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How hard would it be to make an electric semi truck

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True enough... But there is the phrase 'Super fast charge' used at the beginning of the post. Something tells me that charging two, four, or eight times as quickly as Superchargers currently operate will develop... issues, and stuff. When it comes to longevity and durability of battery cells.

Let's hope not! I know you like the idea of battery swapping for this application but I personally think it is a big no no. To swap a very large and heavy battery pack will require a very large and heavy machine - at every station. Then you have to maintain them - bummer, back to ICE age problems! Not to mention the machines are soley for high usage vehicles so a kind of a waste of space - whereas a super-duper-mega-ultra Charger (tm) could simply service other vehicles in the fleet at a slower charge rate. IMO fast charging is improving quickly - and actually having its envelope pushed by companies other than Tesla which is great :)

Competition means everybody wins - if 300KW fast charging is possible in 2015 from a company with less resources and experience than Tesla, then what's possible in 3 - 5 years time when we'll be seeing this product?
 
So, let's say 1.5kWh/mi, driving at 70mph, for 11 hours of 14.
That's 770 miles.
Then you need 1.5kWh/mi x 770mi = 1164.24 kWh of charge per day.
Simplified assumptions:
start with 100% charge, deplete to 0%, charge to 100%.
1 hour to charge to 100%. (1C).
So:
1 + 3 x 1 = 4 full charges requiring 1164.24 kWh implies 291.06kWh/charge.
If you want 80% usable then it'd require 363.825kWh of battery.
So, about 4 x 90kWh packs. Make it 5 x 90kWh packs and get more breathing room.

OK, so how much does a 90kWh pack weigh?
Well, let's use an old 85kWh number of 1323lb and scale:
5 x 1323 * 90/85 ~= 7004lb.

That's actually not totally insane.
But that 1.5kWh/mi...
Let's say that due to the shift away from petroleum, diesel remains pretty cheap and the underlying diesel price is $2/gal. Then at 9mpg the cost is $0.222/mi.
Electricity: at 1.5kWh/mi, then break-even price _even ignoring the charging losses_ would be $0.147/kWh.
Hmmm... Your energy expenditure per mile seems rather low in those calculations. Mine might be rather optimistic as well, but, here they are, around 1,000 Wh per mile higher than yours, I think...

I look at it another way... Diesel fuel is slightly more energy dense than gasoline. Or, 40.7 kWh per gallon. If one gallon of diesel will move the vehicle 6 miles, that is equivalent to 6,783 Wh per mile energy consumption. Electric vehicles are far more energy efficient. So it would take less energy to move the same distance in an EV powered truck. If you presume that a diesel system is 38% efficient (they aren't) and the EV were 90% efficient... That means of the 6,783 Wh expended per mile for the diesel, only 2,577 Wh were used for actual motive force. So, an EV would need to have 2,864 Wh of available energy to travel one mile with the same load. Thus, in order to get a range of 600 miles, you need a bit over 1,700 kWh of battery storage. In practice, you'd probably end up with four separate 400 kWh battery packs. Yes, they would be much heavier than the diesel fuel, and yes, their weight isn't going to drop 99.9% as electrons are 'burned'... So you always have to carry the 'extra' weight around with you, but that is no big deal. How much would they weigh? Idunno. But I suspect it would be nowhere near the 12,500 lbs per axle limit.
 
I am going to make an assumption and say that if the heavy Model S was powered by an efficient diesel engine it would get about 50mpg. A quick google search says that modern semi trucks average about 6.5 mpg. I would bet Tesla could increase this fuel mileage to about 8mpg.
That means a Tesla semi is about 6 times more power hungry than the Model S per mile. Since we know the Model S achieves about 350 watts per mile we can assume that a Tesla semi would use about 2100 watts per mile.
I would think that the Tesla semi would need to achieve at least 500 miles per charge which means it would need about a 1MW battery pack. This pack would probably weigh 7500 lbs and be part of the trailer imho.
Cost of the batteries alone would be about $130,000 for Tesla to make.
 
Competition means everybody wins - if 300KW fast charging is possible in 2015 from a company with less resources and experience than Tesla, then what's possible in 3 - 5 years time when we'll be seeing this product?
300kW will still be 300kW and 1 GW power plants will still be only 1 GW powerplants and power transmition lines will still be exactly the same as they are now.
So, there will have to be some stationary battery storage at ever SuperDuperFast Charging station. Swapping a 500kWh battery in say 5 minutes equals 6 MW average charging speed. You only get 6MW on average with 10MW of peak charging power.
Wanna bet 10 MW charging won't happen in our lifetime?

All the problems batterty swapping has with cars, dissapear with trucks. And fast charging looses almost all its benefits it has ower battery swapping.

I will put my money down on truck battery swapping.
 
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300kW will still be 300kW and 1 GW power plants will still be only 1 GW powerplants and power transmition lines will still be exactly the same as they are now.
So, there will have to be some stationary battery storage at ever SuperDuperFast Charging station. Swapping a 500kWh battery in say 5 minutes equals 6 MW average charging speed. You only get 6MW on average with 10MW of peak charging power.
Wanna bet 10 MW charging won't happen in our lifetime?

All the problems batterty swapping has with cars, dissapear with trucks. And fast charging looses almost all its benefits it has ower battery swapping.

I will put my money down on truck battery swapping.

I'll put an electron on that bet :)

Some compelling points you make here. 6MW is a massive figure to be sure but methinks Tesla would see virtue in a far lower charge rate with a longer charge time - let's say 1MW for fun? 30 minutes to full charge seems reasonable to me.

If Tesla go this route they may need to look closely at the current Super Charger system's tendency to bog down with multiple users. If a driver knows his charge takes 20 - 25 minutes each time he will plan accordingly. IF that number blows out several times across a leg due to congesting at the SC then this would pose a major annoyance.
 
A deal with Walmart may only require chargers at their distribution centers. Grocery distribution may be similar. I assume a lot of trucking is from distribution to store. One or two round trips per day.

Here info on grocery distribution networks in the U.S. in 2010. About 50,000 stores. Lots of trucks.

Grocery | Distribution Network | United States | Canada - MWPVL
As I understand it, Walmart has expanded their network of distribution centers ahead of their network of stores. They don't erect a new store unless it is within 12 hours' drive of the nearest distribution center.
 
Good news for Tesla's future semi-truck efforts...speed limiters may become mandatory on trucks as low as 60mph. This will reduce the battery size required to travel the same range.

The government has an unexpected ally in 68 mph truck speed limit push
Maybe good news for Tesla, but could be bad for traffic in the US. We have 90kph limit (56.25mph) in Japan for trucks and buses and they are now required to have speed limiter at 90kph.
Since then trucks are getting into second and third lanes, still at 90kph, and taking minutes to just take over one slower truck on the leftmost. So we now have very slow highway at night.
 
Maybe good news for Tesla, but could be bad for traffic in the US. We have 90kph limit (56.25mph) in Japan for trucks and buses and they are now required to have speed limiter at 90kph.
Since then trucks are getting into second and third lanes, still at 90kph, and taking minutes to just take over one slower truck on the leftmost. So we now have very slow highway at night.
Yes that would be bad if that law went through. I can see it now, on a 4 lane divided highway, one truck is trying to pass another going like 1 or 2 mph faster. Works in Europe but not america because the places in america are much further apart. And could the truck owner not just disable it? Another thing is on big hills, trucks need to get a running start and go over the speed limit so they are not doing 40 by the time they get to the top.
 
What about using multiple Supercharger's instead of one superfast one? Currently semi's with side mounted tanks can fill up both tanks at once using two different nozzles. So for example if you had a battery on each side, could you just simply use a supercharger to charge each battery separately.
 
What about using multiple Supercharger's instead of one superfast one? Currently semi's with side mounted tanks can fill up both tanks at once using two different nozzles. So for example if you had a battery on each side, could you just simply use a supercharger to charge each battery separately.
Possibly. If the Tesla Depot locations I imagine had certain bays specifically for fully electric semi-trucks. But most believe instead that battery swap would be preferred for long distance trucks, to keep them rolling around the clock.
 
I too would think battery swap. First, imagine the power substation to deliver to multiple superchargers. Second - the batteries could be owned by - say Yellow Freight - and just baton passed across the frequently used highway corridor. Different long-haulers would own their equipment and shuffle them to chargers between arrival of the next truck. hmm sounds kinda like the old stage coach horse swap. Not requiring a given driver to return to swap back, but put fleet management uncharge of one more task.
 
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I too would think battery swap. First, imagine the power substation to deliver to multiple superchargers. Second - the batteries could be owned by - say Yellow Freight - and just baton passed across the frequently used highway corridor. Different long-haulers would own their equipment and shuffle them to chargers between arrival of the next truck. hmm sounds kinda like the old stage coach horse swap. Not requiring a given driver to return to swap back, but put fleet management uncharge of one more task.

The SuperCharger locations would probably be at large commercial Distribution Centers which already have large Commercial power-feeds with large sub-stations because they are located in industrial areas that have ample power infrastructure. The issues that Tesla has had with pulling in power is usually installs at smaller commercial sites that are customer facing. Obviously a commercial site with a Wendy's a and a gas station isn't going to have the same sized sub-station as a commercial zone with multiple Million square foot distribution warehouses with cold storage.
 
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I think we could take an idea from the Pony Express --- just change horses. Instead of a complicated battery swap, just swap the whole tractor-unit. Faster recharge is possible with higher voltage battery units (or internal reconfiguration to give higher voltages) -- while this may have added dangers in public, dedicated recharge stations for semis (and buses) makes this more useful.
 
For passenger cars, recharging along the highway is logical. But Trucks often have distribution warehouses to snuggle into. Brogan is right - those centers have the power, and the time, to battery swap.
Changing the entire cab instead of the battery - sent me shivers . I've looked at how truckers live in their cab, make it home - eat in it, sleep in it, etc. To swap out home for some impersonal cab (like getting into a taxi) sounds like a hard sell,
Lucky for me - I can sit on my cloud and tell others what to do with their equipment and have zero consequences. If I was a fleet manager and had to decide on capital expenditures for all of Yellow Freight, UPS, Fed Ex, etc....I may not be so cavalier with my "they should do it this way"
 
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If the batteries were in the trailers, the trailers could charge up while docked or waiting in the yard, to be managed according to the habits of each particular yard.

If the batteries were in the trailers, solar panels could become the roofs of the trailers, and some of the cost of the electricity would be taken directly from the sun. This would be a new type of material that would replace the plate-thin plastic that currently serves as the roof of most semi trailers. A great proportion of charging would come direct from the roof of the trailer, although not always all of it. It might be enough that they don't need to build out connectors out in the yards, and can leave the yards barren (which is easier for driving and cheaper to install), and the trailers could just top-off while docked to unload and load. Whenever the dock was in disuse, trailers could be backed in early for loading or left late after unloading to continue topping off the charge. The routing computers could assist in this planning, using real-time readouts from radio connected data driven batteries.

If all of this was done, they could also use swappable batteries IN ADDITION, for those edge cases where a dock connection didn't work right or there wasn't the expected amount of sunlight or someone had to drive further around a detour or the sales people did really well or they got routed to timbuktoo, and then they'd go over to the battery swap and swap for charged up batteries. These would only be used as a logistical backup measure plus a few by plan just to keep the capability alive and paid for and maximize advantages of the available capability (of battery swap). There's ample belly room in a trailer for all of this.

If batteries were in the trailers, there's no reason they couldn't, completely at their option, also put motors in the trailers.

Ahhhhh, but then there's the end game: there's no reason there'd need to be a cab. The trailer would have turning wheels in front (8 of them), motors at every axle, and drive themselves. You'd only see trailers going back and forth, without any tractors.

Eventually these would right-size for every particular business model according to the cost factors associated with delivery via a computer-driven vehicle. The cost of the human being would be removed, and therefore, smaller size trailers with smaller loads would not have the same fixed cost sitting in the front tractor any more. It has been calculated that loads that are much smaller than a full length trailer would be optimal, except for the driver cost, which is why freight usually wants to be on the longest possible trailers today with human drivers.

Between now and then, tractors that were self driving could be built to pull existing trailers. Tractors that were human driven could be built to pull new battery-fitted trailers. The possibilities are endless.

Freight is a lot more flexible in a lot of ways than passenger cars. I think this will quickly manifest in the way electric cargo and computer-driven cargo will be built. An opening at the back for a dock and the width and height of the roads are the major limits; it's an extremely flexible ordeal besides those restrictions. I think this arena will have a dozen solutions in the next two decades, and in about four decades, optimal truck sizes will have been sorted out and exist pretty much everywhere driving themselves around -- probably about a handful of predominant sizes, with around 3 dozen legacy sizes still using up their depreciation.

Right after that, humans may evolve to virtual computer programs, and may not need bodies any more, so we might not need cargo any more. But, I think that's an edge possibility: more than likely, cargo will still be needed even in that virtual future, to build and maintain those computers, and the infrastructure for all that cargo and material, and space, and everything else. Plus, most of us will want to "go human" on occasion, inhabit a body, enjoy the pleasures of flesh, feel the fresh air, see the trees and hills and rivers and seas, etc. So, cargo will be needed to transport organic fuel, clothing, all the trappings of the human body, as well as any environmental projects we have to keep the planets happy in ways we want.
 
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Another thing is on big hills, trucks need to get a running start and go over the speed limit so they are not doing 40 by the time they get to the top.
Believe it or not, most modern trucks are much more powerful than older trucks, but are power-limited by the computer settings by the owners so that they won't drive "too fast" and use too much fuel and strip out the gears. Mountain-driving trucks or any trucks that use steep grades usually have their companies set the power higher for the hills. These days, with computers in the trucks, these trucks could be set to have more power precisely when they need it ascending steep hills, and not need any type of running start whatsoever. A governor in all trucks would mean that those trucks that were previously de-powered in order to save money might now be allowed to offset that savings with the slower speed governing, and then assert that power for hills. So, regardless of whether or not I think it is a good thing (I feel very mixed), I think you happen to be wrong.

In fact, the most modern trucks today already allow settings that allow higher power going up hills than in regular flat driving. You can see that in a lot of trucks that have the damndest time getting up to speed in a flat area (or trying to pass another truck quickly), but when they go up a hill, they just keep going up at the same speed as on flat land almost. You can also tell the difference between these trucks, the trucks that have their power turned all the way up, and the trucks that have their power constantly turned way down: loaded trucks going up a hill without power crawl, prettymuch every other truck passing them. Unloaded trucks with low power will go slow. Loaded trucks with power will go a fairly good speed, depending on how much load and how much ultimate power they have -- there are some really powerful trucks out there, but not all of them. And unloaded trucks with full power can go up a hill faster than their governers will allow them to.

I always get mad at the trucks that have their power turned down to almost nothing that have to go through heavy commute area mountain passes --- that's just criminal. If their tractor is set for flat lands only, they shouldn't come to the mountain passes. Either reset the tractor or get a new tractor or don't come where the land rises.

I hate slow driving trucks, and slow driving cars, that clog the freeways and roads. But as long as they are out of the way, I don't care, not one bit. If the number of lanes are reduced ("road diet") or the number of vehicles in the way (self righteous slowpokes trying to control others by blocking them or just not caring), then that's VERY bad. But, if they stay to the right and don't clog, who cares?
 
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Hmmm... Your energy expenditure per mile seems rather low in those calculations. Mine might be rather optimistic as well, but, here they are, around 1,000 Wh per mile higher than yours, I think...

~1.5kWh/mile is too low for 70mph, but I think it's doable at 55mph. The SuperTruck program was pretty close to that with better aero. Tesla just needs to drop misc power consumption and rolling resistance. What's more appealing is a autonomous convoy, which could get down to ~1kWh/mile (or less with better aero). Here's a link to my guesstimates.

Is Elon Musk serious about the Tesla Semi?