So, I asked this specific question of one of my friend/former colleagues who owns a Roadster here in AZ. He keeps EXTREMELY detailed stats on his roadster. He measures very detailed Wall-to-Battery and Battery-to-Wheel metrics. And he has it graphed against ambient air temperature.
As a Volt owner, I was shocked by his experience over the last 2.5 years. In my Volt, I easily see a 25% reduction in range in the summer's heat as compared to the fall's nice temps. In the fall, I can get 45 miles per charge (on about 10.3 kWh) in the Volt. In the summer, that can drop as far as about 26 miles per charge, with essentially the same drive style. I attribute this difference to A/C usage and the Volt's temp mgmt system - liquid-cooled "like" the Tesla (I say "like" in parentheses for a reason - I'll explain). I dont keep the same kind of detailed records, so I can't verify my reasoning, but it seems rational.
But my Roadster-owning friend said he sees a relatively small (only about 5%) battery-to-wheel difference in the summer in his range. And his data doesn't indicate a strong correlation between high ambient temps and range reduction. He did say however, he see a large (about 62%) impact and strong ambient temp correlation on the wall-to-battery numbers. Meaning he sees the charging efficiency drop quite a bit in the summer.
So, how to explain this. Let's talk about the battery charging first. In our brief discussion of this, he said he believes the issue is that the Roadster's TMS works very hard to keep the battery in good temp range while it is applying the high current to charge. Indeed, when I parked next to him at the charging stations, I could hear the cooling system running while he sat in the sun charging. Sounded like an air conditioner fan running. Thus, the charging efficiency drops significantly because the car needs to work very hard to keep the battery temp in range while applying the 40A current (our work charge station is a 208V/50A 14-50 outlet). It'll be interesting to see if this is the case with the S. 62% is huge. Basically charging times would be 50% longer or more.
There was a separate issue he had where the EVSE would stop working when ambient temps approached 118 degrees. The EVSE brick sitting in the sun in such conditions can reach a surface temp of 150 degrees, which seemed to trip it. This is an issue I've called Tesla about specifically regarding the Model S to which they replied this wont be an issue withe the S' UMC (based on learning from Roadster owners).
Now, for the surprisingly small battery-to-wheel impact at high ambient temps. We could only speculate a few things:
1. The A/C in the Roadster was much more efficient than the Volt.
2. I ran my A/C in the Volt much more than he did in his Roadster in the dead of summer.
3. The battery in the Roadster is much better insulated than in the Volt.
4. The Roadster's TMS is much more efficient than the Volt's.
5. The Roadster's battery pack's higher density/chemistry made it less impacted by high ambient temps.
There may be others, but we figured it's likely a combination of the above.
Now, will the S be the same? Will its shape (high surface area) exposed to the baking temp pavement make it more susceptible to the heat? These are big unknowns. BUt the evidence he's collected for the Roadster is very promising. I'm hoping it translates to the S. FWIW, I asked my Tesla rep these questions specifically. Of course, no detailed promises were made other than assurances that the S was tested in the heat of Death Valley and the S was "fine". Unfortunately, that doesn't say much.
Anyways, I wanted to share this as some data to help answer the question. My friend sent graphs. I will ask if he minds my posting them (or if he's willing to post them).
