Odd Exterior "Burnt" Odor assc. w/ 12v Batt or Heater / Precon...

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eNate

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So to cut the quick, my '21 was producing an exterior odor I associate with a burned soup pot (like from my restaurant days, when the double boiler ran dry of water). BMW changed the 12 volt battery and so far the problem is gone.

Of course, the details:

I noticed this odor after the car climatized for departure on chilly days. Once while charging, twice while not. When I turned off climatization during my final charging session before taking it in, there was no odor.

I initially noticed the odor but didn't think it was coming from the car. It could have easily been an environmental odor such as from roofers or asphalt paving, in that sort of "hot tar" realm. But I eventually linked it to running the heater, and turning off the heater eliminated the odor.

I parked the car, turned scheduled climatization off, and made my service appointment. But on the day before my appointment, my 12 volt battery went dead about mid day (based on last update to my MyBMW app). There were zero indications of 12 volt issues prior to this, so it was a complete surprise.

I jumped the car and had it towed in to BMW. The technician saw nothing other than codes associated with a failed 12 volt battery, replaced the battery, and all seems well. Even sniffing under the hood and any trace of the odor has completely vanished.

It's not particularly cool today. I've got the car climatizing and it's producing no odor. All I can do is sit on this and monitor. I just can't believe this odor was the result of a failing 12 volt battery. I'm much more inclined to think that whatever caused the odor somehow caused the battery to fail.

BTW, this 12 volt battery was reportedly changed by BMW in August '21, 18 months ago, shortly after I bought the car.
 
A failing 12 V battery could emit sulphur-containing gases that are very odiferous. However, these gasses smell like rotten eggs, not hot asphalt. When your i3 is climatizing, the DC-DC converter could turn on to charge the 12 V battery which could increase the emission of these stinky gasses. When your i3 is not climatizing but is only parked, the DC-DC converter wouldn't normally charge the 12 V battery, so the stinky emissions would be reduced. However, if the 12 V battery is failing, its voltage could be low enough that the DC-DC convener would turn on for 1 hour to charge the battery. It would then turn off until the voltage drops below a certain voltage before turning on again for another hour. Not being charged continuously might reduce the emission of stinky gasses.

My diagnosis doesn't seem to agree very well with the symptoms, so I'm not confident that a failing battery is to blame. However, it's all I got.
 
Thanks Art and perhaps you're correct in that assessment.

There is not so much as even a lingering odor when I've sniffed under the hood on multiple occasions now. So I'm going to continue on as if the problem is corrected.

At least I know what to expect next time I smell that smell!
 
Oh and here's the extra info I didn't include:

I called for the tow.

While waiting, the tow driver called and convinced me to try charging the battery before he arrived, just to get it out of Park and make it easier to load.

He showed up and I had only just connected my trickle charger to the jump points under the rear motor cover, and that wasn't going to do the trick.

So he brought out his jump pack and fired it right up.

The car went into drive ready with no problem, and I took it for a quick drive down the block. A-OK!

The cluster was full of errors and the SOC showed zero, but I had just charged two days earlier, and I was at 70% per my app's last update at noon. So we discussed, and I opted to drive it the 6 miles to the dealership.

The 12 volt was "flat" flat – he had to jump me one more time before leaving (because I got out and the car shut down). But I wasn't concerned about the 12 volt once the car was powered up.

So I drove, picking a street route "just in case," and at about mile four the car quit. This was a depleted HV battery kind of quit: the BMS really believed the car was out of charge.

So I rolled to a stop in a bus lane at a busy intersection, hazards on, and called for another tow. Thankfully there were some beat up traffic cones nearby, because everybody assumes a car stopped in a bus lane with hazards on is a Door Dash driver who's about to move? I mean, seriously, at one point a bus clipped one of the cones and dragged it into the next lane.

I fastened my seatbelt behind me and got out on the passenger side to set out the cones, fearful that if I let the car turn off again it would completely die, hazards and all, and shift back into park. This seatbelt arrangement was successful in preventing that from happening.

Anyhow second tow arrived an hour later, this one a standard truck. So I shifted the car into park, we put the rears up on dollies, and finished the trip to the dealership.
 
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