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comment by ThurberMingus
ThurberMingus  ·  1274 days ago  ·  link  ·    ·  parent  ·  post: Plutocrat punch down

    What about weight? Gates says "the more batteries you use, the more weight you add—and the more power you need" but power for what? A heavy battery in a truck on the ground needs no power to continue existing. This is true when the vehicle is parked or in motion. At highway speed, air resistance is the main force that the engine has to overcome.

    Extra battery weight is a factor in getting up to speed. Force equals mass times acceleration, so if the mass of the vehicle doubles, the same engine force will accelerate the truck half as fast. Semi truck 0-60 mph times seem to be about a minute. Compared to the many hours of a long-haul voyage, adding a minute or two to get up to speed is no big deal.

You fell prey to one of the classic blunders of Dynamics 101, conflating power and energy.

Double the mass, and you double the acceleration time is power draw is the same. But energy to accelerate to speed al is also proportional to mass: energy = power draw × time = 1/2 × mass × velocity².

And this increased energy cost isn't a one time thing, it applies every time you push the accelerator, every grade, every time someone cuts you off and you have to slow and regain speed.

Electrifying freight trains is the probably the simplest technologically, its existing technology, assembled in a new configuration. Probably the hardest politically/legally though.





WanderingEng  ·  1274 days ago  ·  link  ·  

The simplest example of this, to me, is pro cyclists. They use the lightest bikes possible, and it's because every push, every grade takes less energy. Ride a heavy bike and it will wear you down over time more than a lighter bike.

wasoxygen  ·  1273 days ago  ·  link  ·  

Thanks for emphasizing this distinction. I barely scraped through Statics, so I appreciate the input.

Gates is not very specific about why he is pessimistic about some EVs. I think he's right that converting from liquid fuel to batteries will increase overall mass. So that means more power while getting up to speed. More power means using energy faster, which introduces the range issue. If Gates thinks an electric garbage truck is okay, but not a long haul truck, it sounds like range is the main problem.

But even without the "big breakthroughs in battery technology" that he rhetorically includes, it seems that you can just keep adding batteries to get the desired range (ignoring cost, which does require big breakthroughs). The volume of batteries seems likely to be small relative to large cargo movers, and the extra energy needed to move an additional unit of battery mass seems necessarily smaller than that provided by the battery.