- The energy consumption per mile (Wh/mile) is function of car weight and aerodynamic resistance. The effect on range is negligible **only** if car is moving with constant speed on a level road. For any movement with varying speed, with stop and go traffic and/or on uneven terrain Wh/mi will increase with increased weight of the vehicle.
This is the reason that 60 has lower Wh/mile than 85. As I posted before, according to the page 149 of Tesla Manual weight of 85 is 223 pounds less than 60 (4630-4407=223). Since EPA range is determined not based on steady speed driving, but on 5 cycle test, the Wh/mile of 60 is lower than that of 85.
It is kind of funny that so many people have problem with this. To use the analogy with ICE car, Wh/mile is equivalent to MPG, or to be precise, GPM (Gallon per mile). For some reason the effect of car weight on MPG is universally accepted, but a lot of people get confused with accepting the same relationship for Wh/mile. Just to complete the ICE analogy, the battery pack size in kWh is analogous to the gasoline tank size in gallons.
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All of the above factors influencing your Wh/m figures are already and completely encapsulated in the EPA Range value.
The "Wh/mile" that you mention above that varies with terrain, mass, speed, etc is *instantaneous* Wh/mile as measured in the car. I agree that instantaneous Wh/mile is one of the factors influencing EPA range. Another factor is pack size in KWh.
But the "Wh/mile" you use in your charts is not the same, even though the units are the same. Your "Wh/mile" values are calculated thusly:
([kWh Pack size]*1000*.95)/EPA Range
which isn't the same thing as instantaneous actual Wh/mile as measured in the car during the 5-cycle test.
Let's just take the numerator of your "Wh/mile" values: ([kWh Pack size]*1000*.95) which you are using on your Y-axis. What you are calling "Wh" is actually just "Usable Wh."
This is obviously a linear function of kWh pack size. Here's the chart of that relationship:
- The reason I chose to work with the Wh/mile chart is because it is linear as function of Battery Pack kWh and vehicle mass (larger battery is equivalent to the added weight). The linearity is due to the fact that energy spent on changing of the kinetic, potential energy and friction losses is proportional to mass. So if two cars of identical shape but different mass are taken through exactly that same 5 cycle test under the same atmospheric conditions, the expended energy would be proportional to the mass of each vehicle.
The problem with the EPA range vs the kWh pack size graph that you are attempting to use is that it actually is non-linear.
But you don't seem to see the point of this. The graph I'm using, and the graph you are using, are essentially the same. The only difference is that your graph is applying a fixed, dependent, factor to the Y-axis (kWh*1000*.95). There's no need to obfuscate the y-axis with your "Wh/mile" calculation when EPA Range provides the exact same graphic data. Just to prove the point, here is your graph, using EPA Range instead of Wh/mile on the y-axis (i.e. removing the dependent axes -- the lines are flipped because EPA Range is no longer in the denominator):
It's just as linear with or without using Wh/mile on the y-axis. If you take a linear function, and apply a fixed factor to it, it's still linear.
Range varies with the Battery Pack kWh **and** also depends on kWh/mile, which in turn also depends on Battery Pack kWh (due to added weight )
This is simply circular logic. Again, all of those factors are already included in the simple EPA Range numbers published.
- The calculation you used to obtain the EPA range for the hypothetical 70 is not analogous to what I did. My calculation, as I mentioned before, is just another way of saying that increase in rnage of 60D over 60 is proportional to the increase in range of 85D over 85. This statement is true because all these variants of Model S have similar batteries and drivetrains.
Similar extrapolation between 70D and 70 based on ratio of ranges for 85D and 85 is not accurate because these cars presumably have different gen. of batteries. In another words the number you obtain for 70 is accurate only if you assume that 85D, 85 and 70D have the same gen. of cells, i.e. cells with identical volumetric and gravimetric energy density. If cells are different, this formula is wrong.
This really doesn't matter.. throw it out then. Throw out both blue lines in the graph in my previous post. I'm not using it to prove any points, I only included it, and imputed the range for the S70 so it was a complete parallel to your original graph. But I *am* assuming the packs are different. You are not. Therefore, even if I needed to use the imputed S70 range, my calculation actually is correct, exactly the same way you imputed the range for the 60D. But like I said, the range for the S70 is irrelevant to my previous post. It was included for "completeness".
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- As follows from discussion in #1 above, the point that you marked on orange line – 281 Wh/mile corresponds to the hypothetical efficiency of the 70D **if** increase in the battery capacity by 10kWh would have been accompanied by the corresponding increase in battery (and car) weight. The fact that 70D energy consumption lies below the orange line indicates that increase in the battery capacity did not result in appreciable gain in car weight.
- The improvement associated with the new gen. battery could be measured as an increase in it’s capacity without increase in size (same quantity of cells) and weight (new cells weigh approximately the same as old cells), i.e. 70/60=16.7%
No, I don't think you can draw those conclusions. Assuming the cars are the same weights, it's quite possible that the added weight from the additional cells in the 70D over the 60D
(and we're only talking 86 pounds here) were made up due to manufacturing improvements between the two cars, since there's no control or accuracy on those data sets. I think it's much more believable, if the cars weigh the same, that it's a net 0 gain between additional batteries and other reductions in weight.
- I am not a chemical engineer (and I suspect you are neither), so I prefer to treat the battery as a black box and not try to attribute battery improvement to “newer cell technology” or “other incremental improvement along the way”. I just say that we are dealing with new generation of the battery cell, which has increased volumetric and gravimetric battery density, by about 16.7%. This is consistent with JB mentioning during one of the ERs that there will be 10-15% improvement in battery chemistry.
This is mincing my words. By "other incremental improvements" I was not talking about batteries, I was talking about any other improvements in the drivetrain, energy or battery management, manufacturing improvements, or weight reductions over time, resulting in what appears to be two cars with the same weight, but different "energy densities".
- One final point for people incredulous that Tesla already has new generation of battery cell. Consider that according to the Q1 2015 ER the start of battery production at GF is scheduled for Q1 of 2016. This need to be preceded by the installation and launch of the cell making equipment, which in turn must be preceded by completion of the manufacturing of the said equipment. In order to complete the above, of course, the design of cell manufacturing equipment need to be completed, which in turn means that design of the cell has to be completed prior to that. To make long story short, if Tesla/Panasonic does not have completed design and bench testing of the new cell/battery pack design **now** ,they (and us, investors) are in seriously deep trouble.
I think this is a red herring. You can't base your conclusions on the totally unknown R&D processes and timelines of both Tesla and Panasonic. Sure, they probably have something in the pipeline, and I would expect that GF planning would allow the GF to quickly adapt to any new/developing improvements in battery technology (I think this was referenced above by something Elon said). This is just using an unknown timeline (several timelines) to back into your assertion that they *must* be using the new technology right now. Or in fact, at least a year ago, since they were probably working on releasing the 70D at least that long.
So I still assert, applying Occam's razor here, that usually the most simple solution is usually the correct one. Are we to believe all the things you assume, that there's totally unannounced, newly developed, low manufacturing yield, battery technology, or it's just simply (as stated by a Tesla employee) that they're using 842 more cells in the same battery pack?
