Regenerative Braking Capacity

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

  1. ajdelange

    ajdelange Well-Known Member

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    If it brakes when you back off on the pedal it very probably is regenerating though the regenerated power may be dissipated in a resistor rather than used to charge the battery (i.e. thrown away). Regen is found on many electric vehicles. If you have a controller sophisticated enough to let you control an AC motor as easily as a DC one (and, of course, we do) there is no reason not to attain the benefits of single pedal control, improved mechanical brake longevity and energy efficiency. Some may not care so much about the third (energy recovery) but that is probably the most important to the designers of these cars. My X 100D wouldn't get 300 miles on a charge if it didn't have regen. Range anxiety is still the major factor that frightens people away from BEVs. It is absolutely essential that the manufacturer squeeze every mile of range he possibly can out of his design in order for him to be able to sell his car. In my X, for example, the door handles are flush, the car is shaped like a bullet, the cell and GPS antennas are flat and it has regen.

    It's pretty clear to most what benefits regen grants but I agree that there should be a way for the unsophisticated driver to turn it off if he wants to. I suppose these folks should be able to set the sound system to mono and the displays to black and white too but I don't think any manufacturer offers that.
     
  2. jimcgov3

    jimcgov3 Well-Known Member

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    The way I look at regen braking, isn't so much for the ability to get that extra mile, it is for the longevity of my braking system. If it saves me money in the long run. Sold American.
     
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  3. Hmp10

    Hmp10 Well-Known Member

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    My view exactly. When I switch from driving my Tesla to driving my Honda, the most annoying thing I find is having to use the brake pedal so much. I get especially annoyed at having to keep my foot on the brake at a stop light. Using a brake pedal frequently in heavy traffic was something I never thought twice about . . . until I quit having to do it.

    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. That is one of the reasons that EV's become more energy efficient in city driving than on the open road and are so well suited for things such as taxi service. (Many European airports have fleets of Tesla taxis.)

    Some years ago a Tesla was driven up and down Mount Washington in New Hampshire. The car added 14 miles of range from regenerative braking going down the mountain, and that was before Tesla was using permanent magnet switched reluctance motors, which capture more regenerative energy.
     
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  4. ajdelange

    ajdelange Well-Known Member

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    #24 ajdelange, Aug 6, 2019
    Last edited: Aug 6, 2019
    I think you are underestimating the amount of energy which can be recovered. When battery energy is converted to traction there is some loss in the inverters. Then there are i^2R losses in the squirrel cage, and hysteresis and eddy losses in the rotor laminations so the efficiency of an induction motor is around 90%, Then not all the motor's power goes into increasing kinetic, potential energy or rotational energy; some goes into friction, drag etc. A typical Sankey diagram shows perhaps 70% going into inertia and gravity. When regenerative braking is applied the energy delivered from the wheels is similarly subject to losses in conversion back to electric energy. If we assume 70% for that then we'd have round trip efficiency of 49% i.e. half the vehicle's energy being recovered. On a down hill run of 10 miles the other day with a change in elevation of 1720 feet I saw the battery gauge go up two percent which amounts to about 2 kWh in my car (100 kWh battery) equivalent to about 7 miles added range. 1720 feet is a potential energy change of 3.6 kWh for my car.

    Also I think you are overestimating the difference between PMSRM and IM performance. The SRMs eliminate squirrel cage (they don't have one) i^2r losses and also the rotor hysteresis and eddy losses as the magnetic field in the rotor never reverses. But starting from the low 90's in efficiency you haven't got much head room so SWRMs are only a few percent more efficient than IM's. In the battle to stretch range that can be significant. Adding 4% more miles to a 295 mile range car (Tesla X) boosts its range to over 300 miles and that 300 mile boundary is a significant one.

    Not to say that I don't really like the one pedal driving aspect of regen. I very much do. It just makes driving easier in town and on the highway. "Dynamic braking" was originally (1890) used to save on mechanical wear. It is only with modern switching and control electronics that the other advantages could be implemented as they are in a modern EV.
     
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  5. BlindPass

    BlindPass New Member

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    So true. Safer, too, with more stopping as soon as you take the foot off the pedal.
     
  6. Hmp10

    Hmp10 Well-Known Member

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    Good news for those of us who are going to find ourselves behind the wheel of a 3-ton Rivian.

    Since you are apparently technically quite versed, can you explain why the 180 kWh battery pack will have the maximum power output of the motors cut from 754 to 700 hp?
     
  7. ajdelange

    ajdelange Well-Known Member

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    No, I can't. I read this as implying that the trucks with 180 kW battery packs have smaller motors (or the same motors derated) than trucks with the smaller packs. So first question is whether I am reading that right. If so then I don't really have an explanation. The larger battery is, of course, heavier and is installed with the intent of getting more range. Adding more battery means adding more weight to the vehicle and as we have been discussing one cannot recover all the kinetic energy (or potential) energy put into the mass of the car as it accelerates and decelerates and goes up an down hills. Thus to double the range of an EV one must more than double the battery size or increase the distance obtained from each watt hour. Limiting the maximum draw obtainable from the battery would increase the average miles per watt hour but as he motors would only rarely deliver their maximum rated power I don't think that's a very good explanation.

    The only other thing I can think of is the battery size is doubled by stacking two single batteries on top of each other which doubles the mass but does not double the surface area. Perhaps they need to reduce the maximum heat generated by the batteries because it is harder to get heat out of the larger pack than the smaller one. Again I'm not too impressed with that explanation.
     
  8. cllc

    cllc Member

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    I think when you compare the heat losses that would be transferred to the brake discs and pads on the car instead of hysteresis in the motor you would see a better trade off to put it towards the motor and battery. Don't forget the batteries weigh close to 1,ooo pounds which is more than your standard ice cars, It takes more to stop the car with out the regen, Like towing around 700 pound trailer with no trailer brakes.
     
  9. ajdelange

    ajdelange Well-Known Member

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    Sorry, but I can't figure out what you are trying to say. We want the car to use all its energy store (battery or gasoline) to get us from one place to another. Cars are not 100% efficient so some of the stored energy will be lost however clever we are. We can do various things to minimize some of those parasitic energy drains such as slow down (minimizes drag and rolling loss), inflate tires properly (rolling loss), and use motor designs that minimize hysteresis and eddy current loss. Given that we have a car that is designed the way it is there is one thing we can do to eliminate energy loss and that is to keep our feet off the pedals to the extent possible. Whenever we touch the brake pedal we are wasting energy. Sometimes there are things we can do about that. For example if we know we must stop at the top of a hill we can take our foot off the "gas" at the point where the kinetic energy of the car is just equal to the potential energy difference between the desired stop point and the current location plus the energy that will go to overcome drag and rolling resistance. If we do this then we will decelerate up to the desired stopping point without having to touch the brake and will, thus, recover a portion of the energy we invested in getting the car rolling in the first place. This works nicely if we always stop at the top of a hill (in fact it was evidently built into the London subway system). But if we must stop at the bottom of Lombard street it doesn't work so well. We will have to hit the brake and in so doing waste the kinetic energy of the car and the potential energy as well. It will go off as heat and (in an automobile) we can't recover it. If we are rolling down the freeway at a healthy clip and some idiot pulls out in front of us we haven't much choice but to hit the brake and waste some of the energy we invested in getting the car up to cruising speed. I used, when gas was over $3 a gallon, instruct my wife to keep her feet off the pedals and apparently her father had also told her that when teaching her to drive. One thing is clear. Any time you touch the brake, money flies out the window. A hypermiler knows this and operates his car with minimum application of the brakes and, thus, presumably, saves money. But even he will have to apply them sometimes (when he must stop at the bottom of a hill, when he gets cut off on the freeway...). What regenerative braking does is allow us to slow down when we have to without using the brakes. The energy that would have gone to heat the brake drums is converted to electricity (and in the process some is lost as heat to hysteresis in an IM configuration but not with a PMSRM and some to warming FETs) and put back into the battery. When we roll down Lombard street we retain a substantial proportion of the kinnetic energy of the car and the potential energy of that hill as well.

    I don't see any trade-offs here. If you are suggesting reducing hysteresis instead of using regenerative braking I think that would be a bad trade as the obvious thing to do is have the benefits of both and that is the approach Tesla has taken with the adoption of the SRM. losses is, of course, a good thing to do and Tesla, and others I believe, have gone that route with the SRM approach



    Again I'm afraid I can't decode this. The battery in my car weighs more that 1000 pounds but all my ice cars weigh more than that. And it takes just as much force to stop a car with regen as without. The difference is that the torque is absorbed by the motor rather than a brake disc.
     
  10. CappyJax

    CappyJax Well-Known Member

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    Ok, so 700HP is 525kW, right. A Kilowatt is a unit of power, a Kilowatt Hour is a unit of energy. 180kWh doesn't mean you can only get 180 kW out of the battery, it means you can get 180kW for one hour, or 360kW for a half hour, or 720 kW for 15 minutes. So, because the motors can suck down 525kW (ignoring transmission loses) a 180kWh battery can deliver that for right about 20 minutes. 525kW/180kWh = 2.9 Then divide 60 minutes by 2.9 and you get 20.7 minutes. So, if you floored the Rivian, the battery would be dead in 20.7 minutes. Well, not really because as the battery heats up, the BMS will reduce the amount of power you can extract from the battery and you would go more than 20 minutes.

    This is related to the C rating of the battery. A 1C continuous battery can only safely deliver its power in 1 hour. A .5C continious battery can deliver it safely in 2 hours. 3C continuous and you get 20 minutes. However, it is not a straight line relationship especially for large battery packs. A battery might be rated for 3C but for a very short amount of time due to heat buildup. For example, a 180kWH battery might be able to deliver 3C or 540kW (180*3), but only for 1 minute. Or it might be able to deliver 2C or 360kW for 10 minutes, 1C for 30 minutes, 0.5C for an hour, 0.25C for two hours, etc. The individual cell might be 1C continuous, but in a pack, it might be .5C. A lot depends on the cooling system.

    Whereas kWh/kg is energy density, the C rating is its power density. A lithium ion battery cell can have an energy density of .25kWh/kg with a power density of 1C continuous, whereas a capacitor might be closer to an energy density of 0.025kWh/kg with an equivalent power density of 10,000C meaning it can discharge safely all of its energy in 1/3rd of a second.

    Depending on how hard you brake and how much city driving you do, regenerative braking can increase your range significantly. We are talking 50% more range or even more than without regen.
     
  11. 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.
     
  12. ajdelange

    ajdelange Well-Known Member

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    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 1005 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.
     
  13. 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.
     
  14. CappyJax

    CappyJax Well-Known Member

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

    cllc Member

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  16. 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.
     
  17. 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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  18. 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.
     
  19. 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.
     
  20. 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".
     

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