Rotten Eggs smells and a hot 12-volt battery

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smytherrrs

New member
Joined
Nov 6, 2021
Messages
3
I saw another post about a “sulfur smell” and hot twelve volt battery, which the owner replaced, so I am curious how common issues are with the 12v battery…

On my drive home today with the windows down, I noticed a distinctive smell of rotten eggs. The natural gas-powered city bus next to me was surely to blame, but it did not dissipate as I got closer to home.

I pulled in the garage, closed the garage door, and almost puked at how pungent the smell was. I got under the car and noticed some steam behind the passenger-side front wheel and small drips of clear liquid every couple of seconds from a tube that appears to be a vent tube for the 12v battery. Fearing the smell or a potential fire would ruin my garage, I moved it outside and went back inside.

After 20 minutes, I heard a distinctive BMW car alarm going off. I walked out and found the car battery mostly dead but the siren blaring and lights on, albeit fairly dim. The key fob didn’t work to unlock the car or silence the alarm. I used the physical key to open the door. The electric hood release didn’t work, so I had to pull the plastic door off below it and pull the cable to manually release the hood. I disconnected the high voltage battery with the sort of fuse box at the front of the hood but didn’t touch the 12v since it is a Saturday night and BMW Roadside Assistance didn’t have any “master technicians” available to confirm it was safe to do so. The car alarm went off after an hour or so, despite the 12v battery appearing to have some juice left, evidenced by the flashing red light under the rear view mirror.

I am anxiously awaiting a tow and very happy I have a month left on my Roadside Assistance package. I am speculating that i3’s have a fairly primitive system to charge the 12v with the high voltage battery and mine is the original 12v from 2016. I speculate the 12v needs to be replaced every 3 years or so to ensure the battery doesn’t degrade as mine apparently has. I will report back with the service center findings!
 
I don't understand how they can design a car that relies so heavily on a stupid 12v auxiliary battery in such a negative way. It boggles my mind what the engineers were thinking.
 
Hello smytherrs –

Wow, I've not seen anyone post about this failure before!

What year is your i3 and is the 12V battery original to the car? They usually last about five years.
 
Arm said:
I don't understand how they can design a car that relies so heavily in a stupid 12v auxiliary battery in such a negative way.
It seems ludicrous that I should need to use my physical key to access the car when the high voltage battery has at least 60% charge.

frictioncircle said:
What year is your i3 and is the 12V battery original to the car? They usually last about five years.

It is a 2017 i3 REx, which I bought used with ~20k miles in December 2019. I haven’t had the 12v replaced, so I am 90% certain it is the original battery, which is coming up on 5 years of age.

A picture of the vent tube that was dripping/steaming:
7264241-D-CCC2-4979-97-B5-75-F1194-A34-F5.jpg
 
My BEV is 2014 with in service date Dec 2014. The original 12v battery last till this June. I changed it on June 2021 with Odometer at 61K miles. There is a vent hose that I needed to disconnect and reconnect during the replacement, but I did not trace the vent hose connection to see where it leads to. The issue that prompted me to change the battery was the Comfort Access no longer works by touching the door handle to lock or unlock, but using the key fob's button is fine. Then within two days, the car displayed Excessive discharge upon startup, but I was still able to drive it multiple days without getting disabled or anything. I decided to change it before it got worse. The old battery measured at 11.70v upon removal from the car, there is definitely no swelling or other physical deformation seen from the outside. I bought the replacement battery from remybattery.
 
keepgoing said:
Then within two days, the car displayed Excessive discharge upon startup, but I was still able to drive it multiple days without getting disabled or anything. I decided to change it before it got worse.

Smart! I had the excessive discharge warning, but it wasn’t the severe type of warning that would have led me to believe I needed to stop driving and seek service immediately. I drove it for a week or two before this happened. Excessive discharge warnings should probably be serviced with some immediacy.
 
smytherrrs said:
Excessive discharge warnings should probably be serviced with some immediacy.
True, but the service might be merely charging a discharged 12 V battery rather than replacing it. The excessive discharge warning occurs due to a low 12 V system voltage which could be due to a good 12 V battery not being charged sufficiently. This happened with our 2014 i3 with its original battery last January. Due to the pandemic, our i3 had been driven much less than previously allowing 12 V vampire loads to gradually discharge its 12 V battery. After fully charging the battery with a battery charger, this warning hasn't been displayed since, and the 12 V battery has been been behaving normally.

So rushing to replace a 12 V battery when an excessive discharge warning is displayed might be unnecessary and wasteful.
 
Hello everyone!

I wanted to share our story about the failed 12v battery since it was scary. We have a 2018 i3. A few weeks ago, we also noticed the "rotten egg" smell in our garage. We didn't know where the smell is coming from at the time (there was no check control messages in our bmw related to the battery). After a couple of days, the smell got so strong, and it started leaking inside the house--you could barely stay inside the house. We thought that we had a gas leak and called the gas company. In turn, they called the fire department. After 20-30 mins of investigating the situation, it turned out that it was the 12v battery of our i3. Apparently, when the battery gets old and needs to be replaced, if you try to recharge the battery, it starts boiling and produces hydrogen sulfide gas, which is extremely flammable and poisonous. The levels in our house were 4x higher than normal. We had to evacuate our family in the middle of the night, and the fire department had to blow out the house with giant fans for an hour to get the toxic gas out. We were told that such levels of hydrogen sulfide are not only unhealthy but can also easily lead to a fire. We were lucky that we were still awake that night, and we acted fast.

The problem is that the car didn't give us any check control messages that there is an issue with the battery (acid leaking). We simply had no idea, so hopefully, if you experience something similar, you should know that it's likely the 12v battery leaking acid (when you try to charge the car). Rather, everything happened rather suddenly and could have had catastrophic consequences. In my mind, this is a serious safety issue. I think there were two opportunities to prevent this catastrophic failure. First, the car should give you a warning that the battery is leaking and needs to be replaced. Second, the issue occurs only when the 12v battery is being charged, so the car could have a safety fail and stop charging the battery if a leak happens.

We called BMW of North America. At first, they were very apologetic for the situation, promised that they will cover all the cost of the battery replacement, look into the issue, etc. The battery was replaced at a local dealership. BMW then asked us to sign a non-disclosure agreement and never talk about the accident with anyone if we want them to cover the charges (close to $900). I felt uncomfortable doing this since I believe that the issue is not just the battery but the failed safety system of the car that can potentially endanger the life of others. I asked them if the car has any fail safety measures to prevent a catastrophic failure of the battery like this one in the future. The people at the dealership were quite incompetent and didn't have an answer. But the BWM representative suddenly started lying and changed his story once I said that I will not sign the non-disclosure agreement. He said that we received a check control message, so it was our responsibility to take care of the battery. I took a video before the car was towed, and the diagnostic that was run on the car by the BMW dealership clearly shows that there were no check control messages related to the battery (only low tire pressure).

In any case, this has been a very frustrating experience for us and I wanted to share it here in case you experience something similar.
 
botzata said:
Apparently, when the battery gets old and needs to be replaced, if you try to recharge the battery, it starts boiling and produces hydrogen sulfide gas, which is extremely flammable and poisonous.
There's no liquid sulfuric acid in an i3's standard AGM battery, so there's nothing to boil. The sulfuric acid is absorbed in glass fiber mats which allows an AGM battery to be mounted in any orientation, even upside-down, because there's no liquid electrolyte to leak out. However, if the sulfuric acid is above 60º C which is below its boiling point, hydrogen sulfide gas (H2S) could be produced when a battery is being charged.

A bit of a mystery to me is why the battery would be charging while an i3 is parked. Maybe the battery's voltage dropped below 12.0 V when the DC-DC converter could automatically turn on to charge the battery. Also, why would the battery be above 60º C? Maybe a partial internal short circuit, high internal electrical resistance, an abnormal electrochemical or chemical reaction, or something else could raise the temperature above 60º C. In any event, the battery has failed and is dangerous, so it needs to be disconnected and replaced ASAP.

An AGM battery is sealed, but each cell has a pressure relief valve that would open if enough H2S, H2, or SO2 is produced. With the original AUX18L battery, all cells vent to a common vent tube to which is attached a hose exiting under an i3. Those who replace the original battery with an ETX18L power sports battery won't have this vent hose directing the H2S, H2, SO2, and other gasses out of the frunk which could allow them to accumulate. An ignition source in the frunk could then ignite the flammable gasses which could result in an explosion. Fortunately, an ignition source in the frunk is probably fairly unlikely.

botzata said:
The levels in our house were 4x higher than normal. We had to evacuate our family in the middle of the night, and the fire department had to blow out the house with giant fans for an hour to get the toxic gas out. We were told that such levels of hydrogen sulfide are not only unhealthy but can also easily lead to a fire. We were lucky that we were still awake that night, and we acted fast.
Yikes! Thankfully, nothing worse happened.

botzata said:
The problem is that the car didn't give us any check control messages that there is an issue with the battery (acid leaking). We simply had no idea, so hopefully, if you experience something similar, you should know that it's likely the 12v battery leaking acid (when you try to charge the car).
No leaking could occur unless the battery had been replaced previously by a flooded-cell battery which is unlikely. There's no temperature sensor on the battery that could have warned you about an overheating battery. About the only warning that could be displayed is when the battery's voltage decreases below a certain level or maybe decreases too rapidly. I and many others have seen such a warning message which could indicate a failing battery or a battery that just needs charging.

botzata said:
Rather, everything happened rather suddenly and could have had catastrophic consequences. In my mind, this is a serious safety issue.
Fortunately, the battery failure mode that you experienced seems to be pretty unusual, but certainly not unheard of, and could happen in any vehicle with a lead-acid battery. I'm not aware of any vehicle having sensors that could have warned you about the problem before it became dangerous.
 
Hello botzata and welcome to the forum!

Thank you very much for signing up here to post such a thorough account of your 12V battery issue. I've been here a while and yours is only the second time I've seen this issue (thanks alohart for directing everyone to this thread).

I would be completely freaked out had something similar happened to me.

In particular I'm very disappointed in BMW NA putting an NDA under your nose before they'd cover your expenses. Granted, you're probably outside of warranty and whatever they covered under "goodwill" (honestly, I've found their post-warranty goodwill to be very helpful over the many years I've been a customer) wasn't too much out of their pocket, but the fact that the BMW rep lied to you when you declined to sign? That's dirty business.

I wouldn't doubt that NHTSA or a pro-consumer organization (Consumer Watchdog, etc.) would find your story interesting.
 
Arm said:
I don't understand how they can design a car that relies so heavily on a stupid 12v auxiliary battery in such a negative way. It boggles my mind what the engineers were thinking.

Well the high voltage battery is over 200 volts and the semiconductors in the car like a voltage of 12 volts or less. While the high voltage battery is turned off, the circuits in the car rely on the 12 volt battery to keep their state and to provide the accessories you like.
The better question is why they made it so small and difficult to replace. The rectangular bucket in the the frunk is a better place to put a larger 12 volt battery.
 
Arm said:
I don't understand how they can design a car that relies so heavily on a stupid 12v auxiliary battery in such a negative way. It boggles my mind what the engineers were thinking.
All EV's have a 12 V battery that performs the same functions as that in an i3. The engineers were thinking that eliminating the 12 V battery and powering everything with the Li-ion battery pack whose voltage can be as high as 400 V could be deadly. Keeping a DC-DC converter running continuously to reduce the high voltage to ~14 V to power the many 12 V loads would discharge the battery pack faster than most people would accept. No engineer has a design that could safely eliminate a 12 V battery in an EV and that would be acceptably efficient.

The 12 V battery in every ICE vehicle could fail in the same ways as that in an i3, yet a 12 V battery has been used for over a century because the probability of a really dangerous failure is quite low.
 
weajd said:
The better question is why they made it so small and difficult to replace. The rectangular bucket in the the frunk is a better place to put a larger 12 volt battery.
Because the DC-DC converter turns on to provide up to 2.5 kW of 12 V power whenever the 12 V battery's voltage drops below a certain limit, the 12 V battery doesn't need to have a large capacity unlike an ICE vehicle whose 12 V battery must be powerful enough to power a starter motor in very cold conditions and to provide power when its engine isn't running. A large, heavy battery would be a waste in a car designed to be as light as possible.
 
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