Mileage Estimation

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Arm

Well-known member
Joined
Jan 7, 2020
Messages
332
I've yet to fully figure out how the on board system calculates predicted full charge mileage. It's very strange. I feel as though the system should take the average efficiency as the predictor but it doesn't.

For example, yesterday I actually had the heat on during my drive home. I was surprised that my mileage dropped 15 miles on the range indicator during use of heat given I've a great pump. None the less, the drive home still returned 4.9 mi/kWh. My average right now overall is 5.1 mi/kWh.

So I came home an the SOC was 76% and range was 97 miles. Didn't plug in.

Plugged in tonight and my current range changed to 86 miles suddenly with a full estimate of 116 miles. I've been getting 135 to 140 miles usually.

2018 i3s BEV btw.
 
I learned not to really look at the GOM since it's guessing based on the last 14 or some similar number of miles. But inaccurate numbers aren't unique to the i3. My 328i has a miles to empty display that varies wildly. When I start in the city, I see about 200 miles or less per tank but out on the highway it will go up to 360 for a full tank.
 
stumbledotcom said:
I learned not to really look at the GOM since it's guessing based on the last 14 or some similar number of miles. But inaccurate numbers aren't unique to the i3. My 328i has a miles to empty display that varies wildly. When I start in the city, I see about 200 miles or less per tank but out on the highway it will go up to 360 for a full tank.

That's interesting... last 14 or so miles? That makes sense but I'm curious what that specific number is based off of?
 
A bit of transparency would be nice, right? My Audi had a "Eco" screen that showed me I was taking a mileage hit for having my windows open or AC cranked up. BMW could do similar with a table showing "this is because you were just driving like an idiot, this is because it's cold as snot outside, this is because you're at the top of a hill..."

While part of the range estimate is your previous 18 miles, there are other factors.

The battery needs to be cooled, so hotter days can also adversely impact your pricier range, just like cool days when the cabin is being warmed. The heat pump is more efficient than the resistive heater, but it still requires electricity to run what is essentially an air conditioning compressor.

Once the battery has a chance to sit, the BMS can fine tune what it calculates is the actual state of charge. So, yeah... the percent charge is also a bit of a guess.

If you use iDrive for nav, that also factors in. Based on the destination, the system looks at elevation gain, temperature, and anticipated traveling speeds, and further adjusts the range estimate (I don't know whether this is dependant on having a working Connected subscription — obviously traffic and weather require this, but perhaps highway speeds and elevation data are referenced off the built in base map).

So there are hidden and known factors contributing, and everything is very in the air. I'll say this though: once I plugged a destination into nav, my 2017 never let me down. It was conservative in its range estimate, but not overly, and as long as I generally struck to the speed limit, I learned I could rely on being able to drive at far as it was predicting.
 
Interestingly, when I charge my car, I can pretty accurately measure how much my battery capacity really is. According to my rate charge (how much I pay for the charge and how much was added to the car), I can kind of accurately compute what my battery capacity is (not usable...actual).

Examples:

10/16/2021 DCFC Charge
Start SOC: 35%
End SOC: 85%
Total kWh Added (according to unit): 15.99 kWh
Battery Capacity = 15.997/0.5 = 32kWh

10/12/2021 L2 Charge
Start SOC: 33%
End SOC: 100%
Total kWh Added : 21.9kWh
Battery Capacity = 21.9/.67 = 32.8kWh

10/10/2021 L2 Charge
Start SOC: 45%
End SOC: 100%
Total kWh Added: 18.3kW
Battery Capacity = 18.3//.55 = 33.2kWh

10/9/2021 L2 Charge
Start SOC: 49%
End SOC: 99%
Total kWh Added : 15.8kWh
Battery Capacity = 15.8/.5 = 31.7kWh

10/1/2021 L2 Charge
Start SOC: 57%
End SOC: 100%
Total kWh Added: 14.2kW
Battery Capacity = 14.2/.43= 33.0kWh

AVERAGE ~ 32.5kWh
 
Arm said:
Interestingly, when I charge my car, I can pretty accurately measure how much my battery capacity really is. According to my rate charge (how much I pay for the charge and how much was added to the car), I can kind of accurately compute what my battery capacity is (not usable...actual).
Of course, there's no reason why your calculation should result in the gross capacity of your battery pack since charging from an indicated 0% to 100% refers to the net usable capacity. At least at the temperatures during these charging sessions, the percentage energy loss in the form of heat (e.g., battery cell electrical resistance, energy used to cool the battery pack while charging) about equals the size of the unusable energy buffers. I expect that your calculation would not be so close to the gross capacity during cold or hot conditions.
 
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