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Survey: Rated Range and Average Energy Tesla Model S

In an attempt to answer the question "Is related range related to driving habits?"

We have heard from Tesla employees that there is a correlation, some drivers say no.

Please fill out this four question survey monkey after charging up your car. A summary of the data will be provided to this forum once collected. Thanks.

http://www.surveymonkey.com/s/569L5MJ

Using 117V charging at 12A (1.4 kW), which is about as inefficient as you can get, the Model S reported 5 kWh added to the battery, whereas the Kill-A-Watt meter reported 8.13 kWh consumed from the Power Company, which means only 61.5-71% of the energy made it into the battery (assuming 5.0 to 5.99 kWh were actually added; the Model S may truncate rather than round). The battery was nearly full, so maybe more energy is waste as heat compared to charging a half-empty battery.

A Tesla rep. has claimed that 40A charging at 240V (10 kW) is the most efficient in terms of minimal energy wasted, but I have not tried to confirm that. Someone with separate EV metering and an HPWC could do this quite easily (charge one night at 40A, charge the next night the same amount at 70 or 80A, compare power consumed in kWh as reported by the utility meter or the utility company's "Daily Energy Consumption" web app.)

@ Bob W – For all the data points I entered in your survey, I had Average selected in the Energy app, my Model S was on and running in Park in my driveway and not connected to my charger. The A/C may or may not have been running depending what the climate control settings were at that particular time. There is no reason a P85 should have a different rated mile Wh/M than a standard 85. If your standard 85 uses 300 Wh/M, your P85 loaner uses 302 Wh/M, my standard 85 uses 306 Wh/M and all the rated mile lines on the Energy app confirm these numbers (the line in my car is clearly displayed above the grid line, not just barely above it as in the loaner), it may be that the Wh/M for forecasting rated miles is unique to each vehicle. This value could be uniquely assigned to each vehicle to standardize the max rated miles displayed to 265 among vehicles with slightly different inherent battery capacity. I will contact the Ownership Experience team and see if they have any insight about this.

BTW, another method I have used to determine the rated miles Wh/M is to note both the rated miles added and the energy added following a charge cycle. This requires that you sit in the car and change the display setting from distance to energy (or vice versa) before disconnecting the charging cable. Dividing the energy added (in Wh) by the rated miles added produces the rated mile Wh/M. You must be adding significant charge to the vehicle in order to reduce the scatter in this data collection due to rounding/truncating errors. The value I get using this method is slightly lower (304.8) than the value I get from your method (307.6). I use an average of the two methods (306.2) as my best guess for the Wh/M in my vehicle.

Well I've got another data point for you. For no apparent reason, my car suddenly jumped to using 305 Wh/mile as the divisor when the rated range was around 188 miles. The line was clearly visible above the grid line.

But after charging, it went back to its usual 300 to 302 Wh/mile, and the rated miles line moved back down.

The calculation just doesn't behave in a consistent fashion, from car to car, and even in the same car. I think it has something to do with the build up of the "reserve buffer" that you documented, something about the fact that you have 0 reserved miles after an extended charge completes, but 5.1 kWh of reserve when you get to 0, and all the calculations are based on when you get to 0.

Bob - have you considered the possibility that Wh/mile depends on temperature?

The temperature here in CA has been pretty consistent. The rated range Wh/mi seems to vary, slightly, with no obvious explanation.

The Rated Range should go down (and rated range Wh/mi should go up), as soon as you turn the A/C or heater on, but it doesn't. I'm talking about the solid line on the Energy App, labeled "Rated Range". Sometimes this line appears just above the 300 Wh/mi grid line (302-307 Wh/mi), and sometimes it moves down and hides it (300 Wh/mi).

@ Bob W – I don’t think the rated miles or the rated miles Wh/M should change when the A/C or heater are turned on. Rated miles are designed to represent a fixed amount of energy use per mile and essentially act like a very finely granulated measure of energy left in the battery. The energy left in the battery doesn’t change when the A/C is turned on; the energy is just used up faster. Likewise, rated miles will be used up faster, compared to actual miles driven, when the A/C is on. Of course, there are times when rated miles don’t appear to be acting as designed, such as the recent movement of the rated miles Wh/M line on the Energy app graph in your vehicle.

The energy left in the battery doesn't change when the AC is turned on. But it does change when the temperature of the battery increases or decreases. It's nothing to do with energy usage; it's battery chemistry.

The rated range is supposed to give you a pretty good estimate of how far you can go, given the "normal driving", based on the amount of energy left in the battery. If you turn the A/C on, I think the rated range number should go down by 10% immediately, as it does on the Ford Focus EV (and maybe the Nissan Leaf as well, not sure), as many tests have shown that using the A/C or heater will reduce your actual driving range by 10% or so.

I really would like to understand how the build-up of the "reserve buffer" affects the rated range displayed. Starting with a full charge (265 rated), do they reduce the rated range display by an extra mile for every mile you drive, until 5.1 kWh have accumulated in the reserve? It does seems like the rated range drops much faster than actual miles driven for the first 10-20 miles, then it drops more slowly. Is that because they're "building up the reserve buffer"? If I do a standard 90% charge, which for me usually stops at 228 rated miles, how many kWh are in the reserve at that point?

I think you need to consider how you think the rated range meter works. And I don't just mean the algorithm, I mean what measurements do you think are taken?

I don't know how the rated range meter works, but I do know how the battery meter for a cellphone works: it uses measurements of the temperature and voltage of the battery. From this, a fairly complicated algorithm (which includes the rate of discharge) is used to project the remaining power that the battery can deliver. I suspect the rated range meter works in a somewhat similar way.

The "reserve buffer" exists because a voltage must be selected at which the meter reads zero. Often this is set at 80% of the nominal voltage. The battery can still deliver power at this point, but in practice the amount of power available once the battery is outside of its standard operating range is somewhat unpredictable.

@ schueppert – You bring up an interesting point in your Aug 4 post about energy left in the battery changing as the temperature of the battery increases or decreases. Tesla specifically had a problem with this last winter when cold weather resulted in abnormally low rated range values. The problem, as I understood it, was that the rated mile display did not take into account the thermal conditioning the system would provide to the battery as the car was driven. In software v4.3 Tesla introduced new software designed to improvement range predictions in cold climates and this eliminated the wild swings in rated range that I observed during the first several miles of driving in cold weather. If outside temperatures have some effect on measured battery capacity remaining, then this should be reflected in appropriate changes in rated miles, but it doesn’t make sense to me that this would also affect the rated miles Wh/M.

@ Bob W – The rated range provides an estimate of how far you can go if you drive precisely as efficient at as the vehicle was driven during the EPA 5-cycle test. The predicted ranges on the Energy app are designed to provide estimates of how far you can go based on current driving conditions during the last 0.1, 5, 15 or 30 miles.

The battery buffer that is created to keep drivers from being stranded when the rated range reaches 0 is built up slowly and linearly as rated miles are consumed. Rated miles are predicted using about 306 Wh/M, but they are consumed using about 287 Wh/M. So for every rated mile that is consumed the battery buffer is increased by 19 Wh (306-287). After driving 265 rated miles a buffer of about 5.1 kWh will have been created (19 x 265 / 1000). So if your rated miles read 228 after a 90% charge, the system has created a buffer of about 0.7 kWh and will add about 4.4 kWh to this buffer as you drive from 228 rated miles to 0 rated miles.

I determined this behavior based on data I collected during two recent trips of 175 statue miles each. I recorded the 30-mile average Wh/M and predicted range from the Energy app, and the total energy from the Trip display, which was reset at the beginning of each trip. Multiplying the 30-mile average Wh/M by the predicted range results in the remaining battery capacity until 0 rated miles. When this value is added to the total energy already expended on the Trip display this results in the total usable battery capacity until 0 rated miles. This value started at about 81 kWh and slowly decreased in a linear fashion as rated miles decreased to about 77 kWh at the end of my 175 statue mile trip. So, there was no battery “buffer” at the beginning of my trip and there was about 4 kWh of battery “buffer” created by the end of my trip.

Your analysis is very clever. But it assumes the rated range meter and the energy app have a shared estimate of the energy (kWh) remaining in the battery. This may not be correct. For example, here is one alternative explanation for your observations that does not make this assumption:

1. The battery management system uses a voltage measurement to estimate a state of charge in the range 0% to 100%. (The state of charge is not displayed anywhere but it is available from the API).

2 The rated range meter reads 265 miles (the EPA rated range) at 100% state of charge and decays linearly to 0 miles at 0% state of charge. The vehicle can still be driven because the battery is not dead at the voltage associated with 0% state of charge, but the remaining range is hard to predict because the battery is operating outside of its standard operating range.

3 The energy app estimates the remaining energy (kWh) using the state of charge and the temperature of the battery. It displays an estimated range using this energy estimate divided by the instant or average usage.

So for example, an 85 vehicle at 50% state of charge would always display 133 miles of rated range, but the energy remaining shown in the energy app would vary somewhat based on the temperature. The implied rated Wh/mi you are calculating would also then vary based on temperature.

I have no idea if this explanation is closer to the truth than your explanation. But it fits the same facts and is arguably simpler. It also makes some testable predictions: that implied rated Wh/mi should vary with temperature, and that vehicles at the same state of charge should always show the same rated range (though you would need access to the API to test that).

I for one think they nailed the rated range, so kudos to the EPA. I usually average under 308 Wh/mi which is theoretically the rated usage, so I always travel as much or more than the dash shows. Even on all day trips, with 8 hours or so of vampire loss and remote cooling of the interior, I get home about 1:1 with rated mileage.
I just can't fret a few miles here and there. My gas cars never get the same mileage every tank, nor does my S. Its life. As long as my gauge is true to a degree I trust it.
So far as I can simply tell, rated is just whatever charge is remaining divided by 308. I deal is whatever charge is remaining divided by 268. Use the trips page on the dash for this to gauge your current average. If you want a range that changes based on driving habits, use the energy page. But if you drive in a respectably efficient manner, you should be able to hit rated numbers and have no worries. Ideal is truly a challenge though.
I'm so over worrying about the range. I so rarely break over 200 miles in a single day its not worth the stress. I know it will get me where I'm going.

Cn;
Get a job with the EPA or Tesla as the perfect test driver. ;)

Being an avid hot rodder, I don't think many nice things about the EPA. But man I have thought about working for tesla on several occasions. But until I lose my current job and house, it's not gonna happen.
If I did work for tesla that car would not have come off the line with incandescent bulbs... Anywhere!

@ schueppert - In order to keep from hijacking this thread, I posted an answer to your August 6 post in this thread: http://www.teslamotors.com/forum/forums/important-new-information-about-....

I finally got around to reading this thread. Interesting stuff. I do have a question about filling out the survey, however. What do you mean by "a full non-range mode charge"? I assume a range-mode charge is a maximum charge, but short of that there are multiple levels to which the car may be charged using the current firmware.

@DouglasR

What I meant with the survey is that a full charge before the "trip" section. The maximum Daily number was the data I was looking for.

I think the case is pretty closed that driving habits in terms of Wh/mile have no relationship to Rated Range.

@TeslaOwnerBlog

I must be dense, but I still don't get what you are asking for. My max charge is between 265-275, but my non-max charge can be anything from 50% to 90% of that, depending on where I set it.

I agree with you, BTW, that driving habits do not affect this number.

Bob W and Rod,

You are two very smart dudes!

Richard

@ Bob W – Per my August 2 post, I contacted the Ownership Experience team about the different rated mile Wh/M exhibited in my vehicle, your vehicle and your recent loaner vehicle. I did not receive a very satisfying answer. Here is what they said:

“Hi Rod,

After receiving your email, I inquired to members of our powertrain engineering team for any additional insight. Although I did not receive specific information to explain, I did receive the following rationale:

Similar to a gasoline power vehicle, there will be slight variations between different Model S even when built with the same options. Even when brand new, 2 BMWs that are tested for horse power and emissions will have slight variations that will become further apart over time and use.

While I’m sure the formula you are using will be able to give a general sense of wh/mile measurements, it’s not the algorithm that Model S uses, so there are likely further variations based on minor discrepancies between these.

Thank you for the continued support and enthusiasm for our Tesla vehicles.”

So, I’ll continue to collect data and try to conjure up ways to test theories that might explain the differences we have seen.

@DouglasR

When charging there are two sections "Daily" and "Trip". Trip is what is called or used to be called a full-range charge. You really should not use this routinely. Its not a big deal to fill up the car completely, but the battery will wear down if you do this say daily.

So the survey was based on setting the slider to the max in the Daily range, which is around 90%.

I think this is confusing, because the terminology has changed a bit from Roadster -> earlier software versions -> today's Model S software

So to clarify again, the most charge you can achieve within the Daily scale.

Interesting thread. You might be interested in the Plug In America Model S battery survery.

http://www.pluginamerica.org/surveys/batteries/model-s/survey.php

It has a rich data set on 261 vehicles. Anyone can download the data set and conduct whatever analysis they like.

I'll be presenting results at TMC Connect next week.

Please add your experience to the survey.


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