Supporting smaller battery sizes is like supporting floppy disks.
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SomeJoe7777
Marginally-Known Member
Tesla's web page on the semi (Semi | Tesla) says that the energy consumption is < 2kWh/mile, and Elon's presentation said 500 mile range.
Assuming the actual consumption averages 1.9 kWh/mile, we can calculate:
500 miles * (1.9 kWh/mile) = 950 kWh of usable capacity, add brick protection buffer and rounding due to modules and you're at around 1 MWh of raw capacity.
To save on cost, there's nothing wrong with smaller battery sizes for applications where they're adequate.
However, I do agree with Elon's prior statements that roughly 200 miles of range is really the minimum for a company that wants to sell cars that aren't "crippled". Given today's battery technology, I really don't see any good reason for OEMs to sell cars with less range than this, at least in developed nations like the US. It's hard for me to believe that Honda, for instance, just started selling their Clarity EV with only, like, 80 miles of range! It sure makes Honda look lame.
Also, I agree with other posters that 500+ miles of range would be really sweet for road trips. This would prompt more people of means to go full EV rather than keeping an ICE around.
mknox
Well-Known Member
I'll bet a good coach builder could make a real badass Winnebago-type RV out of that chassis! Begs the question, though... can the Semi use a regular L2 or Supercharger, or is it limited to the "Megacharger" infrastructure?
Yep. Or to put it as my wife says: "I don't like having to stop to eat when the car is hungry".
I'd expect the Semi to be able to use L2 and Superchargers (where space permits), but really only for emergency or overnight-type charging.
To fully charge a 1 MWh battery from empty in 10 hours, you'd need a charge rate of about 100 kW, at the level of Supercharging. I could potentially see a Semi driver pulling into a hotel for the night and plugging into a Supercharger, as there are many Superchargers at or near hotels. Occasional use of Superchargers to bridge gaps in the Megacharger network could also make sense.
The maximum L2 charge rate supported by charging stations today (mostly Tesla Wall Connectors) is about 20 kW. For the Semi, that'd be about 50 hours for a full charge! This is akin to our cars being able to charge on standard 120V outlets, not great for regular use but good to have as a backup.
By contrast, a 200 kWh Roadster or Model S/X would be able to get a decent charge (not full) within 1.5 hours or so on today's Superchargers, or 10 hours from empty to full on a 20 kW L2. That's much more do-able.
EV-lutioin
Active Member
Two sides to this: Range and charging time. If it takes 30-60 min. to charge then more range is better and worth the extra cost for those who need it and can afford it. However, if you can charge in 5-10 min. then longer than 250 mile range is less important. Thankfully, the Semi/Roadster reveal introduced the possibility that both range and charging time will be improved in the near future.
If charging time is significantly reduced, then it really opens up the possibility of making very affordable EVs with less range. Exciting times.
If I'm forced to stop after driving for less than two hours, a reduced charge time doesn't solve the key problem. I don't think the charge time is the issue. In fact, many times on road trips the breaks I take are right around the same time I need to charge, or longer.
Traveling with my family, it often seems as if there's no such thing as a quick stop. What "should" take five minutes may end up taking half an hour. Therefore, it's nice to avoid stopping until we absolutely have to. While it's always ideal if our stops coincide with Supercharger locations, it's best to not have to stop because the car needs juice. In other words, it's best that charging occur incidental to meeting human needs, not the other way around.
That being said, we feel strongly enough about the need to transition away from fossil fuels that we willingly accept some inconvenience from time to time on our road trips.
Yah, but if you take the drag of 0.36 the maximum legal cross section of 8.5x14 ft, a tractor and trailer with low resistance wide tires going 60 MPH with 80k gross weight, it works out to 1.51 kWh/ mile, so a 800 kWh pack might work.
FutureShock
Best Coast Denizen
Yes, but... why would we assume that typical actual consumption would be 1.9 kWh/mile, i.e. the highest possible without making Tesla's spec page a liar?
Seems like if you're posting an energy consumption spec of < 2kWh/mile, as Tesla is, you'd want to allow for some headroom in that figure. For, say, very mountainous/hilly routes (regenerative braking doesn't get it all back) or for bad weather (rain, headwinds).
Average/typical consumption likely wouldn't be right up against the 2kWh/mile figure, since presumably an average/typical route wouldn't be a horrible weather and/or mountainous route.
Thus, to get 500 miles in typical use, maybe you don't need a 1 MW pack. Just like a 100D Model S may be rated at 335 miles range, but doesn't always get it in bad conditions, a '500 mile range' Semi might not get a full 500 miles under bad conditions, only typical.
If the pack is 'just' (whew) 800kWh, then some interesting possibilities come to light. Such as, perhaps Tesla doesn't really HAVE, or even NEED, a major cost breakthrough in batteries for the Semi to work out.mongo said:
Warning: speculation follows...
If 50% of the cost of the $180K Semi is the (absolutely massive) battery pack, that comes to $90K. Which seems possible, given the $60K price delta between the Semi and a typical ICE big rig (approx $120K), plus the fact that the Semi's simpler EV drivetrain is replacing a complex, expensive, ICE big rig drivetrain.
If so, and it's more like an 800 MW pack than a 1 MW one, then the cost is $112 per kWh. That's quite low, but not insanely lower than Tesla's Model S battery pack price with 18650 cells, which in early 2016 (i.e. non- and pre-Gigafactory) they said was "below $190 per kWh".
Tesla Semi doesn't launch 'til 2019, so you get 3 years of incremental battery pack cost reductions over that. If you assume 5% per year, that's 15%.
The Semi's battery packs will have the benefit of Gigafactory economies-of-scale, which could cut 15-35% off the battery pack price (depending on what you think of Tesla's claims on that).
The Semi presumably uses 2170 cells too, which we assume are cheaper than 18650 cells per kWh, if Elon is to be believed.
Add it all up, and $112 per kWh for the battery pack by 2019 seems quite doable... and with NO 'new battery chemistry' major breakthrough, contrary to what I first thought.
I'm certain my numbers are far from exact, and I very much welcome any concrete corrections, but you can see my point: Incremental cost improvements over time + economies of scale + 2170 cells = yeeeehaw!
(also kinda makes me wonder how insane the profit margins are gonna be on that 200 kWh, $200K New Roadster
...).
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800 kWh for 500 mile range is possible based on wide tires and drag of .36 .
Also, allow for residual value of pack at end of life. Repurpose or recycle.
WRT longer range for S/X/3: Our experience is that the current ~240 mile range of our car is fine during good weather. We don't mind stopping every 3 hours or so for coffee and bathroom breaks, However, when there's cold, rain, snow, sleet, head winds, altitude gain, or combinations, the resulting 180 - 200 mile range really sucks.
IMHO, range should be measured at 60 mph, 25F, continuous sleet, 20 mph headwind, and 2000' altitude gain after a 10F overnight soak. In those conditions, a 250 mile range is fine for us, but I've not seen any stats on what the newer cars actually do in such bad conditions when range really becomes an issue.
IMHO, range should be measured at 60 mph, 25F, continuous sleet, 20 mph headwind, and 2000' altitude gain after a 10F overnight soak. In those conditions, a 250 mile range is fine for us, but I've not seen any stats on what the newer cars actually do in such bad conditions when range really becomes an issue.
EV-lutioin
Active Member
That is exactly what I was thinking when Musk kept saying, "...worst case scenario..." at the reveal. I was thinking, dude, we have all experienced a worst case scenario and it includes cold weather, elevation gain, strong headwinds, snow and ice. Take that semi on 1-80 between Laramie and Rock Springs, Wyoming during a strong winter storm and see how much range you get.
"That is 500 miles at maximum weight, at highway speed, so you are doing 60 mph," Tesla CEO Elon Musk told a crowd in Los Angeles Thursday night. "That is the worst case scenario."
bambam4171
Member
Are you implying Tesla can't do it? Have they ever failed to deliver on range? Did they say they deliver it now?
Bad financial decision? Why?
From what I learned (doing some basic research) Diesel semis have a mileage of 6 miles per gallon. That's aprox 4 times of a sedan. If we take the Model S energy usage as a basis it would be 4 times that. 4 x 330 Wh/m = 1320 Wh/mile.
SomeJoe7777
Marginally-Known Member
We might be able to calculate a lower bound on the battery capacity. Given that Elon said that hauling an 80,000 lb load up a 5% grade, that the semi can maintain 65 MPH, we can calculate how much power that would take. If we then assume some battery statistics, we should be able to estimate a minimum required battery size.
Three things will use energy when going up a 5% grade: overcoming air resistance, overcoming tire rolling resistance, and then adding energy to overcome gravity by raising the entire semi+load to a higher altitude. We can calculate the power required for each independently.
Overcoming Air Resistance
Given information:
Coefficient of drag, Cd: 0.36
Air density, p: 1.225 kg/m^3
Velocity, v: 65 MPH = 29.0576 m/sec
Frontal area of the semi, A: 119 ft^2 = 11.05546 m^2
Force (drag) = Cd*A*(p*v^2)/2 = 0.36 * 11.05546 * 1.225 * 29.0576^2 = 2058.282 N
Air drag applies this force to the semi over a 1 mile distance, 1 mile = 1609.34 m.
Work (energy) = F * d = 2058.282 * 1609.34 = 3312.475 kJ
Time to traverse the 1 mile distance = 1/29.0576 * 1609.34 = 55.38448 sec
Power required over this interval = energy / time = 3312.475 / 55.38448 = 59.80872 kW.
Overcoming Tire Rolling Resistance
Given Information:
Mass, m: 80,000 lbs = 36,287.36 kg
Distance, d: 1 mile = 1609.34 m
Acceleration due to gravity, g: 9.8 m/sec^2
Tire rolling resistance coefficient, Crr: 0.00700 (approximately the same as the Bridgestone Dueler H/T)
Frr = Crr * m * g = 0.00700* 36287.36 * 9.8 = 2489.312 N
W = F * d = 2489.312 * 1609.34 = 4006.151 kJ
time, t: 55.38448 sec
Power required over this interval = energy / time = 4006.151 / 55.38448 = 72.33346 kW
Overcoming gravity on a 5% grade
Given information:
Mass, m: 80,0000 lbs - 36,287.36 kg
grade: 5%
Distance, d: 1 mile = 1609.34 m
Acceleration due to gravity, g: 9.8 m/sec^2
h = 0.05 * 1609.34 = 80.467 m
Energy = mgh = 36287.36 * 9.8 * 80.467 = 28,615.36 kJ
time, t: 55.38448 sec (as above)
Power required over this interval = energy / time = 28,615.36 / 55.38448 = 516.6676 kW
Total Battery Power
Total power required over this 1 mile grade: 59.80872 + 72.33346 + 516.6676 = 648.8097 kW at the wheels.
Estimate drivetrain and inverter losses at 15% = 648.8097 / 0.85 = 763.3056 kW at the battery.
I would estimate that a continuous discharge rate should not be over 1 C, thus to pull 763.3 kW out of the battery for an extended period of time, the battery should be no smaller than 763 kWh.
Three things will use energy when going up a 5% grade: overcoming air resistance, overcoming tire rolling resistance, and then adding energy to overcome gravity by raising the entire semi+load to a higher altitude. We can calculate the power required for each independently.
Overcoming Air Resistance
Given information:
Coefficient of drag, Cd: 0.36
Air density, p: 1.225 kg/m^3
Velocity, v: 65 MPH = 29.0576 m/sec
Frontal area of the semi, A: 119 ft^2 = 11.05546 m^2
Force (drag) = Cd*A*(p*v^2)/2 = 0.36 * 11.05546 * 1.225 * 29.0576^2 = 2058.282 N
Air drag applies this force to the semi over a 1 mile distance, 1 mile = 1609.34 m.
Work (energy) = F * d = 2058.282 * 1609.34 = 3312.475 kJ
Time to traverse the 1 mile distance = 1/29.0576 * 1609.34 = 55.38448 sec
Power required over this interval = energy / time = 3312.475 / 55.38448 = 59.80872 kW.
Overcoming Tire Rolling Resistance
Given Information:
Mass, m: 80,000 lbs = 36,287.36 kg
Distance, d: 1 mile = 1609.34 m
Acceleration due to gravity, g: 9.8 m/sec^2
Tire rolling resistance coefficient, Crr: 0.00700 (approximately the same as the Bridgestone Dueler H/T)
Frr = Crr * m * g = 0.00700* 36287.36 * 9.8 = 2489.312 N
W = F * d = 2489.312 * 1609.34 = 4006.151 kJ
time, t: 55.38448 sec
Power required over this interval = energy / time = 4006.151 / 55.38448 = 72.33346 kW
Overcoming gravity on a 5% grade
Given information:
Mass, m: 80,0000 lbs - 36,287.36 kg
grade: 5%
Distance, d: 1 mile = 1609.34 m
Acceleration due to gravity, g: 9.8 m/sec^2
h = 0.05 * 1609.34 = 80.467 m
Energy = mgh = 36287.36 * 9.8 * 80.467 = 28,615.36 kJ
time, t: 55.38448 sec (as above)
Power required over this interval = energy / time = 28,615.36 / 55.38448 = 516.6676 kW
Total Battery Power
Total power required over this 1 mile grade: 59.80872 + 72.33346 + 516.6676 = 648.8097 kW at the wheels.
Estimate drivetrain and inverter losses at 15% = 648.8097 / 0.85 = 763.3056 kW at the battery.
I would estimate that a continuous discharge rate should not be over 1 C, thus to pull 763.3 kW out of the battery for an extended period of time, the battery should be no smaller than 763 kWh.
Your aero drag and vertical energy numbers line up with what I came up with. I think your tire numbers may be a bit little high. I tried to find the Dueler you mentioned, but it looked like a standard truck/ SUV tire.
For less resistive tires I started with this report:
https://www.nhtsa.gov/DOT/NHTSA/NVS...RollResistClass8TractrTrailrStopDistPerfm.pdf
Which has ones in the 6.2 range.
Then I adjusted based on Fuel & Mileage Calculator | Michelin Truck
To get wide base tires.
SomeJoe7777
Marginally-Known Member
Yep, I think the tire rolling resistance numbers should/could be lower. Just don't have the data for 18-wheeler tires, though. Light truck tire seemed like it should be in the general ballpark.
You're telling me!
It's like no one cares what the exact rolling resistance of the tire on an 80,000 lb rig burning a pint of diesel a minute is.
It's interesting to see the weight savings due to the wide tires/ single rim.
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