Wh / Mile vs Actual electricity draw 50% discrepancy

[ajdelange]

In either case the display would be telling me that the car uses 3.6 times more "fuel" to go the first tenth of a mile after starting than it does to subsequently go 0.1 mile beyond, for example, a stop sign. It appears that in either case, there is a "departure tax" to be paid. In the case of the ICE vehicle I would assume that the tax is the bolus of petrol required to start the engine. In the Tesla there is no engine to start. In No. 34 it says that the energy the car used while in park is not counted. But I still see a bolus. My question remains the same looking at it in this way. Where did that bolus of energy go? I am forced to conclude, by the evidence, that the energy used in park, or some appreciable part of it, at least, is counted.

BTW the first 10th does occasionally show consumptions as low as 1200 Wh/mi but it is usually 2500 - 3000 and in one case over 7000.

 

[mxnym]

how consumptions as low as 1200 Wh/mi but it is usually 2500 - 3000 and in one case over 7000.

I have NEVER seen the first reading be above 2000 Wh/mi. If you're seeing that, I understand why you think the energy used between drives is included and even wonder if something is different in your vehicle to cause that.

 

[mswlogo]

I have NEVER seen the first reading be above 2000 Wh/mi. If you're seeing that, I understand why you think the energy used between drives is included and even wonder if something is different in your vehicle to cause that.

I rarely see much over a 1000. And my parked energy varies a lot.

 

[ajdelange]

You’re still using a ton of heat in those first few miles, warming up stiff tires, softening up grease in bearings, oil in motors. A cold vehicle is harder to push, regardless of how much you preheat cabin.

Good thinking but I'm doubtful. For starters a stiff tire is going to absorb less energy as it doesn't deform as much as a softer one and it is the deformation that takes energy. Maybe the differential oil and bearing grease but they aren't going to warm in 0.1 mile enough that their loads are going to disappear from the chart subsequently. More to the point is that my car sits in a garage at 55 °F. If we were talking about a car that soaked overnight at 0 °F I would take the suggestion that lubricants are a factor.

There is, of course, no engine oil. In the ICE vehicle comparison requested above having to overcome stiff engine oil at startup would indeed be part of the departure tax.

 

[ajdelange]

I have NEVER seen the first reading be above 2000 Wh/mi. If you're seeing that, I understand why you think the energy used between drives is included and even wonder if something is different in your vehicle to cause that.

I have wondered if firmware might have something to do with this. The software people have infinite flexibility in what they can do, of course.

On this same thought I posted the details of what I saw and someone rated the post "Disagree". This means that either he thinks I didn't see what I saw or that he doesn't see what I saw perhaps because of firmware differences. Perhaps he could come forward and explain?

I'm on 2018.48.12.1 d6999f5. BTW, 2018.48.12.1 I get but what do those 7 hex digits at the end signify? And while I'm on it I see people talking about V8 and V9. What does that refer to?

 

[gnuarm]

Good thinking but I'm doubtful. For starters a stiff tire is going to absorb less energy as it doesn't deform as much as a softer one and it is the deformation that takes energy. Maybe the differential oil and bearing grease but they aren't going to warm in 0.1 mile enough that their loads are going to disappear from the chart subsequently. More to the point is that my car sits in a garage at 55 °F. If we were talking about a car that soaked overnight at 0 °F I would take the suggestion that lubricants are a factor.

There is, of course, no engine oil. In the ICE vehicle comparison requested above having to overcome stiff engine oil at startup would indeed be part of the departure tax.

You have some misunderstanding of how tires work. Tires flatten until the pressure exerted on the total area of contact is equal to the weight placed on that wheel. That pressure is largely the inflation pressure. The stiffness of the tire exerts pressure along the front, the sidewalls and less so at the remainder of the periphery. The overall contact area with the road will change little between warm and cold conditions. So the major effect is to dissipate more energy in flexing the tires as the car rolls.

But I agree that these effects will be small compared to the overall energy of moving the car.

Clearly the way to test this theory is to increase the idle energy usage to a level that will be very obvious rather than to try to measure and explain such small energy levels. Drive to the store and leave the heater on while inside. Then repeat this measurement on returning to the car. The "cold grease" effect will have been greatly mitigated and the actual energy used while the car is "off" will be much larger and easier to see.

 

[ajdelange]

I'm certainly no expert on tyres and I think I see what you are saying. If the deformation is the same and the material being deformed is stiffer it is going to take more energy to deform it so I would have to reverse my reasoning in #44.

With respect to the suggested testing: that's what I have been trying to do but the results have not always been as expected. I have seen spikes as high as 7000 Wh/mi and as low as 700 - 800. The big ones don't necessarily come when I leave the heater on for a long time. Clearly the answer to the mystery depends on how the counters are initialized. At this point all I can say with certainty is that there is a departure tax billed to the first 10th mile displayed. I can't predict it or explain it.

 

[Yinn]

Let's assume your "huge spike" may be 1200Wh/mi vs a highway average of 337.5WH/mi (July2017+ X100D).

337.5/1200=.28

Let's assume you have an ICE vehicle with a 2008+ EPA highway rating of 28 MPG.

28*.28=7.84

How would YOU explain it when you get in that ICE, reset the fuel economy meter, start the car, and immediately start driving it the exact same way you would drive the X100D only to see an average MPG reading of 2 - 8?

I'd recommend using the exact same explanation to cover what you're seeing in the X100D.

We're not exactly comparing apples to apples here. We simplify it, but these are two different animals.

An ICE generates more heat than it does power. Even in a super efficient gasoline combustion engine, we're looking at maybe 40% (typically 20%) efficiency. Comparitively an EV motor is 80% efficient on the low end. The reason an ICE is less efficient is because it actually makes more heat as a byproduct than it does in propulsion energy. This actually makes the entire ICE package more efficient in the winter. Then why does your MPG drop in the winter? Yes, some of it is due to denser air, friction, etc. but most are due to increased ethanol mixes into the gasoline.

Then you also have to factor that gasoline just outputs more BTUs. A gallon of gasoline is capable of putting out 112,500 BTUs. At 40% efficiency for propulsion and ~60% in waste heat; it's putting out roughly 67,500 BTUs in waste heat. Electricity on the other hand makes 3,412btu's per kWh. On a 80% efficient motor, you might get 20% of waste heat. That's equivelant to 682 BTUs. That's a HUGE difference. The remaining heating needs on a ICE vehicle can be absorbed by the waste heat. On an EV, the heat has to be created by using additional energy because there's simply not enough waste to be utilized.

We can certainly get into debating how the EPA tests, weather, etc. but I feel like the understanding of why it spikes is a big factor. It's not really fare to say Tesla is lying to us when they followed a government standardized test.

 

[gnuarm]

We can certainly get into debating how the EPA tests, weather, etc. but I feel like the understanding of why it spikes is a big factor. It's not really fare to say Tesla is lying to us when they followed a government standardized test.

I don't want to get into all the other stuff as there is little info available and lots of issues. But regarding the "fairness" of citing the EPA numbers... Yes, the EPA sets the procedures for measuring the mileage numbers to be displayed in the window sticker. But that's the only thin the EPA mandates. The EPA doesn't tell Tesla they can't provide other data that more accurately reflects the experience drivers are likely to have.

The discrepancy between what you are told with this EPA figure and what you will experience is huge. Potential buyers need to know this. But then not many will buy a Tesla until a trusted friend has one and they can get the straight skinny. Then the real range of EVs will be found lacking by many.

 

[ajdelange]

I just had a thought as to where some of the departure tax may be coming from. The easiest place to get energy use numbers from is the logged SoC. The energy used between time A and time B or point A and Point B is the difference between SoC's at A and B. Entirely speculation here but lithium batteries are very coulomb efficient. You put a coulomb in and you eventually get that coulomb back out. But the energy you withdraw is the product of the charge you withdraw and the battery voltage. Thus if you arrive home at night with a charge of 250 amp hours on board and battery voltage 400*250 you have 100000 watt hours of energy. If the battery cools you can still get the 250 amp hours out of it but the voltage will be lower because the temperature is lower. If the voltage drops 10 volts you will be able to get only 250*390 = 97500 watt hour out of this battery and thus the SoC dropped 2500 Whr. I have no idea if Telsa reckons SoC this way nor whether the battery voltage is 400 warm nor how much the voltage drops for a given temperature change. The general laws of physical chemistry suggest that it might be about 10 volts from 400 in going from 20 °C to 10 °C.

 

[mxnym]

I'm on 2018.48.12.1 d6999f5. BTW, 2018.48.12.1 I get but what do those 7 hex digits at the end signify? And while I'm on it I see people talking about V8 and V9. What does that refer to?

This is the version I'm on, and I'm not seeing what you are, so that is odd. The hex digits are presumably a checksum for the package version installed. V8 is the previous GUI interface, but I don't believe there is much overlap, if any. For instance, I believe 18.36.2 was the last public V8.1 and 18.39.5 was the first public V9.0.

 

[mxnym]

We're not exactly comparing apples to apples here. We simplify it, but these are two different animals.

An ICE generates more heat than it does power. Even in a super efficient gasoline combustion engine, we're looking at maybe 40% (typically 20%) efficiency. Comparitively an EV motor is 80% efficient on the low end. The reason an ICE is less efficient is because it actually makes more heat as a byproduct than it does in propulsion energy. This actually makes the entire ICE package more efficient in the winter. Then why does your MPG drop in the winter? Yes, some of it is due to denser air, friction, etc. but most are due to increased ethanol mixes into the gasoline.

Then you also have to factor that gasoline just outputs more BTUs. A gallon of gasoline is capable of putting out 112,500 BTUs. At 40% efficiency for propulsion and ~60% in waste heat; it's putting out roughly 67,500 BTUs in waste heat. Electricity on the other hand makes 3,412btu's per kWh. On a 80% efficient motor, you might get 20% of waste heat. That's equivelant to 682 BTUs. That's a HUGE difference. The remaining heating needs on a ICE vehicle can be absorbed by the waste heat. On an EV, the heat has to be created by using additional energy because there's simply not enough waste to be utilized.

We can certainly get into debating how the EPA tests, weather, etc. but I feel like the understanding of why it spikes is a big factor. It's not really fare to say Tesla is lying to us when they followed a government standardized test.

No, they're not exactly the same, but the examples using numbers that I regularly see help support the two points that I intended to illustrate with the example:

1. The majority of the power consumed in either case is consumed by propulsion of significant mass from a standstill.
2. Due to (1), the consumption from a stop after an "average" meter is reset will appear higher than some expect.

As far as EPA tests, they were only used as a point of reference in this illustration. However, while I didn't previously indicate anyone was lying, and I'm not sure anyone else in this thread is, either (asking about a discrepancy and whether or not it is normal is a far cry from accusations of lying, and comparing behavior of meters in different products isn't much closer to one). I do have an issue with the fact that advertised range is apparently based on the highway rating vs the combined rating. As far as I am concerned, the fact that isn't explicitly mentioned where range is discussed is misleading to say the least.

 

[ajdelange]

At this point I'm just going to keep observing and try to take better notes.

 

[muncheroo]

Thanks for the insightful comments as always. Still a bit cheaper than driving a similar ICE vehicle at 13 cents per kWh but this certainly softens the argument for energy savings. Hopefully future updates will help improve this problem.

 

[mswlogo]

Here’s test.

Some cold morning.

Preheat your car for 20 minutes.
Get in, turn Off heat. Drive to destination and note wh/mi

Next morning
Don’t preheat.
Get in, turn Iff heat. drive to same destination and note wh/mi

I bet they come out very close.

 

[mswlogo]

Good thinking but I'm doubtful. For starters a stiff tire is going to absorb less energy as it doesn't deform as much as a softer one and it is the deformation that takes energy. Maybe the differential oil and bearing grease but they aren't going to warm in 0.1 mile enough that their loads are going to disappear from the chart subsequently. More to the point is that my car sits in a garage at 55 °F. If we were talking about a car that soaked overnight at 0 °F I would take the suggestion that lubricants are a factor.

There is, of course, no engine oil. In the ICE vehicle comparison requested above having to overcome stiff engine oil at startup would indeed be part of the departure tax.

Guess this would be a good test for myth busters.

I’m standing by a cold tire is harder to roll than a warm one.

The tires deflection doesn’t decrease because the rubber is harder (not much any way). It’s harder to bend that rubber when cold. You’re thinking the whole tire turns into a hard plastic wheel when cold. I don’t think it works like that. Also your pressure is lower cold so the tire is deforming even more when cold. So you’re bending more harder rubber when cold.

How much this accounts for initial load I’m not sure. But I bet a cold tire takes more energy to roll.

Maybe at 70mph a cold tire is better. But at low speeds (like when your starting out) it’s worse.

 

[CyberShy]

True efficiency calculations are energy IN divided by DISTANCE traveled for a given sample period.

This accounts for all HVAC, pre-heating, AC-DC Charger losses, cable losses, temperature variation, parasitic drain, etc.

Do not trust the wh/mi the car shows. It’s a lie. At least, it doesn’t account for those completely, especially when it’s not moving.

 

[ajdelange]

You’re thinking the whole tire turns into a hard plastic wheel when cold.

No, I'm not. See No. 47. Perhaps I was but not since No. 47.

 

[mswlogo]

No, I'm not. See No. 47. Perhaps I was but not since No. 47.

Looks like we are in agreement.

I just vaguely recall when young that pushing a car in winter was hole lot harder than in the summer.

 

[ajdelange]

True efficiency calculations are energy IN divided by DISTANCE traveled for a given sample period.

This accounts for all HVAC, pre-heating, AC-DC Charger losses, cable losses, temperature variation, parasitic drain, etc.

We may, for whatever reason care about overall efficiency, but in terms of trip planning and progress monitoring (the subject of this thread) we don't care a whit about charger losses, transmission line losses, transformer losses, heat lost at the power plant etc. What we care about is what's in the battery, however it got in there, and the rate at which we will be pulling it out. From this we can get a rough idea as to how far we can go or, more practically, whether remaining range at our planned destination is within out comfort zone.

Do not trust the wh/mi the car shows. It’s a lie.

Why would the manufacturer put displays in the car intended to deceive us? In fact the displays are well thought out and extremely useful. This forum abounds with advice on how to extract useful information from them but you have to know how to interpret them. I'm catching on fast but then I've had 40 years experience working with similar displays and they are pretty much second nature to me. My wife, however, wouldn't have a clue as to what any of them could tell her.

The displayed Wh/mi are not a lie. They represent, at any point in time, the Wh drawn from the battery (simple numerical integral of battery voltage times battery current) divided by the miles driven since the counter was started. These numbers are used to predict range or margin. Because they are based on the driving conditions experienced on the current trip they are better numbers to use in computing destination estimates than the fixed rated Wh/mi number especially as the planners can take into account terrain. But they cannot take into account (until you actually do it) your decision to increase or decrease speed or a change in weather. These numbers are estimates. That word has, to those familiar with estimation theory, many implications attached to it. I wouldn't expect most drivers of these cars to know anything about that sort of thing and so they would rely on their experiences with the trip planners and in car displays to reach a decision as to how "accurate" their estimates are. Many who post here seem to think they are pretty reliable. Tesla seems to have done a good job. Perhaps you haven't done that checking and are thus not aware of this.

At least, it doesn’t account for those completely, especially when it’s not moving.

Evidently it does. Yesterday morning the display showed 900 Whr/mi going down my 0.1 mi driveway at 10 mph. Thus I used 90 Whr. At 15 mpH it would have taken me 24 seconds to do that (0.00666 hour) so the average power consumption would have been 13.5 kW. Sounds about right - there was perhaps an inch of snow - and the car wasn't pre-heated. When I experiment with preheating off the charger my more typical reading at the end of the drive way is 1500 and in one case 7000. These imply power consumptions of from 22.5 to 105 kW. This would either have to be sustained down the whole drieveway or more likely, peak at some point as I accelerated and then decline. Believe me, if the power meter were reading this high in my driveway I would have noticed it. The obvious conclusion is that energy used prior to departure is considered in the first reported consumption number. Common sense says that it should be but many here, such as you, assure me that it isn't. My conclusions are based on observed numbers. I find it interesting that those who disagree never offer numbers to support their position.