Results tagged “Tesla Roadster” from Tom Saxton's Blog

EV Efficiency: Tesla Roadster and Nissan LEAF Compared

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How do the Tesla Roadster and Nissan LEAF compare in energy use?

Tesla Roadster owners have been driving electric for a couple of years now and have built up knowledge about how much energy is required for many different routes and driving scenarios. New Nissan LEAF owners could perhaps benefit from what Roadster owners have learned, especially in the near term while charging stations are few and far between.

On August 4, 2011, we did a test to answer a couple of questions:

How does energy use in a Nissan LEAF compare to a Tesla Roadster?

Does knowing how much energy a Roadster uses for a certain drive help a LEAF owner plan the charge needed for a long drive?

The Plan

To take a first stab at figuring things out, Cathy and I joined up with her parents, Jim and Barbara Joyce, to drive a Nissan LEAF and a Tesla Roadster on an interstate freeway up a mountain pass. We wanted to compare just the two cars and eliminate as many other variables as possible. We drove up together so we had identical road and weather conditions, put the cars on cruise control to minimize driver differences, and restricted ourselves to using the fan but not air conditioning. From Roadster data collected on previous drives and also a recent LEAF drive up the same pass, we were pretty confident it could be done from the Joyces' home even cruising at 70 mph. We were right.

snoq-70-cars.jpgThe Route

We started at the Joyce residence near where Washington State Highway 18 meets Interstate 90 at Exit 25. Their LEAF started with a full charge. We drove to I-90, recorded trip and energy data at the stop light at the base of the on-ramp, accelerated up to 70 mph, then locked on cruise control. We exited I-90 at Exit 52 and recorded trip and energy data at the bottom of the off-ramp. We puttered around the summit for a bit, got some lunch, then reversed the route, again recording data at the bottom of the on-ramp getting back onto I-90 and again after exiting the freeway back at exit 25.

The Results

The graphs below show energy use for both vehicles up the pass from exit 25 to 52, a distance of 27 miles with a 2,000 foot elevation gain, then the descent back down from exit 52 to exit 25.

snoq-70-energy.png The graph shows that the LEAF used about 6% more energy than the Roadster on the way up and about 13% more energy on the way down. Both vehicles used about twice as much energy on the way up as the way down, although that ratio depends on the slope and speed. For a sufficiently steep road and slow descent, an electric vehicle can actually gain net energy driving downhill. At 70 mph, we did not see a lot of energy production, just low energy driving. At slower speeds, more energy would have been produced on the steep sections of the descent.

The LEAF averaged 2.7 miles per kWh (376 Wh/mi) on the way up and 4.8 mi/kWh (233 Wh/mi) on the way down, for an average of 3.3 mi/kWh (305 Wh/mi).

The Roadster averaged 2.8 miles per kWh (355 Wh/mi) on the way up and 5.5 mi/kWh (206 Wh/mi) on the way down, for an average of 3.6 mi/kWh (271 Wh/mi).

How Much Charge is Needed to Drive a LEAF Up to Snoqualmie Pass?

The LEAF doesn't give an indication of the state of charge to any useful precision, so we could only measure energy use from the trip miles and miles per kWh supplied by the LEAF. In terms of how much charge we used, the LEAF started with a full charge and ended back home with one bar showing and 4 miles on the generally worse-than-useless guess-o-meter. This included under 10 miles of driving between the freeway and home. It was a little surprising that the LEAF charge got so low given that the home-to-home energy use was only about 18 kWh, but the reported 24 kWh capacity of the battery is probably measured at a discharge rate that's lower that what's needed to climb the pass at 70 mph. Also, we know the LEAF hides some reserve charge from the driver.

From this data I conclude that starting from a full charge in Snoqualmie or North Bend, a LEAF can easily make it up and down the mountain at the speed limit without climate control. With climate control on, a bit slower speed may be required.

With a DC Quick Charge to 80% at North Bend, it could probably be done by anyone starting in the greater Seattle metro area.

Having Level 2 charging at the summit would be a big help. Even Level 1 would make a difference for someone spending the day skiing at the pass and wanting to get home with little or no charging on the way back.

Driving at lower speeds would use less charge. Really efficient driving, including better use of regenerative braking on the way down, would further decrease the charge needed.

Comparing the Nissan LEAF and Tesla Roadster

The curb weight of the Roadster is about 2,700 lbs, compared to the LEAF at 3,350 lbs. So the LEAF weighs about 25% more than the Roadster. The LEAF has a more aerodynamic shape, but has a much larger frontal cross-sectional area, so I would expect the LEAF to also have more aerodynamic drag. At freeway speeds, one would expect the aerodynamic drag to be a bigger factor in energy use, but doing a significant climb increases the importance of vehicle weight.

Because of how these two issues interact under different conditions, these numbers tell the story only for this specific drive on this route at this speed. Other drives are likely to give different results, so more tests are needed to get the full picture. It would also be interesting to do the same drive with multiple LEAFs and Roadsters to see how much variation there is between vehicles of the same model.

Data Method and Repeatability

We did everything we could both to minimize the difference between the two side-by-side drives and also standardize the drive so it could be repeated later under either similar or different conditions.

It was warm enough that we had to run the car fans to stay comfortable, but we were able to avoid use of the air conditioning.

We were able to maintain 69 to 70 mph pretty well, with a couple of exceptions. Below are graphs of the Roadster's speed versus time. The LEAF speed profile would be similar, with one exception on the descent, described below.

snoq-70-ascent.pngOn the way up, a few minutes after we got onto I-90, we ran into a clump of traffic we had to maneuver through, which slowed us down a little for a few minutes around the 10-minute mark.

snoq-70-descent.pngOn the way down, just a couple of miles from exiting I-90, the Roadster got boxed in between an RV at the same speed in the center lane and a slower vehicle entering just ahead of us. Rather than speed up to jump ahead of the slower vehicle (which would have used a bunch of extra energy), we slowed down sharply to let the vehicle in ahead of us. The LEAF was far enough ahead that it avoided this problem.

Tesla Roadster Energy Reporting and Efficiency

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For the month of November, I drove the Roadster 762.2 miles. That's mostly with just me in the car driving a variety of city and highway miles. I tend to drive enthusiastically most of the time, but the month also included a roundtrip drive to Longview, WA on cruise control at 55 mph.

During the month, I put about 247.8 kWh into the car from the wall (213.3 kWh metered from my garage plus approximately 34.5 kWh from an unmetered NEMA 14-50 outlet in Longview). That's 325.1 Wh/mi and includes charging losses, battery pack self discharge, heater, headlights, etc. That's my wall-to-wheel number and is based completely on things I can measure.

From July 25th to August 27th, I drove the Roadster 696 miles and pulled 234 kilowatt hours (kWh) from the grid, giving us 336 Wh/mi. That included some hot weather and four 1/4 mile runs at Pacific Raceways.

On individual charges, I see efficiency vary from 240 Wh/mi to over 400 Wh/mi, and obviously much higher for things like drag racing.

I charge consistently at 240V and 40A at home. In Longview it was 230V and 40A. Because of charging overhead, I assume I would get slightly better charging efficiency if I charged at home at 70A. So, my numbers are just that, my numbers. Another driver would get different numbers depending on driving, weather, road conditions, and charging habits.

The EPA estimates documented in the paperwork for our car say 260 Wh/mi city and 290 Wh/mi highway. I've seen information from early 2008 Roadsters that had the EPA numbers and 340 and 360 Wh/mi.

You may have heard Roadster owners talk about numbers well below my 330 Wh/mi numbers. These are most often the number reported by the car's info screen which are not wall-to-wheel numbers, and in fact are (as far as I know) not at all documented as to what that number means. I have figured out some things about the numbers reported by the car, which I'll now explain.

For the month of November, the Roadster's trip meter says that I used 207.9 kWh, and thus 272.8 Wh/mi. But what does that mean? Did I push 207.9 kWh into the motor, or is that net of energy pushed back into the pack from regenerative braking (regen)? Does it include energy used to run the accessories and/or running the coolant pump and fans during charging?

On the "Energy History" screen, the Roadster tells me my "net energy used" for the month was 233 kWh and that I got 26 kWh from regen. What does "net" mean? I would assume that "net" means "net of regen," i.e., power from battery pack minus power into battery pack from regen. Except, if I compare those numbers to what the trip meter says, I notice that 233 - 26 = 207, which is suspiciously close to the energy use number reported on the trip meter.

From that, I infer that the trip meter's number is net energy use from the battery pack (power drawn minus regen put back in), and thus the so-called "net energy" from the energy use screen is really the gross energy pulled from the battery pack including energy that went into the pack from both wall charging and regen charging.

Do these numbers include the energy spent on accessories? Is the difference between what I put in through charging (247.8 kWh) and the car's reported net energy use (207.9 kWh) just charging losses or does that also include accessory use? I have no idea.

The only number I can stand behind, and the only number I can compare with other electric vehicles, is the wall-to-wheel number. The efficiency number reported on various of the Roadster's info screens is useful for understanding how driving style and conditions affect efficiency and for predicting/optimizing range, but is seemingly useless in any other context.

I believe the same is true of any efficiency number for the Leaf given out by Nissan, or any other EV manufacturer or driver, unless that number is as clearly defined and directly measured as the wall-to-wheel number.

It used to be that the Tesla screen reported an energy number after each charge that was much lower that what was actually drawn from the wall. I suspect that was the energy that actually made it into the battery pack, but I never saw it defined by Tesla. More recent firmware versions are reporting a number that is close to the number I read from the wall meter (and averaging multiple consecutive readings together agrees to within 1% of the wall reading). This is a big step forward for drivers who want to monitor their actual wall-to-wheel energy use and efficiency, but don't want to go to the expense of installing a dedicated meter. It would be a real benefit to the Tesla community if Tesla would (a) define the number they currently report and (b) make the energy drawn from the wall across multiple charges easily available.

Regarding range on a single charge, my personal record is 192 miles driven with a passenger in 100+ degree weather starting with a bit less than a full charge and ending with 10 miles of range left. On the trip back from Longview in cool weather, I drove 136.9 miles using cruise control at 55 mph using 55% of the battery. To the extent that you can extrapolate that to the full battery, that figures out to about 249 miles of range. On the trip down to Longview earlier the same day, also using cruise control at 55 mph, it was raining and colder, so I had the wipers, headlights and heater on and used 65% of the battery pack, for an extrapolated range of 208 miles.

My car is a 2008 Tesla Roadster with firmware version "3.4.15 15" (upgraded from "3.4.13 15" on 11/15/2009).

Edited at 10:23 pm on 12/13 to correct typo in second paragraph.

Sammamish to Mt. Rainier

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On Monday, the weather was too nice to not be out driving so I decided to take a spin up to Mt. Rainier to see if the roundtrip could be done on a single charge. Thanks to Todd Laney for planting the seed by suggesting it as a possible Roadster owner drive route to follow our adventure on Sunday.

I used Google maps to plot a route and get mileage/time estimates. Google estimated the drive at about 2:30 and my goal was to make the roundtrip in four hours. On the way back I took Highway 18 instead of Cedar Grove Rd SE, which wasn't that scenic and smelled of landfill.

I left our house at 4:00 pm, with a full standard mode charge at 194 ideal miles. I reset the trip meter so I could easily track mileage and energy used. I left later than I had hoped, and I paid for it by being stuck in traffic pretty much the entire way from Issaquah to Enumclaw. That didn't help with my four-hour goal.

Give or take a few stunning views of Rainier near Black Diamond, the drive didn't get fun until I left Enumclaw (and the traffic) and got onto SR 410, a two-lane road through forest. I was pushing to make up lost time, but mindful of the risk of deer crossing the road, so I kept the speedometer at or a little above the speed limit. I didn't see much traffic and only passed one vehicle, a van going well below the speed limit.

I neglected to record state when I entered the park to begin the ascent up the mountain, but I recall the battery being around 90 ideal miles and the trip odometer would have been around 70. At about 4 miles from Sunrise, the trip and battery state crossed at 79 miles driven and 79 ideal miles left in the battery pack. If I were on level ground, I would have been considering bailing at that point, but I knew I had used extra energy climbing that I would get back on the descent. Also, I was still driving in standard mode, so I could switch into range mode to get another 25 miles of range.

The best part of the drive was the winding ascent up the mountain, wonderful conditions for the Roadster, although the road was pretty uneven in spots. I was having fun but not being crazy about it. Two thirds of the way up the mountain, I came up behind another car and resigned myself to taking the rest of the drive at his pace, which would have been fine. However, as soon as I came up behind him, he immediately pulled over to let me pass. I don't know whether he was scared of the red sports car, or just really nice. I waved in appreciation as I passed.

I arrived at Sunrise at 6:08 pm, just a few minutes later than my goal time. I let the car sit for a few minutes while I tracked down a restroom and snapped a quick photo. Ready for the return drive, the trip meter said I had driven 83.7 miles and used 28.01 kWh. The touchscreen altimeter said 6300 ft, temperature at 60 F, with the SOC at 47% and 69 ideal miles (standard mode).

0256-090921-tws-IMG_3554.jpg

For the descent, I got regen nearly the entire way. I stopped twice on the way down, once at the viewpoint just below Sunrise where I got my cell phone signal back and called to update Cathy on my progress, and at the bottom to record data. At the park exit, 14.0 miles from Sunrise, the battery state was up to 71 ideal miles, a net gain of two ideal miles from the reading at the top.

I was treated to a beautiful sunset with a thin crescent moon just above the horizon as I was finishing up the drive on SR 410 approaching Enumclaw.

I had a TomTom GPS navigation device programmed for the route home, mostly to show me remaining miles and ETA. When I left the summit at 6:20 or so, it was predicting I would get home just before 9:00, which was obviously wrong since I made the trip there in just over two hours. After exiting the park, the ideal miles tracked the remaining miles pretty closely, showing a buffer of about 10 miles the entire way, while the ETA dropped steadily.

I got home at 8:20, missing my four-hour goal by about the length of my stops. The trip meter read 168.8 miles and 41.76 kWh. Our house is about 100 ft elevation, so the trip involved climbing then descending 6,200 feet. The battery pack was showing a temperature reading of four (the highest of the blue ticks), PEM and motor at 3. After sitting for 20 minutes to stabilize, the state of charge read about 5% of the battery left, 9 ideal miles.

Charging back up to full (standard mode) from 240V/40A took 5:20 and put 44 kWh back into the battery pack. The meter on the wall indicated it pulled 50.0 kWh hours, or 296 Wh/mi from wall to wheel, about 12% below my average of 336 Wh/mi.

If Watt-hours per mile doesn't mean anything to you, at 9 cents per kWh for "green power" from Puget Sound Energy, it cost me about $4.50 to drive 168.8 miles, or about 37.5 miles per dollar of green electricity.

My take-away from the drive:

  • The roundtrip can be done on a single standard mode charge, but if you have a passenger or are farther away than Sammamish, I'd recommend a range mode charge.
  • If you do it on a weekday, start early enough to avoid the evening rush hour.
  • Take a credit card: it costs $15 to get into the park via the automated kiosk. Cash might also work.
  • Driving through the woods at sunset on a warm day leaves the front of your car covered with a thick layer of bugs.
Updated July 12, 2011: Fixed link to route map.

Elon Musk Explains the Roadster Price Increase

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The following email was sent to Tesla Motors customers on Tuesday, January 20th.

From: Elon Musk, Tesla Motors
To: Tesla Roadster Customers
Date: Tue, 20 Jan 2009 12:45 pm PST
Subject: The Importance of Options

A much fuller account of the history of Tesla is worth telling at some point, but for now I will just talk about the essentials of why we needed to raise prices on options.  Fundamentally, it boils down to taking the tough steps that are difficult but necessary for Tesla to be a healthy company and not fall prey to the recession.

When the initial base price, for cars after the Signature 100 series, of $92k was approved by the board a few years ago, it was based on an estimated vehicle cost of roughly $65k provided by management at the time.  This turned out to be wrong by a very large margin.

An audit by one of the Series D investors in the summer of 2007 found that the true cost was closer to $140k, which was obviously an extremely alarming discovery and ultimately led to a near complete change in the makeup of the senior management team.  Over the past 18 months, observers will note that Tesla has transformed from having a senior team with very little automotive experience to one with deep automotive bench strength.  We now have executives with world class track records running everything from design to engineering to production to finance.

To bring the cost of the car down, we have reengineered the entire drivetrain, which is now at version 1.5 and will be at version 2 by June.  The body supplier was also switched out from a little company that was charging us nutty money and had a max production of three per week to Sotira, who supplies high paint quality body panels to Lotus, Aston Martin and others.  In the process, we had to pay several million dollars for a whole new set of body tooling, as the old tooling had been made incorrectly.  The old HVAC system was unreliable and cost almost as much as a new compact car, so also had to be replaced.  The wiring harness, seats, navigation system and instrument panel also had to be modified or replaced.

After reengineering and retooling virtually the entire Roadster and completely restructuring our supply chain, we are now finally coming to the point where the variable cost of the car (to be clear, this excludes fixed cost allocation) is between $90k to $100k.  With a lot of additional effort by the Tesla team and the help of our suppliers, we should be at or below $80k by this summer.  There is some variability here due to exchange rate shifts.  Although we gain an automatic currency hedge by selling in both Europe and the US, we are still vulnerable to the Yen, which is very strong right now.

Obviously, this still creates a serious problem for Tesla in the first half of 2009, given the $92k to $98k price of most cars delivered over this time period.  The board and I did not want to do a retroactive increase of the base vehicle price, as that would create an unavoidable hardship for customers.  Instead, apart from a $1k destination charge increase to match our true cost of logistics, we only raised the price of the optional elements and provided new options and a new model (Roadster Sport) to help improve the average margin per car.

The plan as currently projected, and which I believe is now realistic, shows a high likelihood of reaching profitability on the Roadster business this summer.  By that time, we will be delivering cars that have a base price of $109k plus about $20k or so of options (having worked our way through the $92k to $98k early buyers) at a rate of 30 per week.  We are fortunately in the position, rare among carmakers, of not having to worry too much about meeting 2009 sales targets, as we are already sold out through October and have barely touched the European market.

My paramount duty is to ensure that we get from here to there without needing to raise more money in this capital scarce environment, even if things don't go as well as expected.  I firmly believe that the plan above will achieve that goal and that it strikes a reasonable compromise between being fair to early customers and ensuring the viability of Tesla, which is obviously in the best interests of all customers. It's also important to note that the price increases will affect 400 customers, all of whom will take delivery after Jan. 1 and receive a $7,500 federal tax credit. We made the pricing changes to ensure the viability of Tesla in the long term, regardless of government incentives, but we hope the credit will offset the increase for most customers.
 
There is one additional point that relates to the government loans that Tesla is seeking for the Model S program, a much more affordable sedan that we are trying to bring to market as soon as possible.   A key requirement is that any company applying be able to show that it is viable without the loans.  If we allow ourselves to lose money on the cars we are shipping today, we place those loans at risk.  Mass market electric cars have been my goal from the beginning of Tesla.  I don't want and I don't think the vast majority of Tesla customers want us to do anything to jeopardize that objective.

Elon Musk
CEO & Product Architect