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Resonant hum

Posted: Sun May 02, 2021 11:52 am
by willmhull
Hi guys and gals,

I have a 2017 i3 (no range extender), I just bought it, and I have noticed that when I am coming to a stop without braking, there is a loud resonant humming coming from the front end of the vehicle. However, it doesn't happen every time, and sometimes it is louder (really quite loud) than others. I would imagine it has to do with a problem with the auto regen braking system.

Wondering if there is a known issue or what this might be?

Thanks,
Will

Re: Resonant hum

Posted: Sun May 02, 2021 12:46 pm
by alohart
willmhull wrote:
Sun May 02, 2021 11:52 am
I have a 2017 i3 (no range extender), I just bought it, and I have noticed that when I am coming to a stop without braking, there is a loud resonant humming coming from the front end of the vehicle. However, it doesn't happen every time, and sometimes it is louder (really quite loud) than others. I would imagine it has to do with a problem with the auto regen braking system.
Regen braking occurs only on the rear wheels, so it's unlikely that the sound would originate from the front if regen braking were the source. If the battery pack charge level is so high that the battery pack can't absorb the full regen braking power, the force of regen braking would be mimicked by the automatic application of the friction brakes on all 4 wheels. If the friction brakes were the source of the sound, you should hear the sound when pressing the brake pedal.

So I doubt that the sound is related to regen braking, but I don't have a clue what might be causing it.

Re: Resonant hum

Posted: Sun May 02, 2021 2:57 pm
by willmhull
Thanks for the reply Art, these are some good points. I will try to recreate it with some braking.

It definitely could be the friction break. I remember hearing a similar (but harsher) sound when I had a small rock lodged between a pad and rotor on a car a while ago; although, I rarely fill the battery to capacity. Is the friction brake engaged only when the entire pack is full or with each individual full cell in the battery?

Will

Re: Resonant hum

Posted: Sun May 02, 2021 6:29 pm
by alohart
willmhull wrote:
Sun May 02, 2021 2:57 pm
Is the friction brake engaged only when the entire pack is full or with each individual full cell in the battery?
The battery management system attempts to keep the charge levels of all cells close to equal at the high charge level end, so a full pack would also imply each cell being full. However, according to some British i3 owners who attended a briefing by BMW engineers early on, cell balancing doesn't occur when the displayed charge level is less than ~80%, or maybe it occurs only very slowly, and occurs only when no current is flowing in or out of the battery pack; i.e., when the battery pack is idle. So if you haven't allowed the battery pack to rest very long above an 80% charge level, some cell imbalance could exist.

When charging, the charging power tapers off as the charge level approaches full. I assume that regen braking power would also taper off to avoid damage to the battery pack. So at higher charge levels, an i3 might be mimicking pure regen braking power at lower charge levels by blending lower regen braking power with friction brakes. The brake rotors on our i3 have rusty surfaces which allows me to hear when the friction brakes are being used. However, I don't typically charge above 90%, so I probably haven't experienced much regen braking mimicking.

Re: Resonant hum

Posted: Mon May 03, 2021 10:10 am
by willmhull
After driving using the brakes to stop (some downhill neutral braking and braking while stopping short), I do not think that it is the breaks. Unfortunately, I cannot recreate the sound.

I was wondering what else is in the front of the vehicle? For example, is there a compressor that runs the active battery cooling? I wonder if there is a leak somewhere? I have heard that compressors can create a similar sound when there is a leak in the system?

Re: Resonant hum

Posted: Mon May 03, 2021 2:02 pm
by rtanov
The brakes sound very different if the car has sat for a few days and the rotors have accumulated some rust on the surface. When the pads scrape that rust the sound is very different and much louder. So, it still could be (could be) the regen activating the brakes after the car has sat and the rotors have rusted.

Re: Resonant hum

Posted: Mon May 03, 2021 2:04 pm
by alohart
willmhull wrote:
Mon May 03, 2021 10:10 am
I was wondering what else is in the front of the vehicle? For example, is there a compressor that runs the active battery cooling? I wonder if there is a leak somewhere? I have heard that compressors can create a similar sound when there is a leak in the system?
The A/C compressor is adjacent to the electric drive motor in the left rear of an i3.

A cooling fan for the A/C condenser and the motor and motor electronics heat exchanger is low in the front. A small vacuum pump for the power brakes is in the left front. Both operate only when needed. Can't think of much else that's in the front and that might cause such a resonant hum.

Re: Resonant hum

Posted: Mon May 03, 2021 2:47 pm
by EvanstonI3
Since the "hum" only occurs when you are decelerating, is it possible there is some loose wheel well cowling that, when moving forward is pushed back, but when braking and weight shifted forward, somehow contacts the tire or rotor? Perhaps the infamous deteriorating strut boots are falling apart and rubbing on the back of the tire?
I would start looking in the wheel wells for something slightly out of place.