The Tesla Model S Controversy: Understanding the Effects of Temperature on Electric Vehicle Efficiency and Range
by Vivek Gowri on February 18, 2013 4:22 PM EST- Posted in
- Auto
- Electric Vehicles
- Tesla Model S
- Nissan Leaf
Thus far, plenty of ink has been spilled regarding the Tesla Model S and the fallout from the New York Times article, and it even showed up in our latest podcast. I feel like my perspective on this topic, as someone who has worked in the EV space as an engineer for the last four years, as well as driving a family owned Nissan Leaf for the last year and a half, is a bit different from our editors who discussed it, as well as most of the others who have written about it.
I’m not particularly interested in getting involved with the back and forth or analyzing what either party said - I feel like the Times reporter didn’t really understand how EVs work, nor how to drive them, and I don’t really agree with Elon Musk’s Matlab data infused response (it felt too vitriolic to get across any point other than Tesla being angry about the article). And just to go back to the point about not understanding how EVs should be driven, you have to tailor your driving style to suit the powertrain in order to get the maximum out of the EV - if you don’t want to, you’re going to be disappointed. Consider it like needing to switch keyboard shortcuts when you move from Windows to OS X - it’s a slight mental recalibration that has to happen for you to use the platform to its fullest. But that’s another story for another time.
What I feel like is getting lost here is actual EV performance in cold weather, or hot weather, or really anything in the way of hard numbers. We all know that battery performance is reduced in more extreme environmental conditions, and that all cars, regardless of powertrain type, consume more energy (fuel or battery) in those extreme climates. Unfortunately, quantifying these general ideas is a bit more difficult. That’s where I come in.
Technology Background
First, a bit of background. I spent the last few months working in Argonne National Laboratory’s Advanced Powertrain Research Facility (APRF). They’re an extremely knowledgeable group of engineers and scientists whose job it is to test advanced technology vehicles in their temperature controlled dynamometer facility, which can sustain temperatures anywhere between -5 F to 100 F. Most of this research is done as a part of the Department of Energy’s Vehicle Technology Program.
Everything from pure electrics to plug-in hybrids, normal hybrids, clean diesels, direct engine turbos, and advanced transmissions (dual-clutch, CVT, etc.) go through the labs with an extended set of instrumentation to collect various speed and consumption metrics. The resulting data sets are truly comprehensive (2 million data cells for 20 minutes on the dynamometer) and allow the researchers to draw conclusions about the various powertrains and their behavior. It’s a pretty impressive setup.
The APRF has released some of the data to the public in something called the Downloadable Dynamometer Database. The D3 pages for the Nissan Leaf includes data from the three different temperatures used in EPA’s 5-Cycle fuel economy test: 20F (with a heater load), 72F (climate control off), and 95F + sun lamps (with an air conditioning load). In all cases involving heating or air conditioning, the climate control system is set to 72F. Using this data, we can take a more in depth look at what the actual penalties of environment are on efficiency and performance, and really see the impact of temperature on energy consumption and range.
Before we start, I should probably explain the terms used in the graphs that follow. UDDS is an urban driving cycle (it stands for Urban Dynamometer Driving Schedule), CS stands for cold start (the first start after the vehicle has been sitting for a number of hours), HS is hot start (any subsequent start after the vehicle has warmed up), HWY is short for HWFET or Highway Fuel Economy Test (also seen as the Highway Fuel Economy Driving Schedule or the EPA’s highway fuel economy cycle), and US06 is a supplemental federal test procedure (SFTP) to provide a more aggressive driving schedule than the relatively tame HWFET cycle. A comparison of the three driving cycles (speed versus time) is shown above.
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shokunin - Monday, February 18, 2013 - link
There's several factors that affect range in an EV, the heater/HVAC as described in the article.Active heating and cooling of the battery pack. The ability to maintain temperature of the pack is paramount to keeping the battery operating within thermal limits, whether from weather or excessive current draw (driving fast) creating heat.
Driving style and speed. The faster you drive, the increased wind resistance uses more energy the faster you drive. This is also due to the fact that many EV's simply have one gear, there is no "overdrive" gear on an EV.
For the Model S, it takes more energy getting the car "up to speed" say 40-50 than it is staying at that speed. Ironically, in the city driving with lots of stop lights, it will use more energy than coasting along the freeway.
The NYT writer did a poor job explaining these and explaining the fact that one has to change habbits when it comes to an EV. Fully charge the car overnight, and when you are at a Supercharger, charge to full. You don't fill up your car to make it to the next gas station, you just fill it up to full. When you take a road trip on an ICE, you fill up whenever you've got a 1/4 tank left. He sort of missed the common sense part of charging.
mchart - Monday, February 18, 2013 - link
I had thought the high-end sports car like EV's had at least a two gear transmission.I know for a fact that there are plenty of high-'output' racing EV's that are linked to a two speed transmission. A 'driving applicance' (Car that people who just need A-B transportation buy) doesn't need gearing though. Even more so in the US given the fastest most people ever go is 60-70mph.
Funnily enough; I'd only ever buy an EV right now for performance reasons. Not for going 'fuel-less' reasons. The essentially straight as an arrow torque curve of an electric motor is amazing for racing application.
DParadoxx - Monday, February 18, 2013 - link
While I appreciate seeing this sort of article on AT you needn't insult Musk's article. He identified inaccuracies in the NYT article and provided clear and easy to read graphs to back up his argument. Several of your graphs by comparison are crowded and difficult to read, even for an engineer.relztes - Monday, February 18, 2013 - link
Well, Musk's article really was exaggerated. Broder spent less than 2 miles out of 500 over 75 mph, but Musk made a big deal out of it. Same for the half mile around the parking lot, which would have to be the world's weakest attempt at draining a battery. He points out the 40 miles for which the heater was set at 72-74 F after Broder noted the declining range, but ignored the last 60 miles of that leg where the heater was set at 64-66 F.Now Broder did seem to exaggerate about how slow he was driving, and some of his decisions were pretty stupid. The decision to unplug at 35 miles was by far the dumbest. I'm really curious about Broder's claims that Tesla gave him bad advice, because he mentioned some really stupid things. Tesla might want to reevaluate the employees answering the phones. Even so, you'd have to be an idiot to trust some of that stuff. Who could possibly believe that frequent acceleration and braking will extend the range of your car?
Anyway, I completely agree with Vivek's assessment of Musk's article as vitriolic. His legitimate points were drowned out by his cries of "OMG HE DROVE 80+ MPH!!" when only a small portion of the drive was even over 70 MPH.
jonvaustin - Monday, February 18, 2013 - link
The fundamental problem is basically the whole article. If a EV is to be successful the range has to be there for the right price, a good number of miles off a charge, and about the same experience as we currently have.We have what 100 years of cars under our belt now and for the longest time operation has been a similar experience that is taught in a persons teens and carried throughout his or her life time. For a EV to be mainstream for that consumer to have to take into account temperatures, this or that is foolish. Why it will never take off.
A good example is Apple with the iPhone 4. It got crappy reception from improper antenna placement and the solution was to tell people to hold it the right way? That is fail. Case in point future models fixed the issue. To tell a consumer that you have to hold the phone a certain way and not the way you naturally hold it was not to popular. The same is with EV cars.
Another case is why we all use the slow inefficient QWERTY keyboard. Why? I learned it as a kid because my parents learned it that way. There are a dozen better solutions and ZERO catch on primarly because of that reason.
To ask me to learn about battery dynamics and to drive differently then I currently do in my gas based car will never work for the general population to be successful. What needs to happen is a radical breakthrough in energy storage tech so it becomes a mute issue.
Luke212 - Monday, February 18, 2013 - link
New York Times article link is broken in first paragraphGalcobar - Monday, February 18, 2013 - link
One area neither side in this dispute has addressed -- that I've read -- is whether extremely cold temperatures affect the accuracy of the car's internal sensors.Particularly, the level of charge remaining in the battery.
Elon Musk's defence is based largely on the level of charge remaining in the battery according to the log at various points, while reporter John Broder says his behaviour was guided by what the car was telling him (and what he then relayed to Tesla customer service).
As an aside, The Times' public editor -- a position meant to provide self-criticism to the paper and effectively act as an ombusdman -- wrote an interesting piece on the controversy: http://publiceditor.blogs.nytimes.com/2013/02/18/p...
relztes - Monday, February 18, 2013 - link
Roughly estimating the power consumed by the heating system in the Leaf, I get 3.4 kW (on the UDDS HS drive cycle at 20 F). It's going to be a pretty big limitation if EV manufacturers can't figure out how to reduce this. They need to consider adding insulation, using heat pumps, and recirculating as much cabin air as possible. Obviously there isn't much room for insulation, heat pumps have limited temperature ranges, and some outside air might be necessary for proper ventilation, but they need to do something or they aren't going to have a very popular product in cold weather regions.On second thought, it looks like Tesla isn't doing too bad. The range was only degraded by about 20%, so if they just factored temperature and climate control settings into their range calculations, people might not be too disappointed. A 200 mile range only looks bad if the car claims it'll go 240 miles.
chipkin - Tuesday, February 19, 2013 - link
The 2013 Leaf SV and SL models switched to a hybrid heat pump / resistive heating system. I am the EV sales specialist for a large Nissan store in the Chicago area (where we routinely see -10 F winters and 100 F summers). I keep in close touch with my customers, and the cars have performed well within their expectations. The lease price for a Leaf is low enough that if you have a 50 mile or less daily round trip (like 80% of all US drivers), the car is effectively free once you figure in the savings in gas and maintenance expenses (no oil changes, filters, belts, etc...).vangman - Monday, February 18, 2013 - link
Vivek, could you explain why you consider the Top Gear episode was a sham?