Regenerative Braking Capacity

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

  1. SafeWill

    SafeWill Member

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    @Hmp10 very interesting reading, I didn’t know that battery power could be used to assist the motor in decelerating the car, and finds it brilliant.
    I received some clues as well on a different forum that the Leaf uses the brakes when regeneration is not strong enough to decelerate the car to a complete stop.
    Just read yesterday that the next OTA software update on the 3 would include the single pedal driving (complete stop of the car). That would definitely resolve my deceleration problem when it gets cooler with a linear predictive deceleration. Wonder which system Tesla would privilege though (using battery power or brakes).
    I do agree that touching the brake pedal to activate the regeneration does not make sense, it is an extra unnecessary step.
     
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  2. Mr_funnypuns

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    So my venerable old Prius has regenerative braking... and I learned a few things about using them, and how to evaluate them. Regen has its biggest use in city stop-and-go driving, where your speed constantly fluctuates between 0 and some modest speed of 45 miles an hour or so. You are constantly bringing a car up to speed and slowing it down. The Prius indicated regenerative braking energy harvested with little green leaf icons on the display, alongside mpg data. Each leaf represents 50 Wh of reclaimed energy. I would seldom see more than 300 Wh harvested in any 15 minute increment (6 icons). On rare occasions I’d see eight. I learned that the energy harvested actually represented waste. IT meant that I was applying my brakes more in stop-and-go traffic than I should... and when I focused on avoiding the leafs by driving more conservatively, mileage went UP.

    Now, the real benefit of regen, as has already been noted, is that it saves the mechanical brake system. This is true. My Prius brakes generally lasted ~100,000 miles, compared to ~60K on a non-hybrid car. Now, mechanical brakes have to be exercised... I found this out one year when I had a seized caliper.. during winter driving. Made a point of stepping on the brakes firmly once or twice a drive, and never had a problem again. A good brake computer could be programmed to do this automagically and prolong brake life without user intervention.

    ANyway, I am really looking forward to more technical data... and the opportunity to begin doing math on it.
     
  3. ajdelange

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    The main benefit of regenerative braking is that it improves the range of vehicle appreciably. Depending on conditions of course but I'd guess by as much as 20%. Now I could argue that this is of no benefit whatsoever as I charge from solar so wasted energy doesn't cost me (or the environment) anything and that most of my daily driving is less than 240 miles (the nominal range of my vehicle less the 20% I estimate I'd lose w/o regen). But to the manufacturer, of course, being able to advertise as high an EPA range as possible is very important.

    The second, or perhaps major if you want to discount energy saving, is that it makes driving easier if it is implemented properly.

    Finally, and frosting on the cake, really, is that it saves on friction brake pad wear. This last becomes the only advantage, however, if you don't care about range and energy efficiency and are one of the few who loathes regenerative braking (and there are some) for whatever reason.

    Depending, again, on where you drive and how regen is in use a lot. At every stop sign and red light and whenever traffic slows in urban situations but also in rural/freeway driving especially in traffic where speed has to be adjusted fairly frequently and even when there is no traffic if the terrain is at all hilly,
     
  4. Mr_funnypuns

    Mr_funnypuns Active Member

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    #94 Mr_funnypuns, Nov 1, 2019
    Last edited: Nov 1, 2019
    The R1T weighs in by spec at 7250 lb (3288 kg). At 45 mph (72 kph, 20 m/s), there is 0.5 * 3288 * 20^2 = 657 kJ (1J = 1 kg m^2/s^2) or about 182 Watt-hours (1 WH = 3.6 kJ) of kinetic energy in the moving truck. We approximate energy consumption of the truck at 450 WH per mile at 45 mph, Consuming 27kWh over a one hour trip. To increase efficiency by 20%, regen would need to recover 5.4 kWh over that hour. Assuming 50% efficiency in recovery, this equates to 2 * 5000 / 182 = 54 times over that hour, or about once a minute, slowing down from 45 mph to zero, then speeding up again. That’s an ugly hour of driving! I suppose its possible, but I don’t recall ever being in that kind of traffic in my 35 years of driving.

    Hilly driving will recover more. E= mgh, g=9.8, so it takes 32 kJ (8 watt hour) to lift the Rivian up 1 meter, so we need to see an altitude change of just 1,350 m ( 4000 ft) to recover 20% in an hour. 4000 feet in 1/2 hour at 45 mph corresponds to a grade of around 1% for 30 minutes. That’s something I’ve seen. And regen works nicely in this situation to improve economy.

    However, in most situations, regen adds roughly 5% to fuel economy in my experience with hybrids. I don’t imaging its too different with EVs. There has to be data somewhere though.

    I did this too fast... anyone want to check my math?
     
  5. ajdelange

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    Numbers look about right. I will just mention that I visited a wind farm in Lowell VT and drove back to Newport VT, a distance of 23 miles. Consumption was 133 Wh/mi. Normal consumption is about 320. Thus we recovered 320/133 = 2.40 or 140% of the energy we used. Is this an unusual case - well not for someone who works there and lives in Newport but it was for us. I expect you'll want to know that the elevation difference was 1592 feet. I just saw a report from a guy who came down from Mount Washington and arrived with his battery fuller than when he departed. So you can indeed get more than 20% under unusual circumstances. The 20% number I threw out was a WAG based on gut feel from driving around rural Quebec and Vermont. Maybe I should have said 15%. If you are driving in rural Kansas it isn't going to be that much.

    It is going to be much different with a hybrid. A hybrid is a very different animal (in my very limited understanding of them). I do note you are posting from a flat part of the country,
     
  6. Mr_funnypuns

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    I am indeed posting from a flat part in the country. But I’ve got experience with hybrids & hills. I’ve driven in hills (e.g. black hills. / Rushmore), and there I come down, the little hybrid battery very full indeed, and for the next couple of miles, the car leaves the ICE shut down.

    I figured 20% was a WAG... but regen has this magical standing... and so a clarification of the physics involved helps those that are interested grasp the scale of what’s going on, and helps us operate our vehicle;es better. In this case, regen isn’t a cure-all of poor speed control. Just like any other vehicle, your best bet is to keep a constant speed. Regen helps to salvage energy from your mistakes, and from unusual circumstances you correctly mentioned.

    Most of the time a Regen indicator tells you when you are wasting kinetic energy. The best place for kinetic energy is in the forwards motion of your car. Drive to keep it there for best efficiency.
     
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  7. Feathermerchant

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    As previous poster said at the end, coasting is better than regen. Why? Well there are losses in recovering kWh from the momentum of the vehicle and putting them into the battery. Then there are more losses using that energy from the battery. It goes like this:
    Tires->axle/transmission->motor->inverter->battery
    Then the reverse to get the energy back out of the battery.
    I have found using less regen by planning my stops or slow downs where possible and polite does increase efficiency.
    Right now my Model 3 requires brakes to stop below about 4 mph so the brakes do get a little work.
     
  8. EVian

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    That’s not quite true. If you’re consuming solar that could be being fed into the grid and reduce the amount of fossil fuel needing to be burned then it is costing the environment something.
     
  9. Feathermerchant

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    Now I could argue that this is of no benefit whatsoever as I charge from solar so wasted energy doesn't cost me (or the environment) anything...

    So did you get your Solar installed for free? No? Then you are wasting an investment and maintenance.
     
  10. ajdelange

    ajdelange Well-Known Member

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    QUOTE="ajdelange, post: 3594, member: 481"]Now I could argue that this is of no benefit whatsoever as I charge from solar so wasted energy doesn't cost me (or the environment) anything ....[/QUOTE]

    The point was that what regen is worth to someone depends on that person's perspective and, indeed, on the circumstances. During the summer I charge from hydro so there is no environmental impact if I waste energy (except for the relatively tiny losses of transmission). During the winter it's from solar so there is none there either. The utility does not need my solar. The utility does not want my solar. The only reason they accept it is that they are forced to by government and they make it as difficult as they possibly can for the homeowner and the situation is likely to get worse. And personally, I don't care about this aspect of it as I think anthropogenic global warming in probably the biggest scientific hoax ever foisted on mankind. My love affair with regen is solely based on the fact that I feel it's cool tech. You can argue, however, that I won't get paid for the solar energy I put into the car and waste and that is true. I drive about 9000 mi/yr at 300 wH/mi so that's 2700 kWh. If the regen recovery is 10% of that it amounts to 270 kWh for which the utility would pay me about $30. If regen amounts to 20% then $60. I guess I still don't care.

    No. As my wife will be happy to explain to you the capital is gone whether I use the system or not and to whatever extent.
     
  11. EVian

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    Whether your utility company wants your generation or not, whatever you generate and don’t consume will go to the grid for others to use. If you use it instead then that energy not going to the grid from your generation will probably come from fossil fuels.

    Whether you believe climate change is human made or not, do you believe that the climate is changing? The facts on that are incontrovertible. There are communities around the world that will not be able to exist in the way they do today if climate change continues (for example due to sea level rises etc.). We can sit around and do nothing about it, or we can try to do something that may or may not help, but more people agree that it has to be done than believe it doesn’t. Surely it’s all worth doing?
     
  12. Feathermerchant

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    ajdelang - No. As my wife will be happy to explain to you the capital is gone whether I use the system or not and to whatever extent.

    Sounds like your wife is an accountant or lawyer.
    I am speaking pragmatically. If I spend more on something than I need to, it's wasted capital. If I own something and don't use it, that's wasted capital.
    Solar is impractical for me because I like trees and have lots of them that shade my roof. Instead I have a very efficient heating and cooling system that cost a lot less anyway.
    I use about 6,000 kWh per year and at 9¢ / kWh (<$550 / year) it would be hard to justify solar or any other alternative anyway.
    Google says estimated cost of a 6kW Solar installation is $17,700 before incentives. That is a 32 year payback without incentives.
     
  13. skyote

    skyote Well-Known Member

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    I too looked at solar, but the financials didn't make sense right now, even with incentives.

    I love technology & can afford to do it, but logic/reason prevailed. I also follow PV tech, & believe that the next 5-10 years will bring worthwhile investments.
     
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  14. ajdelange

    ajdelange Well-Known Member

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    No, she's a Scott.

    And you are going to buy (or are at least seriously considering) buying a $100,000 electric truck?

    How do you mange that? I picture you huddling around a wood burning stove with kerosene lanterns! I use 39,700 a year in my winter place and 23,400 at my summer house. And they have very efficient heating and cooling systems (geo heat pumps). And they are not McMansions.

    Who cares? I'm long enough in the tooth to give more consideration to the fact that there aren't any pockets in the shroud than to worry that the solar company rounded here and there in estimating how soon the system would be making me money. Electric cars, electric trucks, solar systems, backup generators, geothermal heat pumps are all cool tech and I am going to enjoy them while I can. And I am, frankly, a little surprised at seeing pragmatism extolled in this forum. I would have thought it populated with more tech geeks.
     
  15. Mr_funnypuns

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    Technology as art is a legitimate reason to buy an EV, solar arrays, geothermal, supercomputers, or a 1985 Pontiac Fiero. Objective valuation is another valid reason for purchasing technology. Technology as a symbol and statement of sociopolitical ideology is also a valid reason. There are others.

    We don’t make the decision to purchase as a group, but as individuals, and so our valuations of a purchase are also unique, and valid so long is the individual does not indulge in wishful thinking that attempts to deny the reality of what an EV is and is not. Wishful thinking tends to result In confirmation bias that overvalues/undervalues the the decision that is made, and is a detriment to the ability of the rest of the public to learn from our thought processes.
     

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