Regenerative Braking Capacity

Discussion in 'Tech: Batteries, Charging, Alternative Energy' started by Aslan, Mar 8, 2019.

  1. Hmp10

    Hmp10 Well-Known Member

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    Which is why in an earlier post I wrote, "it's true that regenerative braking doesn't put much energy back into the battery pack during a lot of driving, but it becomes more significant in heavy city driving."

    Most of my driving is on uncongested highways, so I get little range enhancement from regenerative braking, as I simply don't brake or slow down much relative to miles driven. I understand that if I did more city driving I would see considerably more energy recovery.
     
  2. ajdelange

    ajdelange Well-Known Member

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    #32 ajdelange, Aug 7, 2019
    Last edited: Sep 22, 2019
    The C rating of a battery is actually it's capacity in coulombs divided by 3600 in order to convert the units to the more convenient ampere-hours (1 ampere-second = 1 coulomb). When we say we are going to discharge (or charge) a battery at, for example, 0.8C that means we are going to charge it with cuirrent numerically equal to 0.8*C. For example, the battery in some Tesla's is rated 100kWh and as the battery voltage is about 385 V and stays pretty level over the discharge cycle that's equivalent to 100000/385 =259.7 ampere hours. Charging (or discharging) this battery at 0.8 C means 0.8*259*7 = 207.8 amperes. Multiplying by the 385 volts that given 0.8*100 = 80 kW. Clearly it is going to take 1/0.8 = 1.25 hours to discharge it at that rate. Given that the power meter on these cars goes up to 300 kW we assume these batteries can be discharged at up to 3C and perhaps a bit beyond this. That's 300kW equal to 407.8 HP. Given that the new (V3) chargers will charge at (IIRC) 350 kW that implies that they can be charged at 3.5C (at least at low SoC).

    Another interesting question here is as to what HP means when quoted by an EV manufacturer. Is it derived from a modified SAE J1349 test? Is it with the inverter duty cycle 100% at the speed which produces the most power or is it at some limit set by the manufacturer in order to optimize performance and reliability under some set of weights they have chosen. Too little info here. So not only do I not know what it really means when they say that motor power is reduced from 754 to 700 HP but I don't really have any good understanding as to why they would do that. I can see why they might want to lower the available peak draw from 3.5C to 3.2C if there is a problem, as I suggested earlier, with getting heat out of the larger pack but most of the time the vehicle will, judging by the Tesla story, be operated at about 0.1C - 0.2C.
     
  3. CappyJax

    CappyJax Well-Known Member

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    A larger pack will produce less heat at the same power draw than a smaller pack. That is because each cell is delivering less of the same share to the draw.

    The V3 chargers are currently at only 250kW. To get higher charging rates, the battery packs will likely need to be in the 800V range to get the amperage down.
     
  4. CappyJax

    CappyJax Well-Known Member

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    Do you have any hills where you live?
     
  5. cllc

    cllc Member

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  6. cllc

    cllc Member

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    #36 cllc, Aug 7, 2019
    Last edited: Aug 7, 2019
    My point is that I would much rather utilize the regenerative braking than to not have it than to put all that heat back into the brake pads and rotors.An average ICE vehicles motor and fuel gas tank doesn't weigh near as much as just the battery on your EV car, The ICE car gets lighter as you drive ,stopping your ice car is easier and takes less energy than stopping your EV without the Regen braking over time and that adds up.I don't think we are disagree here.
     
  7. jimcgov3

    jimcgov3 Well-Known Member

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    I appreciate the Greek that is being thrown around.....but what has happened to this thread...??? My God.
     
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  8. ajdelange

    ajdelange Well-Known Member

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    #38 ajdelange, Aug 8, 2019
    Last edited: Aug 8, 2019
    It's not because each of the cells delivers a smaller share (there are twice as many cells in a battery twice the size of a reference battery) but because the current per cell is half when the same power is drawn. Assuming all or most of the heat loss is i^2R loss then each cell gives off 1/4 the heat and as there are twice as many of them thermal losses are half what they would be. Example: you have a single cell fully charged to 2.4 V and discharge it at a 1 ampere rate (2.4 W). It has an internal resistance of 10 mΩ. The heat dissipated is 10 mW. To increase the duration your device can operate you add a second cell in parallel but continue to take 2.4W. Half an ampere is drawn from each cell so the power dissipated in each is 0.5*0.5*10 = 2.5 mW. As there are two cells the total dissipation is twice this: 5 mW.

    Yes, that's correct.


    To be clear on this: You do want to get the voltage from the stall up in order to reduce the current in the cable and connector and indeed the internals of the stall itself but you do not want to reduce amperage to the battery cells as the length of time to charge is inversely proportional to that. Manufacturers are exploring higher voltage batteries. I think Rivian may be one of them.
     
  9. ajdelange

    ajdelange Well-Known Member

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    I think you are saying that having regen is better than not having it. That's certainly true. And I think you are saying that it is especially valuable in a heavier vehicle as a heavy vehicle puts more demand on its braking system. Also true.
     
  10. ajdelange

    ajdelange Well-Known Member

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    The OP asked for experience or expertise WRT regenerative braking. I can't really claim the latter but I experience regen every time I drive the car. Note that this thread is titled "Tech: Batteries..." Thus readers should expect to encounter some technical terms. It wouldn't help the OP if someone posted "Ah, regenerative braking. It's FM".
     
  11. Hmp10

    Hmp10 Well-Known Member

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    Only the roadway flyovers. I'm in south Florida, and the land is flat as a pancake within a 100-mile radius of my house.
     
  12. ajdelange

    ajdelange Well-Known Member

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    If you live in a flat place there is no need for regenerative braking or, indeed, any other kind. The car will slow down from drag and rolling resistance if no power is supplied to the wheels. All you have to do is take your foot off the accelerator at a point at the appropriate distance from where you wish to stop and the car will smoothly decelerate to 0 speed at that spot. Unless you have a tail wind...
     
  13. Hmp10

    Hmp10 Well-Known Member

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    Jeez, I'd hate to get caught driving behind that guy.;)
     
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  14. ajdelange

    ajdelange Well-Known Member

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    When gas was over $3 lots of drivers engaged in "hypermileing" the fundamental part of which is keeping one's feet off the pedals. This implies gradual coasting up to stop signs and lights as well as minimal acceleration when departing same. This did indeed lead to a fair amount of grumbling from other drivers.
     
  15. jimcgov3

    jimcgov3 Well-Known Member

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    I see that humorous posts are also discouraged. If you would be so kind as to point me to a YouTube video or a website that explains the math side of this, I would greatly appreciate it. I will continue with the witty banter though...No point in being serious all the time.
     
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