Reported energy consumption [kWh/100miles] is utterly completely totally wrong - Jaguar I-Pace EV400 Forum
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post #1 of 55 (permalink) Old 02-16-2019, 06:30 PM Thread Starter
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Reported energy consumption [kWh/100miles] is utterly completely totally wrong

Thanks to ryzvy for motivating this thread with her thread "Energy consumption".

I just did a simple test, where I hammered it on the freeway for 7.2 miles and then drove home via surface streets for 7.2 miles, noting the SoC and the reported kWh/100miles at each step.

I'll spare y'all the gory details but here's the upshot: 14.4 miles used 11% of battery capacity. Assuming 84kWh battery capacity, that's a consumption of 0.11*84/0.144 = 64 kWh/100miles. The reported consumption was 35.4 kWh/100miles.

Yes the numbers are small (few miles, not much SoC delta) but the contrast is too large to be chalked off to some measurement variance. The culprit is the reported kWh/100miles. Its calculation is totally wrong. It's fine if you keep constant speed, but the more you vary your speed the wronger it gets because it's dominated by the time you spend driving efficiently and it incorrectly downweights less efficient driving because its calculation of the average is incorrect.

I'm dumbfounded that JLR could be making such a rookie mistake and I'm wondering if they did that on purpose to make people feel like they have great numbers when in fact their efficiency is far lower. The long term averages, as well as the averages for short trips with highly varied consumption within the trip, are completely totally utterly wrong. Bad JLR.
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post #2 of 55 (permalink) Old 02-16-2019, 07:49 PM
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Well, the opposite of that would be driving in my Sprinter. You fill the tank up and it says 800kms range. You get on the highway it drops to 600kms range. You go up a slight climb it drops into the 400ís and then then go downhill and it jumps up to the 700ís. Now THAT is completely useless.

Jag uses an average based on your trips and it doesnít change to much based on the occasional sprint. Hop out on the highway for 90 miles at 90 miles an hour and the next time you get in the range will be less than you are used to seeing. Piddle around in the city and it will go back up. It is always adjusting and a 14mile run isnít long enough for it to average out the previous trips. Eventually it will learn your driving style.
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post #3 of 55 (permalink) Old 02-16-2019, 08:31 PM
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Honestly all EVs do what we joke as ďguess-o-meterĒ because there is no real answer. We are just used to not having as much of a consequence when you fill up so you donít notice. But there is no way to look into future. Thatís why I for one reset my miles on full charge then compare to SOC along with range.

Tesla seems to lie and always give a consistent range on start but truthfully varies a lot. Leaf would adapt too fast to more recent data. The i3/i8 actually have a cool trick that IF you have a destination the computer not only will optimize EV use but will adopt the range based on topography and road speeds (Iíll see the remaining range increase or decrease when I set destination if itís a significant portion of remaining charge).

So far in 3 days I am finding that the Jaguar range guesstimate is not bad if I am not making drastic style changes, but then again I donít live in a very hilly region of country.
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post #4 of 55 (permalink) Old 02-16-2019, 08:46 PM Thread Starter
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I'm not talking about the GoM or anyone's driving style. I'm talking about a calculation JLR makes based on measured parameters. Not predictive estimates.
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post #5 of 55 (permalink) Old 02-16-2019, 10:14 PM
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Sorry my bad!
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post #6 of 55 (permalink) Old 02-16-2019, 11:11 PM
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@sciencegeek I think you are looking at it wrong from the following perspective. In range prediction you do not use straight averages but rather a weighted average based on "intent". What you did most recently is weighted heavier than what you did in the past because empirically you are more likely to continue dong what you are currently doing than what you did first. In other words if you had reversed your test you would have far different results.

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post #7 of 55 (permalink) Old 02-17-2019, 01:18 AM Thread Starter
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I'm not talking about range prediction. I'm talking about the consumption figure for the completed 'journey' and the historical trip average in kWh/100miles and how it's incorrectly calculated. It's not a prediction.
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post #8 of 55 (permalink) Old 02-17-2019, 10:01 AM
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You're also assuming that the percentage used shown in the system is linear but I'm willing to bet it's probably not. Battery charge and discharge rates vary wildly depending on the state of charge as evidenced by the fact that most EV's accelerate faster when they have a full charge as opposed to a partial charge.



You can control for this by trying the same experiment at various charge levels... I'd probably do one at 100, one at 70, one at 45, one at 25 and one at 10 and see what the average looks like... the reason for the curve in the starting charge amounts is to control for the varying discharge rates across the total charge amount. Depends how much free time you have and how much you like driving your I-Pace
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post #9 of 55 (permalink) Old 02-17-2019, 10:11 AM
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Actually, I believe the reported average consumption is fairly accurate, but the reported battery percentage is very inaccurate. We already now that the reported percentage (we like to call it SOC) is not linear. If you charge the car and let the battery have time to balance properly, you will notice that you can drive fairly long before the SOC goes down to 99%. Also, there are reports saying you have a similar situation when you reach 0%.

I think we can assume around 3kWh between 100-99 % and the same from 0%-stop. That's leave only around 79 kWh for the remaining 99%-0%.

In addition, it looks like the SOC is not linear in the interval 99-0% either, so any calculation involving a small SOC interval is doomed to be flawed.

If you charge the car completely full and run the battery all the way to 0% is displayed you can calculate the used kWh my multiplying the average consumption with the distance. Add the last 3kWh from 0%-stop and I think we will
be fairly close to the claimed 84.7kWh. At least if you do this when the ambient temperature is not too far away from around 70 degree Fahrenheti (20 degree Celcius).
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post #10 of 55 (permalink) Old 02-17-2019, 10:35 AM Thread Starter
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The reported SoC is quite accurate, at least in the midrange. See attached chart of a 7.2kW charging session yesterday. The charge added explains 99.5% of the variance in SoC (that's the R^2 value).

I'm also attaching a spreadsheet where you can see that over a couple of back-to-back ~60 mile drives which center on a SoC of 50% the calculated consumption based on SoC was way higher than that reported by the car.

=> SoC is not the issue here; the issue is with the reported consumption [kWh/100miles].
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