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Why isn't regen symmetrical?

Discussion in 'Model S: Battery & Charging' started by doctorwho, Sep 11, 2015.

  1. doctorwho

    doctorwho Member

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    It appears to me from watching the energy gauge that it takes way more energy to move the car than is returned to the battery during regen. Why is this?
     
  2. hpham007

    hpham007 Banned

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    Thermodynamics..
     
  3. doctorwho

    doctorwho Member

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    OK so some of the battery energy is converted to heat and is lost but why isn't all the kinetic energy recovered?
     
  4. Yggdrasill

    Yggdrasill Active Member

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    The driveline is something like 85% efficient. That means that if you use 100 Wh in an acceleration, 85 Wh actually goes into moving the car. And the same is true when you regen, those 85 Wh result in something like 72 Wh put back into the battery. Of course, on top of this, the car consumes energy when it is moving. Air resistance and tire resistance means that the car will slow down without a constant input of energy, so that 85 Wh you spent on accelerating is always dropping. If you move a given distance and only 50 Wh remains, you will only recover something like 42 Wh.
     
  5. scaesare

    scaesare Active Member

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    If you account for the losses (mostly referred to above), it is.

    If you match the acceleration and deceleration rates for a specific distance, I find them pretty close on the power meters...
     
  6. hiroshiy

    hiroshiy Active Member

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    A little bit related topic. The recovered power by regen is in AC. Battery is DC. How does the car convert AC to DC to charge the traction battery?
     
  7. trils0n

    trils0n 2013 P85

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    The inverter in the Drive Unit converts Battery DC -> Motor AC to drive, and Motor AC -> Battery DC to regen.
     
  8. Saghost

    Saghost Active Member

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    This is a solid answer. The only thing I'd really like to add is that the battery isn't perfect either - it is probably about 90% efficient on the round trip, so the 72 Wh you got from the inverter to feed into it turn into about 65 Wh you can use later.
     
  9. StaceyS

    StaceyS Member

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    Another issue is traction:
    When you stomp on the accelerator and feel that force push you back in your seat, that's cool.

    Would you want that when you take your foot off the pedal (or it accidentally slips off)? Also, in RWD cars, that would almost certainly break the rear end loose, or (most likely) be seriously diminished by the traction control.

    If you look at the numbers on the power meter scale, the power side goes up to +320 wh (on my p85) on accel, but I think it tops out at around +60 wh on regen. I believe this is probably software limited purely for comfort and control reasons. Theoretically, you could pull as much regenerative force from the motor as you can put in accelerative force. I don't think it would make for a very comfortable driving experience, and it would really mean your right foot needs to stay glued to the accelerator pedal.
     
  10. hipringles

    hipringles Member

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    ontop of everything already said, when you stop, aerodynamics are slowing you down. When you accelerate aerodynamics are fighting you. so there is more loss right there.
     
  11. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    There is less kinetic energy than there is power in the motor(s), so acceleration is always going to be more powerful than deceleration.
     
  12. hiroshiy

    hiroshiy Active Member

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    Perhaps my ignorance, but inverters can rectify regen AC to battery DC?
     
  13. trils0n

    trils0n 2013 P85

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    Inverters convert DC->AC, Rectifiers convert AC->DC. Both are present in the drive unit. (Tesla, and most owners, usually just call the part of the drive unit that does those processes the "inverter" even though I guess it is technically an inverter and recifier)
     
  14. kennybobby

    kennybobby Member

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    Electrical resistance

    A primary reason why it is not symmetrical is due to the electrical resistance of the conduction bars in the rotor.

    The current flowing thru the wires in the stator of the induction motor creates magnetic fields in the iron laminations that are focused into the rotor thru the pole pieces.

    This magnetic circuit induces electrical currents to circulate in the copper buss bars of the squirrel-cage rotor, and these induced currents create their own induced magnetic fields which are aligned to oppose the originally applied fields.

    When you are accelerating, the inverter switches the 3 phase currents into the stator such that the torque generated by the interaction of the applied and induced magnetic fields causes the motor shaft to propel the car.

    When you are coasting in regen, the source of the magnetic circuit changes from the stator to the rotor, and the energy to spin the rotor comes from the car as it back drives the gearbox thru the tires and wheels. The inverter now switches the electrical path of the stator windings such that the previously induced current and magnetic fields of the rotor bars will now cause a voltage to be generated in the windings that is fed back to the battery.

    The induced magnetic fields will collapse over time as the circulating induced currents decay due to the electrical resistance of the copper in the rotor buss bars. If the buss bars were made of silver then more regen energy would be available to harvest.
     
  15. Duma

    Duma Member

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    True, there are losses in the battery, inverter, and drive train that reduce the battery energy that ends up as kinetic energy, and further reduce the kinetic energy that makes it back into the battery. However, the reason the energy recovery is so hard to measure is that the kinetic energy is relatively small compared to the energy stored in the battery or the energy lost to rolling resistance and air resistance when traveling even 1 mile. For a model S carrying 400 pounds of people and stuff, the kinetic energy at 60 mph is only about 220 Wh. Compare this to the approximately 280 Wh used to travel 1 mi at 60 mph or the capacity of the battery. We only see battery measurements in increments of 1%, which is 790 Wh of (available) battery capacity, or rated range, which is 290 Wh/(rated mi), so the total kinetic energy is barely detectable from the battery information displayed (the kinetic energy is less than 1 rated mile). Trying to observe how much of the kinetic energy is recovered is basically impossible due to this lack of precision.

    Any observable asymmetry is likely due to the rolling resistance and air resistance. This is energy expended during acceleration in addition to the kinetic energy gained. During deceleration, some of the kinetic energy goes into overcoming the rolling resistance and air resistance and so is not available to be returned to the battery, especially if the acceleration/deceleration is gentle to minimize electrical losses associated with higher currents.
     
  16. hiroshiy

    hiroshiy Active Member

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    Thanks for the answer. So, for regen, Tesla rectifier in the drive unit can take 60kW, while on-board rectifiers in Tesla chargers can rectify only up to 10+10=20kW?
     

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