What’s your Max Kappa? Usable Capacity?

[UpNorth]

Hello all

Im new to owning a BMW and trying to figure out how to assess the battery capacity of my i3.

Also I’m trying to understand what a normal level of battery degradation is for an i3. Particularly the 120ah.

I own a Tesla M3 and have had a Renault Zoe as well, so I’m broadly familiar with the caveats around calculating battery health. And I can see the health warning about Max Kappa numbers. But what else can we use?

I have a 2019 120ah i3s with 50,000 miles.

I have found the secret menu and can see that my car has a Max Kappa of 36.2 kWH.

I checked this after charging from 10% to 100%. (Not sure if that makes any difference).

I understand the 120ah had a max usable 37.9 kWH.

Does this mean that my car has 95.5% of its original battery capacity? That would be amazing!

For comparison what’s your Max Kappa? What’s your percent of original capacity.?

Or should we just ignore these numbers and drive our i3?

Thanks

UpNorth

 

[alohart]

I have a 2019 120ah i3s with 50,000 miles.

I have found the secret menu and can see that my car has a Max Kappa of 36.2 kWH.

I checked this after charging from 10% to 100%. (Not sure if that makes any difference).

I don't think any of us knows for certain what affects Batt. Kapa. max, the BMS' current estimate of the battery pack's usable capacity. I have seen it vary on our former 2014 i3 BEV ±2 kWh over a short period of time, a pretty high percentage of its 18.8 kWh of usable capacity when new. I would say that the consensus is that a series of Batt. Kapa. max readings taken over a period of time under as similar conditions as possible would be more considerably more indicative than a single reading.

I understand the 120ah had a max usable 37.9 kWH.

Yes. However, that seems to be a conservative value. On our former 2019 i3 BEV and current 2021 i3 BEV, both with 120 Ah battery packs, the cell capacities were as high as 125 Ah when relatively new.

Does this mean that my car has 95.5% of its original battery capacity? That would be amazing!

Not necessarily, but it's probably close. At 35k miles, the BMS had measured the average gross cell capacity of our 2019 i3 to be 119.67 Ah which it calculated to be a 95% state of health. This suggests that the BMS is using a new nominal cell capacity of 119.67 / 0.95 = 126 Ah. BMW assumes a nominal cell voltage of 3.7 V which would make the gross capacity of our 2019 i3's battery pack 3.7 V x 96 cells x 119.67 Ah = 42.5 kWh, more than the advertised new nominal gross capacity of 42.2 kWh!

Other than voltage, current, and temperature, all other values are calculated estimates made by the BMS. We really don't know how to interpret these values other than to assume that they're estimates that shouldn't be taken too literally.

The free mi3 (iOS) and Electrified (Android) apps display much more BMS data than the service menu. You'd need to buy an OBD to WiFi or Bluetooth adapter to connect these apps to the OBD port.

For comparison what’s your Max Kappa? What’s your percent of original capacity.?

I quit looking at Batt. Kapa. max because it can vary so much and is only a single data point. I use the mi3 and eFlow apps to display much more data which should provide a more complete picture.

Or should we just ignore these numbers and drive our i3?

I enjoy looking at these numbers. However, doing so is certainly not important for any owner of a 94 Ah or 120 Ah i3 whose battery packs are proving to be very stable with slow deterioration, especially compared with early i3 60 Ah battery packs, many but certainly not all of which degraded more rapidly than expected. Having experienced significant battery pack deterioration in our 2014 i3 BEV, I might be more sensitive to this than most. However, I've been very happy with the battery packs in our former 2019 and current 2021 i3's and am becoming more relaxed about monitoring the condition of our battery pack. You and I probably have more important concerns.