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How can a Tesla run for so long?

  1. Nov 5, 2016 #1
    Forgive my ignorance, I really don't know a lot about this topic.

    A Tesla Model S has a power of 315 HP (234 kW) and the largest battery they come with is 100 kW*hr.

    So shouldn't it only be able to run for (100 kW*hr)/(234 kW) = 0.43 Hr?

    I'm assuming the answer to this is the car doesn't use the full 315 HP when driving around town... The problem is when I look at dyno charts for it, the power still seems to be too high for the battery even at low rpms. Any enlightenment about this would be appreciated.
     
  2. jcsd
  3. Nov 5, 2016 #2

    billy_joule

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    Power curves are given for WOT (wide open throttle). Do you drive a car? Try driving around with the throttle wide open all the time, it's impossible, except for maybe on an autobahn.
    normal driving is mostly part throttle, which produces part power and consumes part energy consumption. This applies regardless of the engine type.
     
  4. Nov 5, 2016 #3

    russ_watters

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    In order to drive at highway speed, a sedan needs only about 15-20hp.
     
  5. Nov 6, 2016 #4
    Power consumption is a function of drag, electrical/mechanical losses, and acceleration. If you remove acceleration power consumption comes from only drag plus electrical and mechanical losses. For normal operation acceleration is usually not maximal for lengthy intervals. Power consumption is always at a much lower level that maximal output. Now if you increase drag by going faster or accelerate and decelerate maximally for long periods power consumption goes way up and run time goes way down. That's always true regardless of the energy source, electricity or internal combustion.

    A big percentage of power consumption goes to drag and losses. Decreasing weight or reducing aerodynamic losses reduces drag and thereby reduces power consumption providing better run time in cruising conditions. It's a big part of efficient design in an electric car or any other motorized vehicle. If it were possible to design a car with zero drag and zero losses (which it's not) no power would be consumed in operation at a constant speed and the car could run indefinitely without refueling as long as speed remained constant.
     
  6. Nov 7, 2016 #5

    CWatters

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    Dyno charts tell you how much power it can deliver at a given rpm. Not how much power is actually needed to move the car at that rpm.

    For example when rolling down hill it might not consume any power or the power consumption might even be negative (regenerative braking).
     
  7. Nov 9, 2016 #6
    Another point is that for an electric - or hybrid a lot of the kenetic energy is recovered in regenerative braking, so the majority of the energy is used in the drag. In a pure combustion vehicle the energy is just lost. When the hybrid cars first came out it was surprising, but understandable. that they had better "around town" economy than highway. Stop and go driving they use much less energy, than at highway speed due to drag. And - drag increases with the square of the speed, so driving at 60 uses 4 x the energy as driving at 30.
     
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