If your car battery looks like this, shut your car off immediately and step away from your car.
Redditor /u/One-Platypus3455 shared a concerning yet educational video to the /r/MechanicAdvice subreddit earlier today, showing the OEM battery on his 2020 Honda Accord venting a mysterious gas that emitted a rotten eggs smell.
OP’s thread is linked here, with his smartphone video below.
“This was from yesterday,” OP mentioned in the comments.
“My dumb a *** was driving (75 miles in all), thought the air from outside was just polluted…I went through a drive through and when I went through the local McDonald’s drive through, I smelled a terrible odor, and it smelled like rotten eggs. As the smell progressed, the car started revving itself slightly to ensure it didn’t turn itself off. Got home, lifted the hood and saw that.”
His short, 13-second video shows his car battery venting a gas from both sides.
And, as mentioned, it smelled like rotten eggs.
When installed properly, car batteries are one of those set it and forget it type of car parts you really only need to replace typically every 3-5 years.
Except collecting dust, car batteries should not make any noises or emit any kinds of gasses and vapors like OP’s battery did.
Battery acid, the stuff inside a battery, is made of between 30-50 percent H2SO4, better known as Sulfuric Acid.
When the wrong type and/or amount of current goes to a battery, that battery acid can heat up beyond its boiling point (638 F to be exact) and can convert Sulfuric Acid to H2S AKA Hydrogen Sulfide (what’s responsible for that rotten egg smell.)
What most likely happened here is his alternator is shot, and it’s not converting Alternating Current to Direct Current (AC to DC) which is the exact type of current a car battery uses.
There are two things OP and anyone in this situation can do.
If your car is under warranty, or you don’t mind spending on a mechanic to get your car fixed, get your car towed to the repair facility of your choice and tell them the symptoms that you saw.
Most likely, they’ll recommend getting the alternator and battery replaced. If you want, you can source your own replacement parts if you’d rather not pay dealership prices.
The alternative solution is to definitely NOT drive with this car battery anymore. Instead, wait for it to cool, put on eye and skin protection (glasses and gloves), and remove the cooled battery.
Don’t just replace the battery as, if you don’t fix the cause of your boiled battery, you’re going to ruin the new one, too.
I’d recommend asking people on make and model specific car forums and Facebook pages if you need to be pointed in the right direction.
Do your homework, find out the best way to find out the root of the problem (which, most likely, will be replacing the alternator, but don’t take my word for it), and fix that issue first.
Places like Autozone, O’Reillys, and etc. can test your alternator for you.
You don’t want to use those external battery charger alternator testers as those need your car running to check which, you do not want to do with a boiling battery.
Beyond a potentially explosive situation with the car battery, hydrogen sulfide in general is nasty stuff to breathe in.
It can cause lung irritation and other respiratory issues if breathed in for an extended period of time.
So, if you ever observe your car battery boiling and emitting gasses, shut it off immediately and wait for it to cool before proceeding.
A venting battery is serious business.