July 2010 Archives

Tesla Roadster Charging Rates and Efficiency

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Note that the graph cuts off the last three hours of the 16A charge. The 120V charging graphs aren't shown. A full standard mode charge at 120V/16A takes about 38 hours and at 12A it takes about 60 hours.Note that the graph cuts off the last three hours of the 16A charge. The 120V charging graphs aren't shown. A full standard mode charge at 120V/16A takes about 38 hours and at 12A it takes about 60 hours.Updated: April 17, 2011 to add 120V charging data.

The Tesla Roadster offers a wide variety of charging options, from 120V/12A up to 240V/70A. Charging at higher voltage and current charges faster, but most of the time charging speed isn't an issue. If you drive a typical commute and charge at night, even the lowest power will get the car fully charged overnight. At least with the early Roadster firmware, charging at 120V was pretty inefficient because of the fixed charger overhead, but what about charging at 240V at various amperage limts? My theory was that charging at higher current is more efficient because you spend less time paying the charging overhead, but another owner challenged that assumption with the theory that higher current is less efficient because it generates more heat and thus increases the amount of energy spent keeping the battery pack cool.

Another aspect of charging is that for any given current setting, the Roadster will charge steadily at that current until it gets near the top of the charge, at which point it will start to taper off. This reduces your charge rate near the top of the pack. This aspect of charging isn't documented in the owners manual.

If I don't care about charging time, what's the best amperage for energy efficient charging? If I'm on a road trip and want to squeeze the most range out of time spent at a charging stop, how should I space my stops and how long should I charge at each one? I've collected enough data to shed some light on these questions.

Methodology

I performed a series of charges at various current levels from relatively low battery states up to a full standard mode charge. For each charge, I collected time, voltage and amperage once per minute, and state of charge once every 10 minutes. From that, I can compute energy used during every segment of the charge and the total energy used.

To track our energy use for driving, we have a dedicated electric meter for each of our EVs. To validate my energy calculations, I verified that the total energy calculated per charge matches the value computed from the meter readings.

All charging was done overnight in cool weather with a 2008 Roadster. The 16A charge was done with firmware version "3.5.17 15", all other runs were done with firmware version "3.4.17 15". The 16A charge stopped at a lower state of charge (96%, 188 IM) than I normally see (98%, 193 IM). I don't know if this is due to the lower current limit, the new firmware, or a one-time fluke.

Charging Efficiency Results

Is it more efficient to charge at a low rate or a high rate? Here are the results:

Charge Rate Wh per Std % Wh per Ideal Mile
120V - 12A 807 414
120V - 16A 723 371
240V - 16A 589 306
240V - 24A 544 282
240V - 32A 527 274
240V - 40A 512 266
240V - 48A 524 272
240V - 70A 516 268

As you can see from the table, there's not much variation in charging efficiency when charging at or above 240V at 32A, but energy use rises noticeably at lower power levels.

Road-tripping and Charging Rates

Also of interest are the charging rates at various current levels. This is especially important when charging away from home.

Charging at higher currents is faster than lower current, but by how much? Is it worth it to drive 55 mph in order to make it to a 40A charge point instead of driving faster and stopping sooner at a 24A or 32A charging spot? Tesla gives us a table on charging rates, but it's pretty low resolution.

How far can I charge before I start getting diminishing returns because of the current tapering that happens near the top of the charge? Tesla is silent on this subject.

If you care about getting the most out of your charging stops, you may be in Range Mode, so this table shows both standard and range mode values for when current begins to taper off.

Charge Rate Ideal Miles
per Hour
Current Tapering Begins At:
Std % Std IM Range % Range IM
120V - 12A 3.3
120V - 16A 5.1
240V - 16A 13 93 179 82 205
240V - 24A 20 94 180 82 205
240V - 32A 28 93 178 82 207
240V - 40A 36 93 178 81 204
240V - 48A 42 91 174 80 201
240V - 70A 61 84 161 75 188

Let's assume I want to get the most range for time spent charging, and don't need to charge all the way to the top. From the above table we see that if I'm charging at 48A or lower, I can expect to see the charging rate start to taper off at around 80% or a bit over 200 ideal miles (range mode). If I'm lucky enough to be charging at 70A on the road, my charge rate will start dropping around 75% or 188 ideal miles. I'll keep charging above 40A until I hit that 80%/200IM mark, so if my next charging stop is only 40A, I may as well keep charging to that point.

I'm sure there's some variation from car to car, and the pack and ambient temperatures will change charging behavior, so don't plan your trip to depend on these exact values, but this is at least a rough guide.

Charging Profile Graphs

Let's start by looking at how the state of charge varies over time using different current limits at 240V. All charges are standard mode all the way up and normalized so that all the charge sessions are shown from the same starting point, around 36%.

Tesla_SOC_v_Time.png
You can see how more current yields a faster charge, and that the rate of charge starts to drop off as the battery pack gets near the 100% mark.

Note that the graph cuts off the last three hours of the 16A charge. The 120V charging graphs aren't shown. A full standard mode charge at 120V/16A takes about 38 hours and at 12A it takes about 60 hours.

Now let's examine current draw and state of charge throughout each of the current settings. In each session, the car draws an approximately constant amount of current until near the top of the charge when it begins to taper off. The following graphs show current drawn (in amps) and state of charge (as standard mode percent) as a function of charge time in hours. Each charge begins at a slightly different level, but all start below 40% so they have a nice long stretch of steady current draw.

Tesla_Charging_240V_70A.png
Tesla_Charging_240V_48A.png
Tesla_Charging_240V_40A.png
Tesla_Charging_240V_32A.png
Tesla_Charging_240V_24A.png
Tesla_Charging_240V_16A.png
You may notice that at 32A and 40A, the rate at which the SOC increases doesn't drop off as much as you might expect from how quickly the current drops near the end. I attribute this to the SOC calculation stabilizing near the end of the charge. It's difficult to know how much charge is in a battery while you're charging it. My guess is that the SOC is an estimate that gets better near the end of the charge. Regardless, the less current you're drawing, the less power you're putting into the battery. I've seen behavior that leads me to believe that if you stopped the charge within the tapering zone, you'll see the SOC continue to rise for a bit as the software gets a better estimate of the charge in the pack. However, you're still getting diminishing returns on charge time once the current starts to taper.

Another way to look at the data is to plot amperage draw as a function of state of charge. This will show us how the different charge limits compare with respect to when they start backing off from the full allowed current.


Tesla_Amps_vs_SOC.png

From this, we can see that there's isn't a penalty for charging at higher amps. Although it starts tapering the current earlier, it hits the lower amperage levels at about the same point as charging at those amperage values would start tapering.


Charging in Range Mode

Each of the above graphs show a standard mode charge. In range mode, it makes the bottom part of the charge range available and charges the pack even further: 0% and 100% in standard mode correspond to 11% and 87% in range mode. The same charging profile is in play, so as the battery pack crosses beyond the top of the standard mode charge, the current draw drops even further.

Tesla_40A_Range_Charge.png

Tesla says that the range of the Roadster is 244 miles and that it can be charged from empty to full in as little as 3.5 hours, but those two don't really go together. The 3.5 hour charge time is for a full standard mode charge which is less than 80% of the full range, around 195 miles. Getting the full range mode charge takes longer. For my car, it's about an hour and forty minutes to go from a full standard mode charge to a full range mode charge (and add more time if you start below 10% in range mode). So, if you're on the road trying to make good time, waiting the extra 1:40 for another 25 ideal miles is not worth it unless you need the full range to get to the next charging stop. Charging to the top of range mode only makes sense if you're charging overnight and don't care how long it takes. So, on an extended road trip, a full range mode charge is probably only useful at most once per day.

Topics for Further Research

I would like to add data for some more scenarios, most notably 120V/12A (the slowest of the options, which requires three days for a full standard mode charge).

It will be interesting to see how these graphs change over time as the battery pack ages.

Charging in a hot environment definitely changes energy consumption during charging because the fan and A/C will kick on to cool the battery pack. It's harder to control for ambient temperature across multiple charges, but it would be interesting to collect data and see how things change. I would not be surprised to see a significant penalty for charging at higher current if that pushes the temperatures high enough to require the A/C during the charge.

These results are for our Roadster, yours may be different. Even the conversion from standard mode percent and ideal miles to range mode may vary between vehicles and across firmware updates. Drop me a note if you want to learn how to do this analysis for your Roadster.

Collecting and processing the data to produce the charts is only partially automated. It would be nice to automate more of the process to make it easier to do the analysis for me and others who are interested in doing the same for their vehicles.






Three EV Lessons for Nissan from Tesla Motors

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In the all-electric Tesla Roadster, Tesla Motors has done an amazing job of designing and producing a car that shows the world how to build a great electric vehicle that is reliable and fun to drive, creating a driving experience that is far superior to that of a comparable gas-burning high end sports car.

Despite having Tesla's example, I'm concerned that Nissan is going to do a poor job with the Leaf. They've already made three missteps which I think need to be corrected before they start selling electric cars.

Overstating the Leaf's Range

Nissan has been saying the Leaf will have a 100-mile range, but they are basing this claim on the LA4 city driving cycle, not on a highway or combined cycle. Tesla says the Roadster's range is 244 miles, and that's a real number. If I drive 55 mph on level freeway, I get energy use consistent with that 244-mile range. From what Nissan has said, I suspect that going 55 mph on level freeway with no heat or A/C will yield somewhere around 80 miles. That's still an awesome range that will meet the needs of many drivers, but it's a disappointment that they entered the game by overstating their range with a number that requires driving even more conservatively than a steady 55 mph.

The vast majority of people who've had the opportunity to drive electric on a daily basis prefer it to driving gas. The only people I've heard of complaining about the electric driving experience are people who purchased an EV with inadequate range for their driving needs. The EV consumer has to take some responsibility to understand their real driving needs and the capability of the EV they are considering purchasing, but any automaker that does anything less than conveying a conservative and realistic picture of the car's capabilities is going to end up with a lot of unhappy customers and a public relations disaster.

Nissan: Get real range numbers out there now. Tesla Motor's detailed page on range information could be better by being far more visible on their site. Make sure the one or two numbers that are most visible to the public are representative of what consumers can realistically expect to get under conditions that are clearly stated. Beyond a simple number or two, also put lots of technical detail out there to satisfy the people who want all of the information and will be the early adopters that clear the path for the mainstream buyers.

Update: I arrived at the 80 mile figure by adding a generous 10% to the 70-mile range for 55 mph with A/C on as reported by Forbes. A MotorTrend article pointed out by mwalsh and evnow on the MyNissanLeaf forum after I published this post quotes Nissan Leaf chief engineer Hidetoshi Kadota as saying normal freeway driving at 60-70 mph without climate control yields a range of 105 miles. So maybe the Leaf's range is better than suggested by the negative Forbes article, but it's still the case that Nissan is not making any of this information available on their web site.

Not Fully Exploiting the Advantages of Driving Electric

Nissan is apparently making the Leaf drive like a gas car rather than fully exploiting the advantages of driving electric. Specifically, they are putting little or no regenerative braking on the accelerator pedal. Tesla does a beautiful job on this. As you press down on the accelerator pedal, the car accelerates more, just as you'd expect. As you let up on the pedal, you get to the point where the car is just coasting before the pedal is completely released. As you release more, the car starts using the motor as a generator to charge the battery, the more you release the stronger the effect. When the pedal is fully released, the regenerative braking becomes quite strong and will slow the car down almost to a stop. (This effect is stronger at slow speeds where you're likely to want to slow more quickly, and lighter at freeway speeds where you want a more gradual slowing to match traffic.) To slow the car more quickly or bring the car to a complete stop, you press the brake pedal to engage the car's friction brakes, just like driving on gas.

After getting used to driving a 2002 Toyota RAV4-EV, which puts only a little regenerative braking on the accelerator with more on the brake pedal, I was dubious of the Tesla scheme. (The Honda Insight and Toyota Prius are similar to the RAV4-EV in this regard.) After driving the Roadster for a few days, I found the Tesla scheme to be much better than the RAV4-EV. It has two big advantages over more closely emulating a gas-burner. For the sake of driving efficiency, I want to slow the car with regenerative braking as much as possible, every time you touch the friction brakes you are wasting energy by converting momentum into heat and brake wear. With the Tesla scheme, I know exactly when I switch from efficient regenerative braking to wasteful friction braking: when my foot moves from the accelerator to the brake pedal. Aside from helping me drive more efficiently, and reducing wear on the brake pads, the Tesla scheme is simply a better way to drive. I can control speeding up, maintaining speed and slowing down all with one pedal. With just a little bit of time behind the wheel, it quickly becomes a more natural and comfortable way to drive. This is especially nice when driving downhill, it's just so easy to control your speed, driving a gas car seems primitive. The only complaint I've ever heard from a Tesla owner about how this works is that they want more regenerative braking on the accelerator, enough to fully stop the car at a light. Personally, I think what Tesla has done is perfect: the mostly one-pedal driving is familiar enough that a first time driver won't have any problem driving the car, with a bit of practice it's a better experience, and the occasional use of the brake pedal keeps my brain-foot connection trained to use both pedals, reinforcing the old skills that puts your foot on the brake pedal instantly when required to slow or stop quickly.

Nissan: talk to some Roadster owners about the pedals. Drive a Roadster for a week or a month. It's important to get this right, it will give your owners a great driving experience sell a lot of cars.

Yielding to Unreasonable Demands for Artificial Traffic Noise

Nissan has yielded to the hysterical calls to add noise to electric vehicles. So far, Tesla Motors has resisted doing the same. All modern cars are quiet when driving slowly; the difference between a pure-electric car and a modern sedan is only audible in very quiet conditions. If quiet cars are a safety issue, then we should be looking at requiring all cars to make a minimum amount of noise at low speeds rather than singling out electrics and hybrids. There is no credible research to suggest that quiet cars are any more dangerous than other cars. Cars are only quiet at low speeds, when both drivers and pedestrians have enough time to react and avoid any problems.

Even if we make electric vehicles noisy at low speeds, they will still be inaudible in noisy environments. If anything, noisy cars that drown out the normal sounds of tires, fans, and pumps are more of a danger than quiet cars. So, if we're really worried about sound-related risks between automobiles and pedestrians, we should have strict laws for all cars that require minimum sound levels at low speeds, and prohibit sounds loud enough to drown out those minimum sound levels. But actually, that wouldn't help either. Just imagine what a parking garage would be like if all cars had to make a constant continuous sound, it would be like having a stadium full of vuvuzelas creating a cacophony that makes it impossible to discern any individual sound while training everyone to ignore the annoying buzz.

Instead of squandering an opportunity to have quieter cars, we should be taking real steps to improve safety for all pedestrians, bicyclists, and everyone else on the road. We should be studying the whole situation to find out if quiet is a real problem for pedestrians, considering all cars -- not just electric and hybrid -- and also the impact of natural or artificial traffic noise on quality of life. Does adding noise to all cars benefit anyone, or does it just crank up the level of background noise and make it harder to hear what's going on nearby? Does adding a constant warning noise to a car just train drivers to expect that pedestrians will automatically scatter out of their way?

I've been driving electric for two years and I have surprised exactly one pedestrian: a woman who was walking backwards into the driving lane of a parking lot while carrying on a conversation with someone across the lot. I stopped and waited for her to realize she was walking into an occupied traffic lane and she eventually saw us waiting for her. She was surprised, but I wasn't, and there was never any danger to anyone. She was clearly embarrassed by what she had been doing and tried to blame her reckless behavior on my quiet car. If I had been going fast enough that her foolishness could have created a dangerous situation, my car would have been making the same tire noise as any other car, which may or may not have been audible depending on the environment.

I'm quite sure that I don't need my neighbor's electric car waking me up at 5 am just because people are scared of unfamiliar technology. I propose that we solve a real problem, like driving while phoning or texting, before we rush into squashing a quiet car advantage in response to uninformed hysteria.

Nissan: Please give your drivers a manual way to alert pedestrians with something less obnoxious than a blast of the car horn. GM did this with the EV1 and owners loved it. Hold off on making a constant noise until there's enough research to show quiet cars are a danger and we have a validated way to improve the situation for all cars -- electric, hybrid, or gas-burning.

Edited July 5, 8:46 am: corrected technical error in description of Tesla's regen algorithm and clarified pedestrian surprise story.

Edited July 5, 3:20 pm: added update on more optimistic Leaf range numbers as reported by MotorTrend.

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