Recent fast battery degradation of 2017 i3 (94Ah) at 120k miles / 8 years

[Philipp]

Hi all,

I bought my 2017 i3 REX (94Ah) in 2021 when it had 103k miles. The battery condition was excellent.

I believe I got a Kappa Max of ~28.8 kWh in 2022 or 2023, at perhaps 110k miles. I remember it was higher than the capacity that BMW advertises. I didn't measure Kappa Max regularly.

Over the last few months, now at 120k miles, I started noticing that my range was decreasing quite a bit. My Bat Kappa Max is now showing as 24.4 quite consistently. Being in California's mild weather, it shouldn't be strongly affected by seasonality.

It seems that the battery had zero degradation for many years but is now falling of a cliff.

Is it worth getting it checked? Would that have to happen at a BMW dealer?

I checked the codes and there are 13 codes related to the Battery management system all saying "Test failed since last DTC clear".

I did hit the underbody of my car twice (once a rock, once a "freestanding" curb piece) which was scary but didn't seem to have immediate effects on the battery (it happened a few years ago).

Overall, is it worthwhile to investigate this? Is there something I can do at home prior to spending significant $$ at the dealer?

Thank you

 

[madasa fish]

Do you leave your car fully charged for any period of time or do you only leave it overnight when not fully charged?
If you do not leave it fully charged for any length of time, the automatic battery equalisation system does not get a chance to work and battery will degrade. You can fix it by regularly leaving the car overnight fully charged or charging and then fully charged

 

[Philipp]

Thanks for your response! I don't have home charging, so I charge overnight at a L2 charger 1-2x a week. On average, the car might remain plugged in for ~8 additional hours after being fully charged.
Other than that, it usually sits at some intermediate charge level (I avoid low charge states though).

 

[alohart]

If you do not leave it fully charged for any length of time, the automatic battery equalisation system does not get a chance to work and battery will degrade.

Battery cell charge level balancing occurs at less than a full charge but probably more slowly. The i3's passive balancing is based on discharging cells at the highest voltages until these voltages match those of lower voltage cells. Cell voltages at higher at a full charge which should allow cells to discharge more rapidly.

There are several smartphone apps that can display the minimum and maximum battery pack cell voltages. Even though over a decade of i3 ownership I have never routinely charged to 100%, the difference between the minimum and maximum cell voltages has never been high enough to indicate a poor cell charge level balance, so charge level balancing seems to work well at 80% or higher charge levels.

 

[alohart]

It seems that the battery had zero degradation for many years but is now falling of a cliff.

The cells in all battery packs degrade over time and charge-discharge cycles. We don't know whether the i3's BMS could hide this degradation for a while by adjusting the sizes of the unusable charge level buffers at low and high charge levels. I'm not aware of anyone demonstrating that this is happening, but it might be difficult to detect. 94 Ah battery cells have been much more resistant to degradation than some 60 Ah battery cells, but even 94 Ah battery cells lose capacity over time.

Is it worth getting it checked? Would that have to happen at a BMW dealer?

That would likely be fairly expensive and might be a waste of money. Unfortunately, few independent U.S. mechanics seem to be working on i3's.

I checked the codes and there are 13 codes related to the Battery management system all saying "Test failed since last DTC clear".

Not sure what that means. If you are not using an OBD scanner that understands BMW's proprietary diagnostic trouble codes (DTC's), it might be worth licensing BimmerLink to display stored DTC's in the hexadecimal format that the BMW faults Website can decipher.

I did hit the underbody of my car twice (once a rock, once a "freestanding" curb piece) which was scary but didn't seem to have immediate effects on the battery (it happened a few years ago).

If you didn't notice anything at the time, there was probably no internal inside the battery compartment. However, I would probably check the battery compartment to learn whether it might be dented deep enough to have caused some internal damage.