An Overheating Aurora
On Scott Carpenter’s three-orbit flight on May 24, 1962, he was instructed to depend on flying his craft, Aurora 7, manually, while hot-rodded features had been added to its thrusters. This was being done since John Glenn’s flight three months before had seen high fuel use from the automated system, while the manual system seemed “slow and mushy.”
Hot rodded thrusters with a craft-gulping fuel? At the end of his flight, Carpenter was to turn on his automatic control system to ensure a dependable reentry. Meanwhile, Glenn had problems controlling his comfort inside his space capsule — HVAC problems that were somehow not addressed.
Soon after Carpenter soared into orbit, things got hot inside Aurora 7. The suit temperature was very warm over the Pacific on the first pass and the astronaut reported sweating. He struggled with arcane valving controls designed for the Mercury spacecraft. By the second orbit, the body temperature was reading 102°F and ground control considered terminating the flight. Yet, the astronaut, eager to continue, assured the ground that those readings must be off. Meanwhile, he chose not to inform Mission Control that beads of sweat falling into one eye were obscuring his vision of the controls. After the flight, he admitted to being very warm — even confessing it was like having a fever and that he noticed difficulty finding the right words to speak.
That’s mental impairment. The problem with achieving comfort in the fully automated Mercury capsule turned out to be more than difficult — it became dangerous. Also, although Carpenter could see his fuel gauges with dwindling amounts, he did not have data showing the rate of use when he tapped the “double-authority thrusters.” Think, spurs — heavy foot — afterburner. Yet, without data on the rate of energy use when he pushed the stick beyond a light touch, he had no immediate idea. But the ground did. They told him at the end of the first orbit to “stop using so damned much fuel.” He drifted for well over an hour — most of the third orbit.
Finally, when he went to turn on the automatic control system just before retrofire over Hawaii, it failed, facing the capsule in the wrong direction. Another failure, the gyros failed! Firing the rockets this way would leave him marooned in orbit. He had only minutes or risk fatal calamity.
Now Carpenter was overheated, low on fuel, and having to orientate the spacecraft by the window the astronauts had demanded. Arguably, Carpenter showed the Rightest of Right Stuff by managing a manual retrofire and reentry with hardly any fuel. Landing 250 miles beyond the planned landing zone, Carpenter was later roundly criticized and never flew again.
What would it have been without manual control and that window?
Now that we’ve rescued Aurora 7, it’s time to circle back to the Model 3. And speaking of hot in the cabin, understanding HVAC cooling controls in the Tesla Model 3 might be useful, not only for comfort but for range. We also make our final case for Tesla providing us, like Astronaut Carpenter, with information to know when our machine might be gulping electrical fuel (even sitting still) and the status of the heating, cooling, and ventilation system.
Summers in Florida are notoriously hot, and the otherwise stunning panoramic glass roof of the Tesla Model 3 is not your friend. You’ve heard about the roof rejecting 85% of the sun’s heat, but don’t be fooled. Even though the tempered glass section is UV-reflective with good heat rejection properties, it remains a single laminated plate of 5mm glass. Even if its metalized laminate creates beautifully orange surface raindrops, the total rejection of solar heat likely only approaches the 85% figure in a moving car where the dark tint absorbing the solar irradiance sees much of the heat buildup removed by forced convection on the outer surface.
Standing still in a sweltering parking lot (static soak) likely results in heat gain at least twice that figure. Midday solar irradiance is approximately 1000 watts/m2 integrated across the spectrum. Coincidentally, the overhead 50 x 33” section is approximately one m2 in area. The rear segment going down to the rear window is approximately another square meter. This means that even with the dark tint and even with an interior sunshade, 300–600 Watts of heat is coming in on your head. Indeed, using an infrared camera and looking from the interior up is immediately telling.
On a June day in 2020 at 4 o’clock in the afternoon and an outdoor temperature of 91°F, I measured the inside glass temperature (with a copper-constantan thermocouple) under late sun at just over 145.8°F (static soak condition). With the interior at 75°F, it still showed a glass temperature inside of 135°F. That’s hot above your head and leads to discomfort due to the radioactive component on the car interior heat balance. Of course, I’m not the only one to notice:
How much is 300–600 Watts? About half the typical power of a small electric space heater. There’s a lot of heat coming in there — so much so that during long summer drives from Florida up to North Carolina to visit Mom, I had taken to wearing a nice straw hat for greater comfort — at least until I installed insulation under the glass roof. Really.
A key thing for me to tell you is I already installed some of the best window tinting available for the Model 3.
In the future, I’d recommend Tesla (or the many aftermarket enterprises) create a simple, flexible, fabric-covered, extruded polystyrene insulation insert (~R-4 ft2·°F·h/Btu) that will fit in the overhead roof with appropriate headliner cover and a snug friction fit. Not only would this have advantages in Florida, but think Phoenix, Las Vegas, and even Palm Springs. Another option would be more complex: exterior deployable firm plastic cover (white reflective) for hot places that can be set in place for summer. Given the shape of the glass and edge tolerances, malleable heat resistant rubber gaskets might pop into place to hold a flat section around the glass gunnels. Either solution will work, but one or the other is needed in hot places. As shown in our thermography, the webbed-window shade is not enough here.
Frustrated by the circumstances, I got the nice insulated window interior covers from HeatShield.
For the top window, you want “Top Window: Item# 1791T-A.” Also good is the upper rear window, which is “Item# 1791R-A.” Mine arrived together as a single kit. They were a snap to install. These likely have an R-value of only about 2, but it is certainly much better to have the window tint, the insulation, and the inside screen to keep me from needing my hat at midday in summer while driving.
To further look into the potential of better control of roof solar heat gain, I staged a simple experiment on July 5th at my home. I parked the car in the sun and placed the nice windshield sunshade (which I strongly recommend) to protect the dash and controls. I then just used masking tape to affix sheets of white paper to one half of the panoramic roof — silly, but fast — and then just left the car with the AC on while things cooked up under hot Florida sun.
I have an interior sunshade in the car, but recorded the glass temperatures under the interior shade. Outdoor temperature at 1:13 PM was 91.5°F (33.0°C). Under full sun, the measured exterior surface temperatures were 76.5°C (179°F) on the standard section of the glass roof and 40.6°C (105°F) under the white paper on the exterior glass. The interior glass surface temperatures under full sun with AC were 57.4°C (135°F) under the uncovered glass section vs. 35.0°C (95°F) under the white covered section. I then used the IR camera looking up at a 45 degree angle to “see” the temperature of the mesh sunscreen at the same time. This was 53.5°C (128°F) under the unshaded section vs. 34.2°C (94°F) under the white section. This is nearly a 20°C difference in the temperature over one’s head.
- If you live in a hot place, have the expanses of glass professionally tinted using a crystalline or ceramic tint with good heat rejection qualities.
- In summer, unless charging to full, complete charging considerably before driving to allow the battery pack to cool off prior to driving and reduce the coolant loop battery conditioning.
- Park the car in the shade when unattended if at all possible.
- If forced to park in full sun, install a front window sunshade or a car cover over the long term.
- Make sure to have the windows tinted professionally to reduce heat buildup. Inside sunscreens on the large expanse of the Model 3 glass roof are highly recommended in hot climates.
- Remember the air conditioning system is fully variable speed which means that setting the interior temperature to a reasonable value and choosing fans speeds of 6 or less will reduce cooling energy. An even better way, as we’ll describe at the end of this segment, is to set the thermostat to LO and use the fan speed to modulate comfort inside. This is the stingiest mode relative to cooling power.
Shortcomings of the User Interface (UI):
Invisible HVAC or Other Parasitic Loads
The most frustrating thing with the Tesla Model 3 user interface (UI) is that you can’t see HVAC status without also turning it on. When asked by customers why there was no way to see heating and cooling system status, Tesla service approached company software engineers about this issue. Their short answer was that an overwhelming number of people preferred it to operate that way. “They don’t want to have to bumble around changing extra settings, they want one-click-and-it’s-on functionality.”
An option to change the information available might be nice, but at some cost.
“If we included every single option that people would want,” one service representative told us told, “there would be too many options and the UI would be too overwhelming for most people. It would also run against the Tesla mantra of “Beauty In Simplicity.”
Still, it would be exceedingly easy to add lighted indicators to the left drive panel indicating:
They could be grayed out when off. Also, nice would be to see the overall car total heating power for compressors, strip heat, and heated seats (kW). These data are already there in Tesla’s logs from the car, so providing it is mostly a UI software challenge. Here’s my example:
Be Wary of AUTO
When one is driving without displays showing HVAC energy use, there can be surprises.
One really unfortunate thing about the Tesla UI right now is that the HVAC energy use is invisible. And even worse than that: it can be burning a lot of power without you having any suspicion.
AUTO may be a poor choice if you wish to conserve power with heating or cooling. Why? As in other automobiles, this popular setting attempts to alter the temperature to the target set in the shortest possible time. That means setting the fan to the maximum, which typically uses about 350 watts more power than moderate settings. When air conditioning, this is doubly bad, as those fan watts become heat that has to be removed from the cabin to achieve comfort. The higher compressor speeds will also mean more kW for the pull-down period.
There’s another surprise relative to AUTO.
This happens when you place the HVAC in AUTO and think you’re smart and have the inside cabin temperature set to 65° when it is, for example, 55°F outside.
“That should keep it off.”
Meanwhile, the heater is coming on to maintain 65°F, with the cooling system periodically chiming in to try to control the humidity.
All the while, you have not the faintest suspicion that the HVAC might be periodically burning 1200 average watts to do what you didn’t plan on. You can’t see the power consumption. If you could, you’d likely try to find a way to get rid of it and put on a sweater vest.
Turns out that setting the temperature to “LO” and turning off AUTO is the key to truly turning off the space heating. But nothing in the Tesla experience teaches the unwary about that.
Critics might say: “So what?”
Really. Let’s assume you are driving at 30 mph around town for an hour like that using only 190 Wh/mi without HVAC. You go from 5.7 kWh burned to 6.9 kWh— a 21% decrease in trip efficiency — and five miles lost range — simply from an oversight. Or it is an engineering-UI-obfuscation/disconnect, depending on how you look at it.
What did you get for the added energy? Nothing. Or at least nothing you cared about.
There are other unfortunate surprises. If you negligently leave AUTO defog on (blue icon), after the reason you turned it on is long past, you can have both heating and cooling going on simultaneously with no
Seems perfectly comfortable inside, without any suspicion that you are leaking away miles as you drive. It would be nice to have such a warning, and one would think AI with the humidity inside being measured would allow the car plenty of smarts to gently remind you that defog has been on a long time.
We can override such warnings of course — and often do in this surprising life. It can be very foggy around North Beach. You might need extended defog.
Kind of like when John Glenn had to override the automatic retro sequence when he was about to blaze into the Earth’s atmosphere at 17,500 mph with perhaps the only thing holding on his ablative heat shield being the metalized straps on his retro rocket pack. Sometimes it is vitally useful to ignore the warning, but having the warning can be plenty important too. (When Scott Carpenter’s “low fuel” warning came on, he covered it with tape so he could see well during the night passes.)
Of course, HVAC electricity use is not the only range challenge in an EV. There are several others that we we’ll cover below. Some are choices; some are beauties of technological accommodation (imagine a company that gives us Dog Mode); others are inevitable (self-discharge of batteries), but inevitable with modifications.
As always, knowledge is power.
How to Operate the Model 3 Cooling System Most Efficiently
A seldom recognized danger to most efficient cooling in the Model 3 is unseen resistance heat as the AC starts to overcool beyond your thermostat setting. Here’s how to avoid it as well as how to control the AC most efficiently. The cooling system is variable speed, so everything you do to lower fan speed with the AC on will save some power—and a lot when you get into a hot car.
To drive Model 3 most efficiently with the air conditioning on, here’s what I’ve discovered:
- First, set the controls to manual AC with recirculation on. AUTO is off.
- Set thermostat to LO temperature (down at the bottom).
- Now choose the lowest fan speed that provides you suitable comfort, modulating that speed lower as the cabin cools down.
When it becomes too cool in the cabin, choose the very lowest fan speed (1) to keep from turning up the thermostat that would bring on resistance heating to modulate the temperature. That is decidedly inefficient. It would seem that you could modulate the compressor on and off when the thermostat is reached, but unfortunately the Model 3 system keeps the compressor on at the lowest speed and then dribbles in resistance heat from the proportional temperature control (PTC) heaters. The other way (and the conventional way) is to choose the highest thermostat temperature that is comfortable for you, but then my secret sauce: lower the fan speed to the lowest value that keeps you and your passengers comfortable.
Here’s a real-world example. Recently driving back from North Carolina with ~150 miles on each leg at 74 mph showed the following: During the morning leg, it was cool and we easily avoided cooling or heating. During the afternoon, it was hot and we needed AC. During the late afternoon, it became cloudy and even with a fan speed of 1 and cooling, we had to bring the thermostat setting up from LO to 72°F.
- 74 mph in the morning without AC: 244 Wh/mi
- 74 mph in afternoon with AC: 271 Wh/mi
- 74 mph in late afternoon with thermostat turned up to 72°F: 281 Wh/mi
So, what can we see in these data:
- Energy use without AC was very similar (~2% better) to our road tests (see Part 2).
- AC set to LO with fan speed modulated adds 11% to electricity Wh/mile.
- AC with a selected temperature that is near outdoor temperature adds 15% to Wh/mile.
So, most efficient cooling is turning on AC and modulating interior comfort with the fan speed rather than the thermostat, particularly when there are low load conditions (cooler outside).
That brings us to “phantom drain.” This is the famous vanishing-range issue in the Tesla world. In the larger universe of energy-efficient equipment and appliances it has often been called standby losses or even Vampire Loads. Yet, in the Tesla community, it is known as “Phantom Drain.” When the car is not moving, there is no way to see what the power draw is on the battery, so the Teslarati is left guessing about how anywhere between 1 and 15 miles of indicated remaining range on the upper left-hand panel mysteriously disappears.
Beyond the vanishing range while driving, Evan Mills, like many drivers, also noticed that his car seemingly lost a good amount of range each day, even with Sentry Mode off and not driven from his home near Mendocino. Tesla told Evan that Model 3s typically lose about 1% of range per day when parked (~3 miles). However, Evan claimed worse. “I’ve noted between 5 and 11 miles (2–3%, even assuming fully charged).” Evan had put the losses in an Excel spreadsheet which he had sent to Tesla. “You’ll notice all were winter (Nov/Dec) last year, so ambient temps were presumably a factor, but, again, it’s pretty mild around here (maybe low 50s at night that time of year). Does this concern you?”
Tesla informed Evan (more correctly) that the losses could be up to 2% or more per day (6 miles of range or more) depending on many factors. One turned out to be how often Sentry Mode was left on. Another was how often one checks on the car from the app and “wakes it up.” There can be other factors: for instance, keeping a phone charger plugged into the cigarette lighter accessory port will keep the entire vehicle from properly powering off. Tesla also noted that both firmware and software changes had been made to the Model 3 overtime to minimize parasitic power drain on the battery. But Evan was particularly surprised to find out how much range Sentry Mode could take away. He lost 10 miles or range in one night with it on, and then 3 miles the next night with it off. Here is a list of others:
- Some third-party apps can drain power, particularly if not configured to allow the car to go completely to sleep. Some apps, such as Stats, endeavor to help the car go to sleep. Some apps, such as Teslafi, even allow a user to put the car into a low power mode remotely.
- Overheat protection: Do you live in a southern state with oppressive heat? If not, consider that the car’s electronics can survive the heat in the shade or with a windshield solar shade.
- Tesla app connecting by Bluetooth. I switched off Bluetooth on my phone and noticed the drain became less.
- Smart Summon-clearly uses power when enabled.
- Keep in mind that checking the car on the app more frequently will cause more drain. In fact, it takes about a day for the car to go into “deep sleep” in which drain falls to a very low level.
Depending on the outside temp, the battery thermal management system might still cool or heat the battery, and this sensibly cannot be disabled. However, it can be affected by keeping the car in the garage and finding shady spots in summer and covered ones in winter.
Putting your Model 3 to Bed
So you want to leave your car unplugged for a while and want to come back without a lot of lost range? Let’s say you’ve left the car in a covered spot and the above energy gobblers are off. Still, if you check on the remaining range in your car remotely using the Tesla app, the car wakes up to about 150W idle power. But, it takes 24 hours for the Model 3 to go into deep sleep mode where the power use is only about 10 watts. “Wakeful sleep mode,” as I’ll call it, in the Model 3 is about 25 watts.
By calling it each day, unwittingly, you wake it up to check on its range, but reset the clock and substantially increase phantom drain. That way, you can worry more — and lose more range at the same time.
Nevertheless, when managed properly phantom drain can be substantially reduced. Indeed, many users have noted that parking a Model 3 at the airport and not waking it up frequently can reach a depth of sleep for the car such that phantom drain is significantly less than a mile a day long term (~0.22 Wh/day).
As example, one owner reported:
“Last month I was on the road for work and my M3 was parked in the airport long term parking for about 10 1/2 days. Total phantom drain was 9 miles. Was rather surprised to say the least. Had summon standby turned off and no sentry mode. Also had climate control turned off.”
Another in a South Florida garage reported a similar experience: “We were away for 4 weeks and our Model 3 MR, which was parked in the same garage and not plugged in, only lost about 20 miles.”
The most teachable experience, of which I am aware, of the need to only infrequently check on a parked Model 3 is from Kim Ahlberg in Seattle, who left his Model 3 at the unheated airport twice for long periods of time in December–January 2019 and then again in the following summer (July–August). He charged the car up to 85% state of charge before departing each time. During the first 42 day period in the winter of 2019, he checked and recorded the car charge level each day. The car lost only 28% charge over the 42 period (85–57%) or about 0.67% per day or 5% per week. This is approximately 2 miles per day lost or about 0.5 kWh.
However, the following summer, Ahlberg left for an even longer period (49 days), but only checked on the car weekly. Able to go more deeply asleep, the car suffered much less phantom drain — only 0.9 mile per day or about 2% loss of charge per week — 1.5 kWh.
The moral of the story? When you’re away, don’t check on the car often.
Sentry Mode draws quite a bit of power, as it has to keep the Autopilot computer running while it is on. Tesla’s official number used to be 20 miles of power drain a night when using Sentry. Tesla is still improving Sentry Mode power efficiency even while continuing to activate more Sentry cameras. There are some reports from customers with Sentry on losing as much as 30 miles of range drained overnight. The Autopilot computer CPUs each draw about 250W of power, and there are two of them operating, as well as additional power consumption from the cameras themselves. The Autopilot computer creates enough heat for the car to use it in winter for assistance in cabin heating.
There are other undocumented goofy things to surface. For instance, say you are legitimately worried about phantom drain (vampire loads) from the car while you have left the car at the airport. Did you leave it in Sentry Mode or Dog Mode or leave Remote Summon active? Those are 250W mistakes — you’ll lose a mile of range each hour. (Dog Mode can be much more, but why are you leaving your dog in the car at the airport?)
Overheat Protection, Dog Mode, Camp Mode
Overheat Protection might be considered a good bet in Florida, but like Dog Mode, get ready for HVAC energy use as the air conditioning comes on in summer to keep the interior temperature from pushing further into the triple digits beyond 105°F. There are a lot of drivers reporting their experiences out there, but often you’ll see about 12 miles of range lost each day, or about 3 kWh.
It is certainly not as energy-gulping as Camp Mode, though, which will just keep the space conditioning on to the way you have it set until the battery falls to 20% capacity. Beware!
Overheat protection certainly has its place, but a shady parking spot starts to look better and better, at least for a long-term parking. Heck, shade will even help a bunch for Dog Mode and Camp Mode. Remember, the AC system in the Model 3 is fully variable speed, so window tinting, window shades, and/or insulation inside will help. Everything you do will help. The exterior shade is perhaps most effective and will save real battery power. If you’re driving, remember that if you’re on manual with the AC, the thermostat setting and the fan speed are the cues for the HVAC power.
But the real fix: maybe Tesla will allow us to see the HVAC power and status without having to access the HVAC control sub-panel and annoyingly change all one’s settings.
That’s the real fix.
What About Winter?
It’s coming, but not yet.
I’ve written up a fairly long segment on the important issue of winter performance, the likely impact of the new heat pump in the Model Y, and a host of tips and tricks. Alas, it is quite warm at the moment and this piece is already long. I’ll follow up with a winter-focused Part 5 (bonus) in December if my sponsors are willing.
To Sum Up
So, where do we end up?
Dear Tesla, thank you for these wonderful electric automobiles. Now, please tell us the condition of our craft. Give us the energy feedback that these machines deserve so we can do better. It would be best if you can show us a display with total HVAC power on the right panel. Waste less and go further on the same charge.
Call it Pilot Mode. Call it Nerd Mode. Call it whatever you like. We’ll be happy.
I appreciate the thoughtful perspectives from helpful service personnel with Tesla in Berkeley, California, who had much to do with the genesis of this article. I also value the input of JB Straubel early on. Also assisting me at Tesla to explore potential changes to the User Interface design: Scott Sims and Adrienne Trane. At ABRP.com, thanks to Bo Lincoln and particularly to Troy Teslike (Teslike.com) for use of data and plots. Brett Williams with Motion Arts kindly provided me with images of the Mercury spacecraft interior. Kris Stoever, the daughter of Scott Carpenter, and co-author of “For Spacious Skies,” helped with insights about Project Mercury. Michael Kluge with EMDS e-mobility in Blankenhain helped with hardware to scan Tesla logs via OBD. Amund Børsund helped to provide data within ScanMyTesla from the OBD link. Ramin with Stats for Tesla provided other help. Reviewing my material, I appreciate guidance from my old pals Evan Mills and Steve Greenberg at Lawrence Berkeley National Laboratory and Jon Koomey from Stanford. Thanks also to Bruce Wilcox, who, like me, loves all efficient machines as well as a fine bottle of wine.
Danny Parker is Research Scientist at the FSEC Energy Research Center where he has worked in the energy efficiency field for the last thirty years.* Beyond better machines, he has a keen interest in low-energy cooling technologies, zero-energy homes, rockets, and good coffee. His sister lives in Fremont, where he became familiar with Tesla. Neighbors and other pals in Cocoa Beach work for SpaceX. Tesla investor.
*Disclaimer: Note that the author’s information and opinions do not imply recommendation, endorsement or favor of specific products or services by the University of Central Florida or FSEC, its research institute.