Comment by nimos | Hacker News Reader

Comment by nimos | Hacker News Reader

I actually think the math really works out well for semis given you can burn through your battery lifespan super quick. Let's say you take two 15 minute breaks + 30 minute lunch per day. That is 1 hour of charge time. Assuming you start from full battery and a 2% per minute supercharge rate (~what wikipedia has for the model s) that gives you an extra 120% battery.

1.9kwh per mile and driving 600 miles per day and you can get away with a 400kwh battery. Less actually but my understanding is you want to not charge to 100% for maximum lifespan. Not sure the optimal cost trade offs...

7 cents per kilowatt hour * 1.9 * 600 = ~$80 a day vs $2.7 a gallon * 600/6(mpg) = $270 a day

$150/kwh for battery with 2000 full cycles is $85 dollars a day without interest(only ~2 year lifespan so w/e and presumably has some residual value also not taken into account)

Seems to work out to about $100 a day in savings?? I feel like I must be off on numbers somewhere. Data on number of cycles doesn't seem to be particularly consistent.

> $150/kw

That's insanely cheap, if you can find a place where I can buy batteries for 150$/kWh, please let me know. The cheapest I found are still >300$/kWh.

> 7cents/kWh

In NY the price is ~0.2$/kWh [1]

Also, a 400kWh battery weighs 2 tons. [2]

[1] https://www.bls.gov/regions/new-york-new-jersey/news-release...

[2] https://en.wikipedia.org/wiki/Energy_density

Tesla doesn't publish it's costs but based on what they've said it is a reasonable estimate[0] Cells in bulk are below 150[1]

7 cents per kwh is a very common industrial rate.[2] Maybe it is a bit low but it's a lot more reasonable than one of the highest residential rates in the country ($.2/kwh).

Let's say 5000lb for batteries + cooling/wires. Semi's routinely have (200gallons *7) 1400lb in fuel or more and a ~500 hp ~3000lb diesel engine. I don't know what the electric equivalent would weigh exactly but Tesla's 362 hp motor is 70lb. Let's say a 500hp one weighs 500lb. 5000+500 - 1400 - 3000 = 1,100. Tractor weights are around 15-20k so ~5-7% weight increase with a pretty generous estimate for cooling/wires and engine weight assuming rest of the drive train is identical.

[0] https://electrek.co/2017/02/18/tesla-battery-cost-gigafactor...

[1]http://benchmarkminerals.com/lithium-ion-batteries-are-now-s...

[2]https://www.eia.gov/electricity/monthly/epm_table_grapher.ph...

I want electric vehicles to succeed more than anyone, but lets not delude ourselves:

Tesla's batteries are not that energy dense, they are currently 140 Wh/kg[1]. A 400Kwh battery will weigh at least 6000 pounds.

A 400Kwh will get you maybe 150 mile loaded range, 200 gallons of diesel in a semi is a 1300 mile range[2]. To compete with a 400Kwh battery pack you only need to carry 25 gallons of diesel.

Charging a 400 kwh will take at least 3 hours on a supercharger, vs a few minutes for diesel. Its simply not practical for "long haul" as claimed. For it to be usable for long haul you need at least 500-600 miles for a days driving then charge overnight (even that is low for a truck driver). 600 miles is a 1200Kwh battery at 24,000 lbs costing more than the truck with some kind of faster charger so it can be ready in 8 hours.

Bottom line diesel is 30x times as energy dense as current battery tech and that not even counting how much efficiency is left on the table for diesel, the various super trucks programs are showing real world 10+mpg on a semi vs the current 6.5mpg standard by doing secondary heat recovery like a power plant among other things.

The Tesla semi might have some use for local urban delivery but I am highly skeptical of any "long haul" highway replacement claims, will be very interested in the real specifications come Thursday.

[1] http://enipedia.tudelft.nl/wiki/Tesla_Model_S_Battery

[2] https://insideevs.com/tesla-semi-truck-battery-is-how-big/

Taking the 2012 numbers you have at your link, the 85KW pack and 600kg, that is 6200 lb to get to 400kwh. The 2015 number on wikipedia is 540kg per 85kwh pack and the model 3 pack is supposed to be 30% more energy dense[0]. The 2015 number gives 5600lbs so I don't think at least 6000lbs is accurate, even the 2012 number is just slightly over.

All else equal you can charge batteries based on a % of their total capacity. Current superchargers can do about 2%/min for a model S in the 0-80% charge range but can't deliver the requisite power for a 400kwh pack. But I also haven't seen very many supercharger stations that would be particularly well suited for truck traffic either, so I'd imagine there will be a new charger/stations as well.

Sure batteries are no where near competing on absolute range of diesel but I don't think they have to. The question is can you get a days driving in without spending hours and hours charging while keeping the battery capital cost/weight reasonable.

You don't need a 1200kwh pack for that because you can take a couple short breaks plus a lunch with supercharging and over double your effective capacity.

We'll see what happens but I'd be very surprised if the announcement doesn't include a new super charger and stations unless they design it for short range point to point only.

[0]https://electrek.co/2016/11/14/tesla-model-3-battery-energy-...

Magna has electric trucks as a short-range shuttle between factories. They charge during loading/unloading. So it certainly seems doable, at least for short-range transportation.

https://investinaustria.at/en/news/2017/11/magna.php

I don't know about long-range. But I would be curious where you get the 1.9 kWh/mile and the 600 miles per day figure from?

1.9 was sort of an average from googling. This seems like the most detailed source which has a slightly higher number. https://insideevs.com/tesla-semi-truck-battery-is-how-big/

600 is just random googling of what is a long but not super long day for most truck drivers.

Aside from energy efficiency, the semi would be an attractive vector to introduce fully autonomous driving:

1. Feasibility: Many trucking routes avoid some of the most complex driving tasks (e.g. dealing with pedestrians, city traffic).

2. Labor Cost: In commuter travel, the driver _is_ the cargo, so the utility gain is offset:

  utility = cost(operation) - cost(occupancy)

For shipping, in contrast, there is no occupancy requirement, so the gain is greater:

  utility = cost(operation)

However, if the truck is autonomous, then DOT restrictions on hours of service for a driver no longer applies. So, the semi drives a longer day: requiring a bigger battery and longer charge times.

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