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regen/charging efficiency

Discussion in 'Technical' started by JRP3, Mar 21, 2011.

  1. JRP3

    JRP3 Hyperactive Member

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    Not true. Regen never gives you back the energy you took out to go up the hill. Going up hill you are fighting friction and air resistance, on the way down you are also fighting those same things which take away some of the energy before regen even comes into play. Then there are conversion losses in the motor and controller. That's why coasting is better than regen when possible. You can see this for yourself by watching your amps going up a hill then seeing what regen amps are at the same speed going back down. I can do 200 amps going up a hill and then get only 50 amps going back down the hill at the same speed.
     
  2. stopcrazypp

    stopcrazypp Well-Known Member

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    I think he's talking about zero in terms of elevation related losses not driving losses overall (which I don't think any car can accomplish). For the 200 amp number you would have to subtract your typical consumption driving at level ground, then compare to what you get from regen going downhill.

    In other words, consumption of driving a loop with elevation changes in an EV should be roughly the same as without elevation changes.
     
  3. JRP3

    JRP3 Hyperactive Member

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    That would only work if all the extra energy expended to go uphill were recovered, and it's not. On the flat I run about 60-70 amps to hold 45mph, to go up the hill I'm thinking of at 45mph takes at least 200 amps, maybe more, while coming down that same hill at 45mph gives me 50 amps of regen. You always consume more energy going up a hill than you get back from regen. The reasons are as I described above.
     
  4. qwk

    qwk Model S P2681

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    I think what he meant to say is that Teslas regen is really efficiently implemented thus not taking such a big hit compared to Leaf, most home conversions...
     
  5. JRP3

    JRP3 Hyperactive Member

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    Tesla can't change the physics of energy conversion. I've already discussed Tesla's poor charge efficiency, regen would be even worse.
     
  6. PeterW

    PeterW Member

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    If I take 70 amps in your example: It requires an extra 130 amps to go uphill(200-70) and 120 amps less to go back down(70 not used +50 regained).

    I admit I took 70 amps not 60 and there is still a difference but(not knowing what that 10 plus amps really means) it does not seem to be a significant difference to me
     
  7. qwk

    qwk Model S P2681

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    I think you are missing the big picture. Yes the li-cobalt format requires a bit more energy for temp control, BUT it's lighter that any lithium tech out there right now.

    In the end what matters is total wall to wheel usage, which the roadster is very comparable with the rav4 ev, and the Leaf. Yes the roadster is lighter, but that is really irrelevant as it also has more that twice the range, and doesn't get penelized as much in cold climates due to the already present temp controls.

    What is going to be very interesting is Teslas Model S efficiency, and wall to wheel numbers, both in hot and cold climates as it's a heavy car.
     
  8. AndrewBissell

    AndrewBissell Member

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    #8 AndrewBissell, Mar 21, 2011
    Last edited: Mar 21, 2011
    Thanks to everyone who tried to defend what I said against JRP3's attack. I meant what I said, but am happy to try to clarify it with an example:

    If I drive a ten mile loop at a constant speed (say 50 mph) on a completely flat road I use a certain amount of energy, say 3kWh. That energy is used to accelerate the car to speed, and keep it there against air resistance and rolling resistance.

    If I drive a loop that goes downhill for 5 miles (and drops 100 metres[1]) then up for 5 miles, the air resistance is largely the same (there could be a small effect from changing air density) and the rolling resistance is the same. So my consumption will be 3kWh plus whatever it costs for the electric motor to lift the mass of the car 100 metres less whatever was regained dropping the car 100 metres. If the regen, battery and electric motor drive were perfectly efficient (they aren't) then this would be net zero and my consumption would be the same on the down-up loop as on the flat loop.

    So the only thing we need to analyse is how efficiently the car regens into the battery and drives from the battery. The air resistance and the rolling resistance are constant between the cases.

    I speculate that the Roadster's induction motor drive and large battery allow higher levels of regen and more efficient regen and drive than the LEAF. I invoke this hypothesis to somewhat account for what I observed.

    Footnote [1]: See I'm British - I freely mix imperial and metric units. I can buy 10 metres of 24" wide cloth. I can drive miles and do elevation change in meters. :)
     
  9. JRP3

    JRP3 Hyperactive Member

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    For one thing over time on each hill those differences add up, and, probably more importantly, the 200 amps uphill was an estimate, probably on the low side, it could easily be 220-250. I haven't driven the car in a few months so I can't remember exactly the number. Also some of those regen amps, especially at higher currents, will be lost in the pack as heat. You can do a simple experiment, drive a loop with hills and track energy used then drive the same distance on the flat and do the same. The route with hills will take more energy. Even easier, find a hill that you can maintain speed by just coasting down, then go up the hill and see how many amps you use to go up it. Then drive the same speed on the flat and compare.
     
  10. JRP3

    JRP3 Hyperactive Member

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    This has nothing to do with the physics of regen. However, the fact that the smaller, lighter Tesla has the same wall to wheels efficiency of the larger, heavier, 5 passenger LEAF should tell you something.
     
  11. qwk

    qwk Model S P2681

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    If Nissan takes the current Leaf and gives it a range of 245 miles like the roadster, do you really think that it will still get 3mi/kwh?
    Now they add a heater to the pack for cold climates and it gets much worse.

    I think you live in the land of denial. You only hone in on one aspect of energy efficiency, and ignore the overall usage.

    The roadster has better regen recoup than any other production EV. Like stated before, it has to do with the induction motor.
     
  12. JRP3

    JRP3 Hyperactive Member

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    What attack?
    The amount of regen the car is capable of is only relevant if you are hitting the max allowed, which is really a factor of what the battery pack can take. In most driving you probably are not anywhere near what the pack can take in either vehicle so it's not an issue. If regen can slow the car more than you want it to, i.e below the speed limit, then it's already more than you need. As for motor efficiency differences they are probably very close, as Tesla pointed out in their discussion paper on the two technologies.
     
  13. AndrewBissell

    AndrewBissell Member

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    @JRP, 2 posts up: Yes it tells us that the LEAF has better aerodynamics. Which is well known.

    On the other hand, in my limited experience, the LEAF comes of worse on a cold day when I think it's wall to wheels falls well below the Tesla Roadster. Probably due to battery heating (or lack of). I.e. The Roadster spends some energy and keeps a well temperature conditioned battery that can source and sink more energy; the LEAF saves on heating/cooling but pays a price with reduced capacity.

    Maybe. Or maybe all I saw was really bad UI (detail-deficit disorder) - lack of a good SOC meter - and a poor range estimator.
     
  14. AndrewBissell

    AndrewBissell Member

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    @JRP3: when people write in that tone, it sounds like an attack.
     
  15. JRP3

    JRP3 Hyperactive Member

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    I'm sticking to what is relevant in the discussion. You are including extra variables that have no bearing. When discussing the poor charge efficiency of the Roadster, bringing up the kwh/mi used by it or the LEAF is meaningless. I'm not saying which vehicle is better or more efficient over all I'm simply talking about the charge efficiency of the Roadster. You are the one who seems to be in denial any time a possible short coming of the Roadster is brought up.
    In what way? The peak efficiency of a BLDC motor is actually higher than induction has better average efficiency, as per Tesla's own paper on the topic. Overall efficiency is very close. Unless you can come up with efficiency numbers comparing BLDC and induction motors you aren't supporting your argument.
     
  16. JRP3

    JRP3 Hyperactive Member

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    It may have a better cd, coefficient of drag, but it has a larger surface area, which increases cdA, coefficient of drag times area. I don't know the cdA of the two vehicles so I can't compare. If you have numbers please post them.
    The LEAF will suffer more in the cold than the Roadster, but I'm not sure how much of an impact on wall to wheels it will have. Part of the reason the colder pack gives less range is because it took less charge, so less energy from the wall.
     
  17. JRP3

    JRP3 Hyperactive Member

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    You said elevations changes should have little effect
    I said:
    What in that was offensive?
     
  18. qwk

    qwk Model S P2681

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    It's only meaningless, because it doesn't support your argument.

    If Tesla were to use the same battery technology in the roadster as in the Leaf, give it the same range, and cold weather efficiency, do you really think it could still get the same wall to wheel numbers?

    Yes, the wall to wheel numbers are the ONLY numbers that matter, as that is the energy needed, used, paid for....


    As far as the regen goes, the Leaf gets pathetic regen unless in Eco mode.

    There are many documented mountain roadster trips with excellent efficiency. The same could not be said for the Leaf. Yes weight is a big part of it, but that goes back to the heavy battery discussion.
     
  19. JRP3

    JRP3 Hyperactive Member

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    No it's not relevant to the discussion about either charging efficiency or regen efficiency.
    Not sure what you're saying here, but with the higher charge efficiency of the LEAF pack it might get better wall to wheel numbers. Still irrelevant to the charge efficiency discussion.
    Not when discussing part of the technology. I'm not sure why you can't handle the fact that Tesla has poor charging efficiency. It's a fact. Doesn't mean the car is bad or that Tesla is evil. It's one aspect of their technology I'd like to see improve. I like Tesla, I own stock, I want to see them do better.
    If it slows the vehicle down as needed then it's enough. If you need more then put it in Eco mode. Still has nothing to do with A. Tesla charge efficiency or B. The idea that regen in any vehicle eliminates the energy hit from going uphill.
    Do you have numbers for LEAF mountain trips?
     
  20. qwk

    qwk Model S P2681

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    I think you are arguing for the sake of arguing.

    Yes, WTW is ALL that matters. If a car has 95% charging efficiency, yet gets 1 mile per kwh, this would be GREAT with your reasoning. It's like me telling everyone how my car uses half the gasoline while idling compared to other models, but only gets 5mpg while driving. People are going to laugh and call me an idiot.

    The two main things that make an EV more efficient are coefficient of friction and, weight. If you really think that a much heavier roadster is going to get better mi/kwh, then you have no clue.

    Technology is making the overall car better, not just one irrelevant thing like charging efficiency. Charging efficiency is probably worse because of the li-co chemistry, but because it's much lighter, you end up with a more efficient EV. Again, in the end, that's all that matters.

    I really don't know how to simplify it more for you.
     

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