Am I calculating Miles/kWh wrong, or is the car wrong?

[jglor]

I've been tracking how many miles I get between charges, and it seems like my calculated Miles/KwH is lower than what the car is reporting, and I can't figure out what I'm doing wrong. I'm not talking about the GoM, I'm talking about what the Trip 1 and Trip 2 screens indicate after actually driving.

Here's my process:
I typically charge the car to 90%. When the charge is complete, the next time I turn on the car, I reset Trip 2 so that I can track the miles until the next time I charge.
The next time I charge the car, I write down the total miles, the current battery % and what Trip 2 reports as Miles/KwH.
Here's an example from earlier this month:
Current charge: 32%, 127.8 miles driven, car reports 3.5 mi/kWh

Because I last charged to 90%, I know I used 58% of the battery (90-32 = 58). I have a standard range Premium, which has a 68kWh (usable) battery. 58% of 68 is 39.44, so presumably I used 39.44KwH. I don't have a smart charger, so I have to rely on FordPass, which doesn't show kWh after a charge is complete. Only Percentage, which is really annoying.

So now I divide miles (127.8) by kWh (39.44) and come up with 3.24. But Trip 2 said 3.5.

So am I doing something wrong, or is the car? Is my math wrong? Did I really drive 127.8 miles or is Trip 2 misreporting? I know the "This Trip" will sometimes double count. Is that also a problem with Trip 2? I guess I need to start checking the main odometer too, just to see if Trip 2 is accurate.

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[bshaw]

Someone might have a more detailed answer, but I don't think you can trust the % SOC displayed -- its also just a best guess the car is putting up.

 

[Tyldum]

Yup, the SoC displayed is not the actual number and the exact number is not exact science either.

It becomes even more apparent if you charge to 100%. You can drive a lot longer from 100 to 99 than you can from 90 to 89.

 

[RickMachE]

First off, the SOC display is not exact. Not only is it rounded, but depending on temperature the amount of battery available can fluctuate.

Second, rounding.

Third, watch the miles per kWh. Do you see the every single tenth at any point? I see it go from 2.8 to 3 without showing 2.9 a lot.

This isn't exact math.

Fourth, regen. You added X to the battery via a charger, but regeneration added more. Go to the top of a long downhill. Reset the trip odometer, then head downhill. You can actually add kWh to the battery, i.e. gain electricity, over what you started with.

Fifth, if you measured the juice you spent from the house to the car, it would be higher too, because of loss. Could be 10 - 13%.

 

[jglor]

So basically, my math is fine, but my assumptions based on SOC are not. Unfortunately, like I said, I don't have a smart charger, and FordPass won't tell the kWh delivered after the fact, so I can't really get an accurate read.

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