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Calculation of Rated Range

A short while ago, there was a discussion in one of the threads regarding the calculation of the "Rated Range" number that appears on the speedometer. I can't find the thread, so I am starting a new one.

The issue, in brief, was whether driving style affects rated range. Here is the text of an email message I sent to Ownership:

Hi,

I wonder if you could settle an argument? In forum discussions, some people claim (and have been told by Tesla reps) that the "Rated Range" number that appears on the speedometer is partly determined by the way the vehicle has been driven recently. Others claim that the number is based on the battery's state of charge and a fixed energy consumption rate of approximately 308 watt-hours/mile. Still others claim that the number is based on state of charge, a fixed consumption rate, and certain other variables such as battery temperature -- but not driving style.

Could you please clarify how this number is derived? If you are uncertain, could you please find out from someone who knows?

My own personal view is that Rated Range WAS based on a fixed consumption rate prior to software version 4.3, but now includes other parameters such as battery temperature, but that driving style does not affect the way that the number is computed (although driving style obviously affects energy consumption, and hence the number of rated miles remaining at any given time). It would be nice to get an authoritative answer.

Thanks for any information you can provide.

Regards,

Doug

Here is the answer I received:

Hi Doug,

Thank you for reaching out to us at Tesla Motors, and for patiently awaiting a response from us. I was delighted to get a clear reply from our powertrain systems architect. He states, with regard to how Rated Range is calculated:
Rated range = the car’s estimate of remaining energy / a fixed whpm. The fixed whpm is different for different vehicles (85kWh / 60kWh) and in different markets based on the regulatory test for that vehicle in that market.

Further information he would not elaborate on, but I hope this aids in ending the discrepancy regarding how we achieve your range predictions.

As always, please reach out to us again if there is anymore hash you need settling.

Kind Regards,

Peter

This is actually a simpler calculation than I had thought, although I had not anticipated that TM would use a different fixed whpm for each market. It does not explain why the relationship between rated miles used and whpm (as determined by the trip meter) seems to vary for a single vehicle. In any event, it does not appear that rated range is at all based on driving style.

Since it took about a week for Peter to get back to me on this question, I assume he got his answer from a fairly authoritative source.

Based on a week of driving (i know, its a small sample size), it appears that the Rated calc is based on 300 wh/mi. That is where the "Rated" line is on the energy app and when i happened to hit that as my average, the projected range (based on average, not instant) was exactly what the rated range was. i have a p85+.

If I remember correctly, general consensus on the previous thread was Rated miles had nothing to do with how an individual drove the car but was a set formula (as indicated above). The Energy App in the car was the only measurement that estimated range based on driver specific consumption.

There was a separate thread on battery degradation and how it is reflected in Rated miles when fully charged. Since many of us see a gradual reduction in rated miles after a charge, some argued that Tesla told them that it is related to driving style. based on the answer you got it looks like that answer was wrong. When new my MS60 showed 190 miles after charge and now it's down to 184 after 6K miles. If this reduction is a reflection of battery degradation we have a problem...

Maybe the P85+ got the Rated back up from 265 to 300?

Sometimes a cigar is just a cigar

Concerning rated miles. I picked up my Model S last Friday and then on Saturday did an extended trip from northern Virginia to Virginia Beach. I set the charge to max range on Friday night. The max charge I got was 270. Drove to Virginia Beach and again wanted max range for the return trip, and again all I got after a nights charge was 269. Have it reset to standard charge now, which is giving me 236, which is fine for local driving. I was under the impression that you could charge it to 300 under the max range setting. Do I have a software problem?

No you don't have a problem. The original estimate of range was Tesla's statement of 300. Now that they have the EPA rating of 265 that is what they have the display showing as the rated range. It is still the same kwh rating.

to Rsusi:
I may have misunderstood your question, but it is my understanding that if you change the settings from rated to "Ideal" you will get your 300 miles.

And,
I'm not sure Tesla's answer is correct. If you drive hard, the rated range goes down faster. It also goes up if you are on an extended downgrade. This means that your driving style affects the rated range. I think the initial rated range is what you should get with a usage of 308 kw/m. If you exceed that usage, the rated range shrinks accordingly and conversely if you stay below it, it increases.

I check this by adding the rated miles to my odometer. Once I have that figure, I continually add the two numbers. If I drive very conservatively, the number increases, if not, it decreases.

Example. My odometer says 6,000 and my rated range is 240. This means that at the end of my charge I my odometer should read 6,240. After 40 miles (indicated on the odometer) my range should be 200. If after 40 miles, when I add the rated range to the odometer it is (for example) I get 6,235, then I know that in 40 miles, I've actually used 45 miles of charge. I know I'm driving over the allotted kw/m. If I slow down, when I add the numbers, I will get over my expectations.

With this being said, your rated range at START UP is what Tesla said it was, but the rep didn't understand your question. The instantaneous range display DOES use your driving to tell you how much has been used BUT the remaining amount is still based on the 308/kw/miles. So the answer is Slow down to increase range and speed up if you're near a supercharger or home.

THe answer that I got from Tesla a few days ago when I asked this question is the same as that received by DouglasR.

So at any given point in time, the rated range is computed by dividing the energy remaining in the battery by 300 (or 308 or whatever number the EPA or other agency in other countries initially used to compute rated range). Projected range is computed by dividing the energy remaining in the battery by your usage over the previous 5,15 or 30 miles.

That being said, I don't see any real value in knowing the Rated range since it is computed using a number that may bear no relationship to how you currently are, have been or will be driving during a trip.

Can somebody explain why rated range is something worth knowing?

mbergman, i think that you are totally right. And you raise a very good question. Its a pretty good guesstimate of remaining battery for those days when you won't be cutting it short. Other option would be to display the battery left in terms of percentage or kwh, but that wouldn't mean anything to most people.

Another good question is: why doesn't the energy app when displayed on the left of the speedometer tell you your projected range? it provides the entire chart and the average usage, but no projected range!

No.
Ideal is the old EPA standard, 2-cycle test. 300 miles.
Rated is the new EPA standard, 5-cycle test. 265 miles.
Projected takes account of your driving style, and varies. The other two do not.

Brian,

We are having a bit of a disconnect. You are right too. We are talking about how the 300 miles and the 265 miles is calculated. Those tests that you refer too come up with a number for the watt hours the car uses per mile (Wh/m). For the "rated" 265 miles, you divide the charge in the battery that is available for driving by 300 wh/m (or 308 wh/m or whatever) which is the result of the 5-cycle test. That is how the remaining rated range at any time is calculated.

But Jnb, if I am not cutting it short, who cares how much battery I have left? And if I am cutting it close, range-wise, and I'm trying to decide if I need to slow down, etc., wouldn't projected range provide me with the most meaningful information?

Neither rated range nor percentage of battery remaining take current energy consumption into consideration, so both seem pretty irrelevant compared to projected range.

I guess I am wondering why projected range isn't more prominently displayed than rated range, or to take it to the extreme, why rated range is displayed at all.

OK, I can see having rated range displayed when you turn the car on, but it becomes more and more meaningless the further you drive, no?

mbergman, I agree with you. It does get more irrelevant the more that you drive. And it would be nice if it adjusted as you drive. Which is why i don't understand why the projected miles are not displayed in the instrument panel when you have the energy app open.

As far as what it is supposed to be, though, I think of it as a "battery remaining" number that is expressed in miles, instead of percentage or hours. Think if it like the battery indicator on your phone. If it says 100% you know that you have a full battery but no idea how long it will last based on your usage. Same for when the battery is at 50%. Here, same idea, but it shows you the remaining battey life in something that is just a bit more useful, miles, although still an estimate. I tend to look more at the green bar that the miles frankly.

@info

I think the rep I corresponded with understood my question just fine. Of course the way you drive will affect the rated range you have remaining at any given point -- I even specified this in my question when I said, "although driving style obviously affects energy consumption, and hence the number of rated miles remaining at any given time."

However, the question had to do with how that rated range number is calculated. As it turns out, that computation is very simple, and does not involve your driving style. The car estimates how much usable energy remains in the battery and then divides by a fixed number. That fixed number depends on the tests used by the jurisdiction responsible for monitoring energy efficiency, but it would be the same for all U.S. cars. So, for example, let's say the fixed number is 308 whpm under the EPA's five-cycle test. Assume further that the usable energy in a full battery is 81,620 watt-hours (81.62 kWh). If you divide 81,620 by 308, you get 265 rated miles. Under the EPA's older two-cycle test, the assumed consumption would be lower, say 272 whpm. Divide 81,620 by 272 and you get 300 rated miles.

Now these numbers are somewhat hypothetical. We don't know the precise number of usable kWh in a full 85 kWh battery. We don't know how TM deals with the 10 or so miles a car can be driven after "rated range" goes to 0. The fixed numbers -- 308 whpm for the five-cycle test and 272 for the two-cycle test -- are basically just estimates. But the principle is the same: all U.S. cars use a fixed divisor to compute rated range from the car's estimate of energy remaining in the battery.

You asked why this matters. For me, it is helpful to know how closely the actual range of the car is reflected in the rated range displayed on the speedometer. I can control energy consumption to a degree by how fast I drive and what accessories I run. If I can keep my average consumption since the last charge to around 300 whpm, then the rated range displayed on the speedometer will tell me exactly how far I can go before the battery is completely depleted.

I actually like seeing and use both rated and projected ranges.

Rated is a nice standard (overall, if I drive fairly conservatively, no fast acceleration, keeping my speed no more than 5-10 mph over the speed limit), then I do seem to stay somewhere close to the rated range. Some of us may need to adjust up or down slightly, but I feel rated range is a good average driving statistic (sort of like a gas gauge). It shows that, if I go back to fairly normal driving, then I roughly know what my range should be. In addition, rated also helps me see how my battery pack is doing over time since it is a consistent number, so I can compare rated miles over time when I am fully charged to see how much capacity I am losing.

When I need to focus on how I am currently driving (which may be different than my normal driving style), then I look to projected range (5, 15, or 30 miles depending on how long I have been in a certain driving style). For example, if I am traveling on a longer trip (highway driving with more consistent and/or faster speeds), then I look to projected miles to make sure I will get where I am going at my current speed and adjust accordingly.

I agree that having projected displayed on the instrument panel energy graph would be a useful addition.

@mbergman, the reason I don't rely on "Projected Range" displayed at the right side of the energy app is that it is rarely accurate. The instantaneous number is so variable as to be practically useless. If I use a 30 mile average, it is better, but it still varies widely as I drive. However, the trip meter showing consumption since the last charge provides a pretty stable number. If I can't get it under 300, I know I need to plan a stop within the rated range.

Yeah, ok, what everybody is saying makes sense. The most important thing, I think, is to understand exactly what these numbers represent. I, for one, didn't understand how rated miles was computed until a few days ago, and I am sure that as I put more miles on the car, I will get a better feel for the relative importance of the rated vs. projected numbers.

@DouglasR

I think you have the a good grasp on the issue. The rated range is based on the EPA estimated consumption rate. The 85 kWh battery consumption rate is slightly higher than the 60 kWh battery. I presume because to the difference in weight of the vehicle. Anyway what shows on the dash under the speedometer is an estimate of how many miles are remaining IF you consume power at the rate of the EPA test for that vehicle. What shows on the touch screen is the "projected" miles remaining if you continue to consume power the same as the average of the last 30, 15,or 5 miles (if you display the average). I am not sure what length of time or distance is used for the instantaneous values.

All that said, the car does not know what lies ahead so you as the driver have to use some common sense and know to adjust the estimates based on your destination. For example my commute to work is a net downhill so my consumption averages in the 230 Wh/mile range while my commute home averages about 335 Wh/mile for a round trip average of about 283 Wh/mile. My projected range on the way to work is higher than the rated range and vice versa on the way home. Once again neither of those numbers are correct predictors of the future, they are based on the past and you have to take that into account.

@ DouglasR – Thanks for this thread and posting the answer from Ownership. It answers a question I was trying to explore in another thread (Need Help from 60 kWh Car Owners) – the Rated Mile Wh/M is a different value for the 85 kWh and 60 kWh cars. This, of course, makes sense since the 60 kWh car is about 200 pounds lighter than the 85 kWh car.

You mentioned, “It does not explain why the relationship between rated miles used and whpm (as determined by the trip meter) seems to vary for a single vehicle.” I have collected a few data points on Rated Miles used. In addition to significant scatter in the Wh/M vales, I found that Rated Miles appear to be “used up” at a faster rate than they are predicted. On one trip, after driving 151 statue miles, the total energy efficiency on the trip display was 308 Wh/M, but we had “used up” 165 Rated Miles. I have exchanged emails with Ownership on this topic and they hope to have an explanation for me sometime next week. I will post the answer here when I hear back from Ownership.

Let me give you a concrete example. A few months ago, I wanted to drive with a friend to Paradise on Mt. Rainier, elevation 5400 feet. The route from my house in Seattle to his in Kirkland, and up to Paradise is about 121 miles, or 242 miles round trip. I was never worried about running out of charge, since I would be passing plenty of public charge stations as I neared Seattle on the return trip. But I wanted to do it on a single charge because my friend is not well, and stopping would be inconvenient.

On the way up the mountain, I could see that my average consumption was quite high, around 400 whpm as I recall, and of course the Projected Range numbers were way too low to get me home. On the return trip, the Projected Range numbers were unrealistically high, and consequently unreliable. However, my average consumption began dropping from over 400 whpm to around 325 whpm at the bottom of the mountain where the terrain began to flatten out. Thereafter, my average consumption continued to drop, but much more slowly. By the time I got to a point where public chargers were available, my consumption had stabilized to under 300 whpm, and my rated range matched pretty closely with the actual number of miles I needed to travel to reach my home. I therefore did not feel the need to stop, and I arrived home with about 18 rated miles to spare, having consumed about 246 rated miles to travel 242 actual miles.

As it turns out, I didn't need to stop. But suppose the trip home had been 30 miles longer, and stopping to charge would have been necessary. At 50 miles from home, consuming energy at an overall average rate of 300 whpm, my Rated Range would have been about 40 miles at the point where I first encountered the charge stations. My Projected Range, however, would have been about 60 or 70 miles, based on the previous 30 miles of (downhill) driving. Had I relied on that too-optimistic Projected Range estimate, I might have skipped a charge and run into trouble.

@Rod & Barbara

Our posts crossed. But I agree with you about the unexplained variability. In the case above, for example, my total actual miles was 242, rated miles was about 246, and yet my average whpm was UNDER 300 (this is from memory, but I think pretty accurate). So clearly the rated miles here had a consumption rate less than 308.

Edit: and of course, my examples involve an 85 kWh car. With a 60 kWh car, the numbers for rated and ideal range will be less than 308/272.

@Rod and Barbara wrote:
"On one trip, after driving 151 statue miles, the total energy efficiency on the trip display was 308 Wh/M, but we had "used up" 165 Rated Miles. I have exchanged emails with Ownership on this topic and they hope to have an explanation for me sometime next week."

According to this summary of EPA testing procedures, "EPA tests for fuel economy do not include electrical load tests beyond climate control, which may account for some of the discrepancy between EPA and real world fuel-efficiency."

Not counting the motor, the Model S has a higher electrical load that most ICE cars. Headlights, radio, heater, A/C, display screens, computer cooling fans, etc. all drain the battery, and this could easily reduce actual range over the EPA estimated range by 10%, as you observed. 32 watts steady load for accessories is 32 extra wH/mile. For a full charge, 81620/(308 + 32) = 240 miles, instead of 265, or for a 150 mile trip you would use up 10% more energy than estimated = 165 miles of rated range used.

Does anyone know if the Model S instrumentation (trip meters and Energy Graph) actually counts energy used by everything or just the motor? You can be sitting still with the A/C or heater on "full", but the center dial seems to read "0 kW" used, and the Energy Graph "Instant" or "Average" number won't move much except to reflect the slowly decreasing state of charge.

With the new 4.5 "shore power" feature, you can see the car draw 1 to 2 KW from the wall when the A/C is running and you're sitting still.

@Bob W

You make a good point. The trip meter does NOT show any energy usage when the car is stationary. It is certainly possible that when the car is in motion, it records ONLY the energy used by the power train. That could easily account for the variability in whpm consumed across a rated mile.

Evidence against that theory is anecdotal: turning on various accessories seems to result in higher energy consumption as measured by the trip meter. I admit, however, I have never looked at this question systematically. The way to do it would be to take two trips under the same conditions (as far as possible), but turning on all accessories, bright screen, etc. for one trip but not the other.

My P85 Sig (w/ 12k miles), is only showing a 'Rated Range' of 240 after a full-charge. I was told by three different service advisors at Tesla, that our 'Rated Range' is based on our driving habits (battery usage, etc), and will vary greatly depending on average speed, grade, etc.

I regularly drive in the HOV lane at speeds of 75-80 MPH on fairly flat roads on a daily bases, with the a/c on. Thus, I assume this explains why my 'Rated Range' is so much lower than some others here.

How does Tesla measure the energy left in the battery? Is it simply a measurement of the voltage and compared to a full battery?

@cerjor - It is apparently quite difficult to do reliably. http://liionbms.com/php/wp_soc_estimate.php

@TikiMan - The service advisors may be right, but "the powertrain systems architect" sounds pretty authoritative. What is your average whpm?


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