The Model S displays two estimates of range remaining based on the Battery State Of Charge (SOC): Rated Miles (RM) and Predicted Miles (PM). The RM are based on a fixed Wh/Mile efficiency and the PM are based on the Wh/Mile efficiency of the last 30 miles driven. The Rated Miles Remaining (RMR) is always displayed in the center part of the instrument panel. There is also a Range display that shows the RMR and Predicted Miles Remaining (PMR). This Range display can be posted on the touch screen, on the right side of the instrument panel, or on the left side of the instrument panel. The Wh/Mile efficiency of the last 30 miles is presented on the Energy display. In like fashion, the Energy display can be posted on the touch screen or either side of the instrument panel. The Wh/Mile efficiency of a RM, which is a constant, is not presented on any display or defined in the Guide for Owners. The Tesla Ownership Experience team has indicted that the RM Efficiency (RME) is about 270 Wh/Mile. My empirical data so far indicates the RME is probably between 266 and 270 Wh/Mile. That is the “quick-start” version of range remaining displays in the Model S, and it’s likely that’s all that is needed for most individuals. However, if you are an engineer, a data hound, or are interested in the precise calculations going on behind the scenes, I will present some interesting and confusing data in succeeding paragraphs and make an appeal for group data collection in order to try to understand RM particulars. If this doesn’t sound interesting to you, I suggest you stop reading now.
Let’s call the maximum RM that can be obtained with a 100% charged Battery (Max Range mode charge) Rated Miles Max (RMM). The Tesla Ownership Experience team has indicated that not all of the 85 kWh in the Battery are available to propel the car. So let’s define the kWh in the Battery available to propel the car as the Usable Battery Capacity (UBC). The following relationship applies: RMM x RME = UBC. Tesla does not define any of these critical parameters in any documentation that I can find. I especially want to know what the UBC is so that I can use it for trip-planning purposes once I get a feel for what sort of efficiency I can expect under particular driving conditions.
There are many different ways to determine the UBC. Here are a few examples:
- The Tesla Model S Efficiency and Range blog presents two different graphs, a Range vs. Constant Speed graph and a Battery Wh/mile vs. Constant Speed graph. These two graphs are related by the UBC. For the Roadster the UBC is 55 kWh and the data on the two graphs are completely constant using this value. For the Model S the data on the two graphs are not constant for any fixed UBC. For example, at 40 MPH the data are constant if the UBC is 82.4 kWh, and at 80 MPH the data are constant if the UBC is 80.5 kWh. I have contacted Tesla asking about this inconsistency, but have yet to receive an explanation.
- The Ownership Experience team told me one time that the RMM and RME were 320 and 270 and on another occasion 300 and 270. These two sets of values result in a UBC of 86.4 and 81.0 kWh.
- The recent Motor Trend article about range testing stated that during the test drive 78.2 kWh were expended and this used 93% of the Battery capacity. This results in a UBC of 84.1 kWh.
- My personal data collection from our Model S indicates the RMM is between 300 and 316 miles, and the RME is between 261 and 272 Wh/Mile. This provides a wide range for UBC from 78.3 to 86.0 kWh.
My suggestion is that we pool data from our driving experiences to more quickly home in on the correct values of RMM and RME. Once we accurately determine these values, we can easily calculate UBC.
The best way to calculate RME is to collect three pieces of data at a specific time while driving. Select the Trips display and the Range display (each of these can be presented on the touch screen or either side of the instrument panel). When one of the trip counters on the Trips display equals 30.0 miles, record the Wh/Mile efficiency for that trip counter, and from the Range display, record the PMR and the RMR. All three of these pieces of data need to be collected during the time the subject trip counter reads 30.0. Once the counter advances to 30.1 the subject values are no longer valid. The RME can be calculated from these data as follows: (PME x PMR) / RMR = RME.
The best way to calculate RMM is to collect data at the end of a charge cycle. At the conclusion of a charge cycle record the following parameters: type of charge (standard or max), SOC, RMR. The RMM can be calculated from these data as follows: RMR / SOC = RMM. As a side note, this particular data collection method can actually be done at any time. For example, the SOC and RMR could be noted every time you begin and end a drive in the Model S. I have done this and found a perplexing trend. The lower the SOC when the data are collected, the lower the calculated RMM turns out to be. This doesn’t make sense to me and I have asked Tesla for an explanation. For now I would prefer to limit this data collection to end of completed charge cycles.
So, if you want to help determine accurate RMM, RME and UBC for the Model S, please post the following data on this tread as you gather it:
- Trip Wh/Mile (same as PME), PMR, and RMR at the trip 30.0 mile point.
- Charge mode, SOC, and RMR following a completed charge cycle.
I will collate the data, and post periodic updates on this thread.
P.S. Please, only post data if you are driving an 85 kWh Model S.
P.S.S. All of the acronyms presented in this post, except SOC, are my personal constructions.
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