • A current meter
  • A car battery charger
  • A 2Ohm resistor
  • Two wires with alligator clamps at both ends
  • Battery electrolyte
  • A 400w DC lamp or 400w AC power inverter

Desulfating The Battery

Once you have the following, you can now desulfate the battery. Here are the steps you’ll need to take:

  1. Take the battery out of the car.
  2. Once the battery is out of the car, connect the positive battery head to the resistor with the alligator clamp.
  3. Then connect the resistor to the current meter and then finally to the positive end of the charger.
  4. After this, connect the negative head to the negative end of the charger.
  5. Turn on the charger at 51 volts. The current must show a number close to zero (however, it’s still fine if it’s not absolute zero).
  6. When the current becomes stable (around 10-20 amps), you can just leave it to charge for the next 48 hours.
  7. After the 48 hours are up, disconnect the charger and then use the voltmeter to see if the voltage has improved. If it’s up to 12 volts, then it has; however, if it still hasn’t worked, then you’ll need to charge it for another 48 hours.
  8. (If after another 48 hours the battery still isn’t working, then it’s done for, and you’ll just need to get a new one. Store the battery for 24 hours; if the voltage has dropped below 11 volts, then you should also dispose of the battery)
  9. If the battery has remained at 12 volts, you’ll need to discharge it and then recharge the battery to make sure its cells are recovering effectively. The best way to do this is to leave the headlights on until the battery is down to 11 volts and then charge it back up to 12 again.
  10. You’ll notice that the more you discharge and recharge the battery, the longer the headlights stay on for. Once you’re satisfied with the improvement, you can end the process!

How To Stop Your Battery Smelling Like Rotten Eggs In The First Place?

Of course, it’s always better to prevent something from happening rather than fixing it when it has. So here are some helpful ways to make sure you’re car battery doesn’t end up smelling like rotten eggs in the future.

Make Sure You’re Checking The Battery Regularly

First of all, you should make sure you’re checking the battery regularly and carrying out maintenance like topping up the electrolytes when necessary. You should also be servicing your car regularly to make sure nothing is wrong with it in general.

Don’t Leave Your Car In Cold Conditions

Sometimes there’s nothing you can do about this, but it’s best to avoid leaving your car out when the temperature is below freezing. Doing so will prevent the battery from freezing and reduce any damage that could end up being caused to it.

Buy The Right Charger

If you’re going to buy a charger for your battery, then you should make sure you’re buying the right charger. Smart chargers are great because they help prolong your battery’s life by making sure the correct amperage is entering the battery.

As well as this, they also tend to come with desulfation and recovery modes, so you can restore your battery even if it’s been damaged.

Make Sure The Alternator Is Working Correctly

You should also make sure that the alternator is working correctly. If the alternator has a problem, then it may end up overcharging your car, which will result in a build-up of hydrogen sulfide. Which, as you now know, causes the rotten egg smell.

Is It Safe To Drive A Car That Smells Like Rotten Eggs?

If your car smells like rotten eggs, then you should avoid driving it and replace the battery as soon as possible. As you know, the rotten egg smell is caused by the battery being broken or dying. So if you continue to drive when it smells like this, then you may end up getting stranded somewhere when the battery fails completely.

Is The Rotten Egg Smell From A Car Battery Dangerous?

While the rotten egg smell that comes from your car is caused by hydrogen sulfate (Which is extremely toxic), if you only smell it briefly, then it’s not going to do any harm to you. And if it’s just coming from a car battery, it’s more likely that you’ll only end up suffering from nausea, headaches, and sickness if you spend a prolonged amount of time around it.

However, it’s important to note if you’re exposed to high concentrations of the gas, it’s definitely able to kill you, so you should always avoid the smell whenever possible.


As you can see, if your car battery is smelling like rotten eggs, then that’s not a great sign. However, if you catch it early enough and try to recondition your battery, it’s entirely possible that you’ll end up fixing the battery. And in cases where the battery can’t be fixed, the good news is replacing it isn’t going to be overly expensive.

If you liked this article, make sure you check out the rest of the website. Otherwise, have a great day!

Leave a Comment