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Does SOC affect regenerative braking?

Discussion in 'Model X' started by lakemirror, Oct 14, 2019.

  1. lakemirror

    lakemirror Member

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    I normally don't charge my battery beyond 90% but took it up to 100 for a trip last weekend. It felt like regenerative braking was not as effective as it normally is until I had burned some juice out of the battery. Since a fully charged battery recharges differently than a partially discharged one, is regenerative braking limited due to lessened chargability? Sorry for the non-technical terminology.

    Is that right? Or was it my imagination?
     
  2. MaryAnning3

    MaryAnning3 Supporting Member

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    That is right. It was not your imagination.
     
  3. Puddles

    Puddles Member

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    A full battery disables regen. There was a little notice about it on your screen which flashed by as you were busy trying to juggle your coffee, seat belt, and cell phone, as you got settled in.
     
  4. lakemirror

    lakemirror Member

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    Nah, it was a Turtle Pecan Cluster blizzard from Dairy Queen

    Thanks, glad I wasn't just imagining it.
     
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  5. CameronB

    CameronB Member

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    Yeah, it's definitely a shock the first time (or two) it happens and you let go of the accelerator and your car doesn't "brake" and behaves like a gas car :). But after a few rounds of that you will inherently come to understand that you won't have regen until the battery can deplete a little bit.

    It all makes sense if you think about it: regen is supposed to put that otherwise wasted energy of your momentum (let's call it engine braking) back into the battery. But when the battery is full - there's no "room" to store more energy. So no regen, Maybe oversimplified, but I think that is the gist of it.
     
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  6. DustWindDude

    DustWindDude Member

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    If you have the "Energy" widget on your dash display, you will usually see a dotted/dashed yellow (orange?) arc where it normally shows the regen amount when regen is disabled (or diminished). I think it also alerts you with a yellow/orange alert icon in the same general area.
     
  7. ewoodrick

    ewoodrick Well-Known Member

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    Regen is when the car uses the power from the motor to charge the battery. When the battery is full, there's no where for the power to go and so it is just wasted. Since it is wasted, there is no regen.
     
  8. KatyF

    KatyF Member

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    I charge to %100 for a sea level trip. I charged to 60% in truckee and had 60% when we stopped in Roseville. So yeah, It was cool to drop 7,000 ft. And average 0 kWh/mile. Keeping regen in mind is new, but pretty nifty
     
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  9. ajdelange

    ajdelange Banned

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    No question that regen is limited when the battery is full (over 90%) and it is also limited when the battery is cold. May be a while before OP sees this in Florida but as cool as 55 °F or so will trigger it. The reason is not quite as stated in the quotation however. It is because when the battery is cold or charged over 90% that charging it at other than a very slow rate will damage it. That's why you will see the rate taper when you charge the car at a SC as the battery fills up and it is why regenerative braking is limited when the battery is full.

    When you depress or release the accelerator the car's controllers send toque commands to the motors. The current drawn from the battery or the current sent to the battery is proportional to torque and speed. When decellerating the controller limits the torque command to that corresponding to the amount of current the battery can safely accept at its present temperature and charge level. When there is a limit it is depicted on the power meter as a dashed line on the negative power part of the arc. I think there should be a chime too as I've had the hell scared out of me a couple of times when I took my foot off and there was no decelleration.
     
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  10. Daisame

    Daisame Member

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    Ummm... Dairy Queen.
     
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  11. Need

    Need Active Member

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    The first time my wife charged her Model X to 100% for a trip, she gave me a call about 5 minutes after she left the house.
    "The car is not working"
    "What? Did it die on the road?"
    "No, it won't slow down before red lights and stop signs"
    "Brake does not work??"
    "No, it would slow down before without using the brake"
    So I just told her to keep driving for 20 or 30 miles and it will work again. ;)

    It is really hard to drive the car without regen braking...always took the corner too fast because I was expecting the car to slow down without hitting the brake.
     
  12. Daisame

    Daisame Member

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    It is interesting how much you get used to the regenerative breaking. When driving an ICE car, it almost feels like it is skating or sliding when you take your foot off the accelerator. It was really disconcerting the first time I drove an ICE after driving the Tesla for a while.
     
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  13. ewoodrick

    ewoodrick Well-Known Member

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    #13 ewoodrick, Oct 16, 2019
    Last edited: Oct 16, 2019
    I am sure of my statement. When a motor is providing regen (i.e. a motor producing power) for there to be power consumed, the power has to be consumed somewhere. With a generator, if there is no load, then the fuel consumption is very low, when there is a higher load, the consumption goes up. This is just the reverse.
    In other situations where motorized braking is used, the power is often fed into a load bank, basically a resistive source that will convert the power into heat. In an EV, the battery is the load bank. But when a battery is near full, there is no where for the power to go. If there is no where for the power to go, then the motor can't translate it back into braking.

    Also said, an electric motor simply converts electrical power into torque power and torque power into electrical power. One side gives to the other. When the battery is full, it can't accept it. Just like if the battery is empty, it can't give any.
    When a battery is cold, it can't give or accept it as high as it can when warm. Hence both the battery is cold and the regen warnings.

    When you depress the accelerator, it isn't send torque commands to the motor, it's just increasing the battery voltage to provide more power.

    This is really easy to test with a simple toy motor and a small incandescent light bulb. Turn motor, bulb lights. Turn motor without a light attached, it's a lot easier to turn.
    Hook battery up to motor, motor turns, hook up two batteries in serial (to increase voltage) motor turns faster.
     
  14. ajdelange

    ajdelange Banned

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    #14 ajdelange, Oct 16, 2019
    Last edited: Oct 16, 2019
    Let's have a look then.


    I think we can accept that for power to be consumed it must be consumed.


    A generator transforms mechanical energy to electrical. The mechanical energy comes from what people in the industry call the "Prime Mover". In your Generac the prime mover is an ICE engine. In your car the prime mover is the roadway. As soon as you take your foot off the accelerator the road starts to supply mechanical power to the wheels where it gets translated to rotational mechanical energy which is transferred through the gearing to the rotor of the "motor". Exactly the same as with your Generac except for the source of the prime power.



    A load bank can only transform electrical energy into heat. But a battery can also transform a portion of it to chemical energy. That's a big difference.


    There certainly is! It goes into the battery. If all the ions have moved to the anode, however, it can't convert any more of that energy into chemical energy and so that energy goes into heat and, I have been given to understand, with Li+ into mechanical energy. Both of those can damage the battery. Thus you don't want to send more energy into the battery once it is charged. If you have the prime mover (the road) spinning the machine's rotor and if the controller is spinning the field faster than the rotor then power will be generated and sent to the battery. As you don't want to do that you command the controller to NOT supply a field rotating faster than the rotor.


    If you have a generator connected to a load and open a switch the power demanded of the prime mover will go down. If you disconnect the battery from the machine that will happen too or if you turn the machine off or down by controlling the field you will reduce the power generated, the power absorbed by the battery and the power required of the prime mover. As the prime mover's power is the product of the torque it delivers and the angular velocity the torque it delivers (which is the torque absorbed by the car) goes down and there is less braking effect. That is what is done in these cars.

    There is no such thing as "torque power". Power is the product of torque and angular velocity.


    Perhaps this misunderstanding is at the heart of the problem. When a battery is really and truly empty, i.e. discharged below the damage point, it can supply no more energy. But you can overcharge a battery quite easily. You may well have done it. A charged battery will continue to absorb energy as long as you supply current to it. You may have forgotten to remove a drill battery, for example, from its charger and come back days later to find the battery warm to the touch. Now it won't have been damaged because the charger, assuming it is properly designed, will limit the current such that it never charges beyond the damage point. The controller in the car does exactly the same thing! It limits the prime power drawn from the road to a level that, when it is converted to electrical energy, protects the battery from damage while still putting as much of it into the battery as it can safely accept.



    The controller takes temperature and charge level into account. Just like the one in your toothbrush.


    This is a very naive view of how an EV motor (or any modern "DC" motor) is controlled. The power delivered to an electric motor depends, ultimately, on the magnitude of the currents to the stator windings and the slip (the relative rotational speed of the rotor and the field). Rate of the rotation is set by 6 switches (two per phase) that send current from the battery, into one phase, and returns to the battery through the other two phases. 3 switches are on at any given time and the triplets (three are 3 sets) are cycled through at the desired field rotation rate divided by the number of poles. In addition, the switches are turned on and off at a much higher rate (PWM) to control the magnitude of the field currents. The battery voltage stays the battery voltage - it is not boosted or bucked. Through some hairy math the controller accepts information on speed, rotor position and desired torque and translates that into field, torque, frequency and PWM commands. There are loops within loops. If you think I am trying to convince you that I understand all this get that idea out of your head. It is very complicated. I summarize this by saying the controller sends torque commands to the machine. And indeed it does but that represents a gross simplification.

    It just occurred to me that perhaps you think the motors in these cars are traditional DC motors. They aren't! The S and X have three phase induction motors (invented by Nikola Tesla which is how the company got its name) and the 3 and Raven now have one switched reluctance permanent magnet motor (synchronous).
     

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