Tesla's $0.07/kWh Megachargers look entirely reasonable!

At the start of last year, I did some back of the envelope math on electric long haul trucks.  And, recently, Tesla announced one.  We don’t have exact numbers (“Under 2kWh/mi”), but their numbers are compatible with what I came up with last year (1.43kWh/mi).  I expect they’ll come in slightly under that (my money is on 1.3kWh/mi), simply because they don’t have significant cooling drag to deal with, and can improve the aerodynamics slightly over a big diesel on that alone.

This is a companion discussion topic for the original entry at https://www.sevarg.net/2017/12/10/teslas-7-cents-per-kwh-megachargers-look-entirely-reasonable/

(Comments from Blogger)

2017-12-10 by Unknown

Hello there Mr Graves. Name’s Joseph Kolodziejski.

I thought you might be interested to know that on the below linked Tesla Motors Club page, I posted a CAD analysis of the Tesla Semi’s battery pack based on a comparative and somewhat painful planar photo overlay:

Lets work out the Tesla Semi-Truck Technical Specs | Page 2 | Tesla Motors Club

In reference to the visual detail:

Due to the size of the layered pack module shapes being very similar in height (close to 100mm or four inches) to that of the Model S/X, and given the typical hexagonal arrangement of the battery cells, there is a surprisingly precise degree of inference that can actually be obtained from the visual evidence, for the range of numbers of battery cells in each module, which apparently in multiples of at least three vertically, in what I assume to be four separate packs which seems to suggest anywhere between 3700 and 3800 cells at maximum.

Multiplying this number by the calculated energy density of the Model 3 LR cells (80.5 kWh / 4416 = 18.23 Wh) and multiplying by the apparent number of modules, 3, gives a number VERY close to 200-205kWh, and the apparent number of separate packs, 4, (12 total) gives 800-820 kWh total, suggesting a usable of 750-800kWh at 94-98%.

(Model S 100 pack and Model 3 are apparently 96% and 97% usable respectively, judging from the Electrek battery pack article and wk057’s work, check them.)

Your assumption for the Megacharger is eerily similar to mine, but I added a “bottom amount” to start (10%) and I made some slight mistakes, I should have written “540-560 kWh” & “1080-1120kW” instead of “480 kWh” & “960 kW”, ideally for the range 60-80kWh to 600-640kWh, with 600-640kWh being 80% of 750-800kWh or a charge range of <10-75% of absolute technical capacity, as stated last paragraph, which may explain the somewhat quickish-sounding 40-minute charge time.

For that capacity, I have judged a larger energy consumption than yours of 1.5-1.6 kWh per mile, which seems to fit neatly as compared to other current electric trucks or aerodynamic diesels (post thermal!) having a consumption hovering very close to 1.5 kWh/mile, but unlike your analysis this is assuming full load for 500 miles, as unambiguously stated by Elon on the day of the reveal.

2017-12-10 by l

Your analysis seems to be truck-focused, and assumes there is infinite power available (please correct me if i’m wrong), which is not really the case.

Would love to see this analysis go one step further to calculate the required increase in generation (america-wide) required for powering the trucks on the road. What would the worst case scenario be (i.e. extended non-POV weather requiring 100% grid charging)?

2017-12-11 by Bob nye

Hi, you didnt take into account losses in charging and dischaeging the externsl batteries, and possibly losses charging the vehicles, which will have the effect of increasing the electricity cost somewhat

2018-05-23 by Joe the Jerk

"…a diesel can slow down without brakes, but it doesn’t recover the energy…"
Do you mean by drag alone, or…?

2018-05-23 by Russell Graves

That’s something I’ve considered doing, and will probably sit down with at some point (total US liquid fuel consumption and power generation requirements), but it’s something that will work tolerably well with the likely overgeneration of solar/wind required to make for a stable grid with high renewable penetration.

2018-05-23 by Russell Graves

You’re correct, but chargers and lithium ion are all fairly efficient, so I didn’t feel like muddying up what are, quite honestly, back of the envelope, order of magnitude level calculations with that much detail. I certainly assume a lot of spherical cows here, but I expect I’m within 10-15% of actual values.

2018-05-23 by Russell Graves

I’m mostly referring to compression release braking (“Jake Brakes”), though exhaust backpressure brakes work as well (as long as you don’t hang the exhaust valves open too far). Any variety of enhanced engine braking will slow a diesel down (and living on a hill, I’m quite familiar with them), but you don’t have any way to recover the energy.