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most efficient regen speed measured

Discussion in 'Model S: Driving Dynamics' started by David99, Nov 1, 2018.

  1. David99

    David99 Active Member

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    A discussion in the Model 3 section was around the subject whether short and strong regen was more efficient than long and lower power regen. In theory the motor/inverter would be more efficient at higher loads thus stronger, shorter regen would gain a little more energy back.

    I wanted to dig a little deeper and used 'Scan my Tesla' to read the efficiency at different levels of regen. It shows the efficiency directly as a percentage so it was straight forward to measure. I don't have a Model 3 so I did these tests with my Model S RWD 85.

    I measured the efficiency at 45 kW of regen power and at 15 kW of regen power. At 45 kW the efficiency is around 87-88%. At 15 kW the efficiency is 83-84%. That alone isn't a big difference. I'd say, small enough to not really care one way or another. But there is more. While I was doing tests, I also noticed that when the car is in D(drive) the car uses aprox 400 Watt more power than when in P or N. It seems the drive unit consumes about 400 Watt just being turn on regardless of motion, acceleration or regen. At 15 kW this is 2.7% loss. At 45 kW that's 0.9% loss. In other words, because there is a constant load, the efficiency looks worse at lower power than higher power. The base load makes up a larger portion. It means the true efficiency difference is aprox 2% less.

    Strong and short regen might have a very slight efficiency advantage on the motor/inverter. OTOH regenerating earlier with less power means your average speed is a little lower (you are starting to slow down earlier). That reduces air drag meaning you have a little more kinetic energy available for regen. All things considered both method should be almost exactly even.

    Bottom line: For efficiency it doesn't matter if you use stronger and shorter regen vs longer and weaker regen. Do whatever you feel like!

    BTW, if you wondered by regen power changes with speed:
    The maximum regen power I was able to measure was 72 kW when going 110 mph and letting go of the accelerator. From there on regen gradually decreases with speed down to zero at about 3-4 mph. Tesla adjust the regen power with speed automatically. This makes sense as higher speeds 'contain' more energy. 50 kW of regen feels light at 80 mph, but it is very strong at 40 mph. So Tesla adjust the regen power down with speed to keep the feel of deceleration consistent.
     
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  2. Jedi2155

    Jedi2155 Model 3 has Arrived.

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    #2 Jedi2155, Nov 1, 2018
    Last edited: Nov 1, 2018
    That's a some very interesting findings in regards to efficiency. I'm wondering how they calculate the "efficiency" number.
    Academically its as follows:

    Recovered Regen Energy and
    Total Regen Energy =
    Kinetic Energy at the start of the regen - Kinetic Energy at the end of the regen event - drag energy consumed during the whole event
    Regen Efficiency = Recovered Regen Energy / Total Regen Energy.

    I'm surprised that Tesla has a method of calculating total regen energy and that its reported on the CANbus.

    Regarding the regen efficiency with fast decel vs. slow decel, I found the answer in an old blog post I'd read a bunch a long time ago. AC Induction motors have a wider efficiency band than PMAC synchronous motors found in most other EVs. While what i said in the other thread may be true for other EVs with PMAC SM, its not true for induction in the Model S. More confusing now since the Model 3 has both....and which one is used when.....

    Induction Versus DC Brushless Motors

     
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  3. mongo

    mongo Well-Known Member

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    Cool data, thank you for sharing!

    It may not be that Tesla is adjusting the regen power, but that the same regen force (deceleration) will cause power to scale with the speed. (they could also be adjusting the g load with reduce jerk at the end).

    Work(energy) is force over a distance, power is work over time. The same amount of regen force at a higher speed is inherently more work, and thus more energy and power. Twice the speed means twice the distance so twice the work/energy preformed in the same time so twice the power.
     
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  4. Jedi2155

    Jedi2155 Model 3 has Arrived.

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    Tesla limits regen based on decel rate (gs), not power. They're definitely adjust the g load at the start of the decel since V9 as V9 feels super smooth relative to V8's regen.
     
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  5. mongo

    mongo Well-Known Member

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    There may be a max regen level based on the drive unit capability along with the target deceleration rate.
     
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  6. David99

    David99 Active Member

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    You can ask the guy who wrote the 'Scan my Tesla' app exactly how he does it. It's coming directly from the inverter I believe.

    The car also keeps track of energy going out of the battery and going into the battery. Makes sense as that's necessary to keep track of the state of charge at any time. It is able to tell exactly how much energy came back from regen. It even has a counter for the total lifetime of the battery. When driving through the city I usually get about 30% back through regen. The more freeway driving, the less regen. On road trips with mostly freeway driving, the regen percentage is around 6%.

    I really hope soon we will have access to the Model 3 CAN bus.
     

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