4 ms·

Not all the energy to power the car comes from charging. Some is generated from regenerative braking.

by dabinat 4y ago

dawnerd 4y ago
The chart even shows this.
- WalterBright 4y ago
  I know. The chart is wrong. The entire conclusion of the article is based on bad math in the chart. Losses due to friction, drag, and heat are NOT magically made up for with regenerative braking.
WalterBright 4y ago
Um, all the energy recovered by regenerative braking was initially supplied by the charging input, which loses 10%. The overall efficiency simply cannot be 90%.
- smt88 4y ago
  > all the energy recovered by regenerative braking was initially supplied by the charging input You're forgetting about gravity. People brake when traveling downhill, which is essentially using gravity to charge the battery (similar to hydroelectric energy).
  - WalterBright 4y ago
    Do you drive downhill both ways to and from school?
- ZeroGravitas 4y ago
  It's not fraction of total energy used, it's the ratio of the original input energy that ends up turning the wheel. Imagine you had a line of 24 people and you asked them to cross a room with traps that kill 2/3rds of them. 66% of the people make it and for the 18 people who survive, send them round an outside corridor and back to the start and ask them to cross again. This time 12 make it, repeat. No more than 24 individual people can ever make it across the room, but even if we stop after a 2nd cycle already we have 30 crossings. One way to express that is that the crossing equal 125% of the initial people input into the system. We haven't broken physics, were just arguing semantics.
mlindner 4y ago
The chart is kind of weird though. If there's 10% from charging loss, let alone from the other uses, how can you get back to 91%?
- WalterBright 4y ago
  It's kind of wrong. With 10% charging loss, you cannot get 91% efficiency.
  - bildung 4y ago
    Obviously both charts start with a filled tank/battery and ignore fuel/energy production. It only makes sense to belabor charging loss if you take the gas equivalents like fuel used in transportation to the gas station into account, and gasoline evaporative losses, and storage tank losses and so on.
    - WalterBright 4y ago
      This assumption does not apply. Let's do the math: d = losses = 10% + 18% + 3% + 4% = 0.35 p = power to wheels r = regeneration fraction = 0.22 p = (1 - d * p) + r * p solving for p gives an efficiency of 64%. Not 87-91%.
      - the_why_of_y 4y ago
        I find the presentation confusing too, but with my limited understanding I think the total energy that's available to be used isn't 100% but 122%; the 100% is the share that is electricity. It links to this: https://www.fueleconomy.gov/feg/atv-ev.shtml https://www.fueleconomy.gov/feg/atv-ev.shtml In the bottom left corner, the image says "Percentage total may exceed 100% because regenerative braking recaptures energy and reuses it." Also, it says: "Energy to wheels: 65% to 69% + 22% (recovered) = 87% to 91%" Switch to the "City" tab, and the result is "94% to 100%". It's not impossible to produce more useful work than the electricity being used - see for example heat pumps, which derive most of the heat from cooling down water (or air) which is "free", so they can generate 3-4W of heat out of 1W of electricity.

Not all the energy to power the car comes from charging. Some is generated from regenerative braking.

dawnerd 4y ago

The chart even shows this.

WalterBright 4y ago
I know. The chart is wrong. The entire conclusion of the article is based on bad math in the chart. Losses due to friction, drag, and heat are NOT magically made up for with regenerative braking.

WalterBright 4y ago

Um, all the energy recovered by regenerative braking was initially supplied by the charging input, which loses 10%. The overall efficiency simply cannot be 90%.

smt88 4y ago
> all the energy recovered by regenerative braking was initially supplied by the charging input You're forgetting about gravity. People brake when traveling downhill, which is essentially using gravity to charge the battery (similar to hydroelectric energy).
- WalterBright 4y ago
  Do you drive downhill both ways to and from school?
ZeroGravitas 4y ago
It's not fraction of total energy used, it's the ratio of the original input energy that ends up turning the wheel. Imagine you had a line of 24 people and you asked them to cross a room with traps that kill 2/3rds of them. 66% of the people make it and for the 18 people who survive, send them round an outside corridor and back to the start and ask them to cross again. This time 12 make it, repeat. No more than 24 individual people can ever make it across the room, but even if we stop after a 2nd cycle already we have 30 crossings. One way to express that is that the crossing equal 125% of the initial people input into the system. We haven't broken physics, were just arguing semantics.

mlindner 4y ago

The chart is kind of weird though. If there's 10% from charging loss, let alone from the other uses, how can you get back to 91%?

WalterBright 4y ago
It's kind of wrong. With 10% charging loss, you cannot get 91% efficiency.
- bildung 4y ago
  Obviously both charts start with a filled tank/battery and ignore fuel/energy production. It only makes sense to belabor charging loss if you take the gas equivalents like fuel used in transportation to the gas station into account, and gasoline evaporative losses, and storage tank losses and so on.
  - WalterBright 4y ago
    This assumption does not apply. Let's do the math: d = losses = 10% + 18% + 3% + 4% = 0.35 p = power to wheels r = regeneration fraction = 0.22 p = (1 - d * p) + r * p solving for p gives an efficiency of 64%. Not 87-91%.
    - the_why_of_y 4y ago
      I find the presentation confusing too, but with my limited understanding I think the total energy that's available to be used isn't 100% but 122%; the 100% is the share that is electricity. It links to this: https://www.fueleconomy.gov/feg/atv-ev.shtml https://www.fueleconomy.gov/feg/atv-ev.shtml In the bottom left corner, the image says "Percentage total may exceed 100% because regenerative braking recaptures energy and reuses it." Also, it says: "Energy to wheels: 65% to 69% + 22% (recovered) = 87% to 91%" Switch to the "City" tab, and the result is "94% to 100%". It's not impossible to produce more useful work than the electricity being used - see for example heat pumps, which derive most of the heat from cooling down water (or air) which is "free", so they can generate 3-4W of heat out of 1W of electricity.