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Car battery smells like rotten eggs? Here’s what you need to do

Car battery smells like rotten eggs? Here’s what you need to do

You get inside the car and notice the stinky smell of rotten eggs, but you’re sure you didn’t leave any eggs inside the vehicle, or nobody has thrown rotten eggs at your car. Believe it or not, the annoying odor comes from the car battery. Of course, another reason might be a broken catalyst convertor which you can detect by smelling the exhaust fumes.

What causes the car battery to smell like rotten eggs?

Your internal combustion car probably has a lead-acid battery which can produce a lot of power for starting the engine. These batteries run by exchanging electrons in a chemical reaction between lead plates, lead-oxide plates, and sulfuric acid; hence, they are called lead-acid batteries.

When car batteries discharge, they produce lead sulfate. Now, if you recharge the battery, this product will dissolve into the battery again. However, if you don’t charge the battery for long, the sulfated residue will stick to the plates forever, and guess what? This will reduce the battery’s life expectancy and capacity.

In addition, the sulfate reacts with damp air creating the pungent rotten egg smell. Overcharging and storing the batteries at high temperatures can also lead to lead-sulfate formation in your car battery.

Is it safe to drive a car that smells like rotten eggs?

Once the batteries start smelling like rotten eggs, they are sulfated so much that they can’t recharge properly, especially if you pop up the hood and find lead sulfate on the battery heads. These are symptoms of a dying battery. Therefore, once you detect the foul egg odor, you have to change the battery.

Suppose you continue to use the broken battery, although the alternator will deliver power to the battery from the car’s motion. In that case, the battery capacity is too low to provide enough energy for starting up your vehicle. Thus, you might get stranded in the middle of nowhere due to battery malfunction.

What happens if you smell the battery?

Your body has an automatic alarm that keeps you away from hazards. Therefore, if your food has gone bad, you can avoid intoxication by simply avoiding the stinky food. The same applies to car batteries. In fact, the smell comes from H2S, a byproduct of sulfate, which is fatally toxic, even in relatively low concentrations.

Don’t worry, though; the rotten egg smell is only detectable at concentrations lower than 20 ppm. At this level of exposure, prolonged exposure might cause headaches, nausea, tearing of the eyes, and of course, the irritating smell.

H2S is odorless at higher concentrations, so don’t try to smell the battery at all, as you will breathe in more of the toxic gas. Exposure to higher concentrations of the gas can cause nose, throat, and lung irritations. In addition, you can tell the battery has a problem by checking the indicator or observing sulfate on the battery heads. Sulfate is a crumbly white material that will stick to the battery heads, where it’s supposed to be a clean metal surface.

How do you get rid of the rotten egg smell?

We will cover how you can revive the sulfated battery, yet the repaired module will not fully recover to the initial capacity; we strongly recommend that you take the vehicle to fix the battery by yourself might violate the warranty terms and conditions. If your car is under warranty, you contact the manufacturer to change your battery.

However, if you drive a car with a mileage beyond warranty and want the battery fixed with minimal cost, you can revive the sulfated battery. The desulfation process might take up to two or three weeks. It requires some equipment, and it will never fully recover the battery. In addition, car batteries cost from $50 to $150, which might not be worth the time and effort. Therefore, we suggest buying a new battery; however, if you insist, follow the steps below:

Recover your car battery safely

Caution! This process involves overcharging the dead battery, possibly overheating, hydrogen ignition, and acid wounds. Check for safety instructions at every step of the process.

Required tools

First, check the following equipment:

An old battery charger with no smart microcontroller: 

  1.  The charger won’t do the trick if the controller is smart because you actually have to overcharge the battery during this process.
  2. A current meter
  3. A voltage meter
  4. An ordinary charger suitable for the car battery
  5. A 2Ohm resistor
  6. A 400W DC lamp or 400W AC with a power inverter.
  7. Battery electrolyte
  8. Two wires with alligator clamps at both ends  

How to desulfate the battery:

  1. Remove the battery from the car.
  2. Connect the positive batter head to a resistor using the alligator clamp, connect the resistor to the current meter, and then the positive end of the charger.
  3. Connect the negative head to the negative end of the charger.
  4. Turn on the charger at 51volts. At this stage, the current meter must show a number close to zero. Sometimes, the current meter shows absolute zero, but don’t worry, the numbers should increase slowly and steadily as some hours pass.
  5. Once the current is stable at 10Amp or 20Amp, let the battery charge for 48 hours.
  6. After two days, disconnect the battery from the charger safely. Check the battery voltage using the voltmeter. If the voltage exceeds 12V, continue to the next step; if not, recharge for another 48 hours. If the battery still doesn’t get to 12V, it is time to dispose of it.
  7. Store the battery for 24 hours and check the voltage. If the voltage has dropped under 11V, dispose of the battery and get a new one. If not, proceed.
  8. Now, you have to cycle the battery a few times to recover the battery cells effectively. You can discharge the battery using the lamps or the car’s headlights. When the battery voltage reaches 11V, disconnect the battery.
  9. Recharge the battery using the ordinary charger until it reaches 12.5V.
  10. Discharge it again. Every time you charge and discharge the broken battery, it should light the lamps for a more extended period. Repeat cycling the battery until cycling no longer improves battery durability significantly.
  11. Now your battery is good to go.


If your car battery stinks like a rotten egg, it is time to change the battery. The unpleasant odor is toxic at high concentrations, so avoid sticking your head under the hood and smelling the batteries. Instead, contact the manufacturer to change the battery for you. Alternatively, you can save a few bucks by desulfation the battery. However, the electrochemical process is hazardous, time-consuming, might revoke the car’s warranty. At the end of the day, it won’t fully recover the battery.


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