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Mountains and steep downhills

Discussion in 'Model S: Battery & Charging' started by DITB, Nov 13, 2012.

  1. DITB

    DITB Charged.hk co-founder

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    As far as I understand it, any touching of the brake pedal on the Tesla S does NOT increase the regenerative charging - am I right? Or is the first bit of pedal travel incurring extra regen?

    I live in an area with mountains around and quite a lot of steep hills - and having to use the conventional brakes to keep the speed down, going downhill, would be a bummer.

    Wouldn't it be awesome to have the regeneration being dynamic, in the sense that downhill (and to some extent, tailwind) would increase the regen? It should be quite simple: Whenever you let go of the accelerator, any speed increase should be countered by increase of regen - to the extent of maximum regeneration on a steep downhill.

    So those of you who are so lucky to already own a Tesla S - please go to the steepest downhill you know of, and see how slow a speed the car can maintain without touching the brakes ...

    This is not only about regenerating and efficiency, but just as much a safety issue - where combustion engined cars have loads of unwanted friction in the engine, it can unload the brakes in mountains - but how about the Tesla? If the car can recharge completely in 4.5 hours, I cannot see why it should not be able to absorb a good mountain downhill drive ...
     
  2. Todd Burch

    Todd Burch Electron Pilot

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    The brake pedal activates friction brakes only. Regen is entirely connected to the accelerator. Most seem to like this setup.

    You can easily exceed the maximum allowable regen on a downhill slope (even one that's not steep), so increasing regen to maintain speed would be impossible. (Well, not impossible but inadvisable). The amount of regen is limited by the rate at which the battery can be recharged without damage. This limits the amount of regen braking force that can be achieved.
     
  3. SCW-Greg

    SCW-Greg Active Member

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    DITB brings up a good point, and no doubt this has been discussed elsewhere, does applying the brake stop regen action, or merely continue the regen effort, plus added stoping power of the brakes too?

    I love how the regen works, takes me back to driving with a stick, and very little braking is need in level driving, IMO.

    However, we live on a small mountain 500+ foot drop/gain coming and going from our house each day. All roads off are very steep, and I'm sure would need brake action beyond max regen setting.
     
  4. metafor

    metafor Member

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    It's interesting that there's such a difference between recharge rate and discharge rate. The Model S is obviously able to discharge at pretty high currents to drive the kind of power the electric motor puts out. Why is it so limited with recharge? Hell, the rate that the supercharger charges the battery is far higher than the rate of discharge.

    It seems an awful waste of energy to have all that downhill force be turned into heat. A (more costly) but elegant solution would be to have a small (relative to the battery) capacitor that could hold the charge while the battery charges. It still wouldn't work for really long extended periods of downhill travel but for up and down roads of rolling hills, it can be quite efficient.
     
  5. stevezzzz

    stevezzzz R;SigS;P85D;SigX

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    #5 stevezzzz, Nov 13, 2012
    Last edited: Nov 13, 2012
    I posted yesterday in another thread about my experiences using regen braking on a long downhill run. Bottom line, there's plenty of regen braking to hold the speed on even quite steep downhill grades: up to 60kW worth of braking, depending on your speed.

    Hack-charging on a generator - Page 8

    In answer to another question, friction braking is in addition to regen braking: stepping on the brakes does not change the rate of regen, which diminishes as you slow down and cuts out completely below about 10 mph.
     
  6. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    I agree with Steve. Even on my 15-18% exit from my house in Pagosa, the regen is more than enough to control speed. On every reasonable hill that I have encountered with less than 98% SOC, the regen is sufficient to control speed. This includes coming off of some significant mountain passes in Colorado.

    Also, friction braking is in addition to the regen. Driving the Model S is like driving a manual transmission car always in 2nd gear. Driving the Roadster is like driving a manual transmission car always in 1st gear.
     
  7. SCW-Greg

    SCW-Greg Active Member

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    Excellent feedback Stevezzz and Cottonwood, thx!
     
  8. strider

    strider Active Member

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    It's a factor of Li-Ion that it can discharge faster than it can charge. The car is also limited by traction of the rear wheels. Even if the batteries could take more regen the tires may not be able to safelt transfer that. Tesla has set up the system to allow the max amount of regen without damaging anything. A Tesla will regen more than downshifting any ICE so if you can find a hill that is steep enough and long enough to overheat a Tesla's brakes there will be a pile of ICE's at the bottom of the hill because ALL of them will have overheated their brakes long before the Tesla.

    I have driven a Roadster for 2 years. Tesla's method of keeping the brake pedal only activating the brakes and leaving regen on the accelerator is BY FAR the best system. No one on the planet has been able to make regen on the brake pedal work properly. There is always a jerk when the car transitions from regen to friction brakes. Tesla does not have this problem. The Tesla will continue to regen when you use the brakes. It is completely seamless. You just have to trust me that it is amazing.

    For the OP, if you want to keep a constant speed, just engage the cruise control :)
     
  9. Liz G

    Liz G P03056

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    So Strider does cruise control utilize regen to slow the car when necessary to maintain constant speed?

    Thanks, Liz
     
  10. strider

    strider Active Member

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    Yes. Though it will not use the brakes (at least not in the Roadster).
     
  11. DITB

    DITB Charged.hk co-founder

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    Thank you for your replies, I think as I understand it, I am quite happy but also looking forward to test this myself.

    I was initially thinking how to factor in headwind, car weight, downhill slope and so on ... but in the end I thought, it is really quite simple:

    No matter how much downhill, tailwind and what weight on board - if 1) the speed is INCREASING and 2) your foot is off the accelerator, then regen should do its best effort to keep that particular speed.

    What I mean is, I hope that the Tesla S regen function is not just a "dumb" fixed resistance, but that it considers whether or not the car is actually accelerating, maintaining speed or decelerating.

    Having to ride the (conventional) brakes on downhills in a modern electric car would be entirely disappointing, but it seems from the replies above (thanks) that this is NOT a problem.

    I assure you that when I get my Tesla S eventually, I will test and report from the hills and mountain roads of Hong Kong :)
     
  12. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Its much easier to drive than to describe. As I have told many people, driving the Roadster is like driving a manual transmission car alway in 1st gear. The Model S is very similar but always in 2nd gear. You very quickly get used to feathering your foot on the accelerator to get just the level of regen that you want. If you use cruise control, it does the feathering for you, and holds a nice constant speed.

    I have not met a descent in Colorado yet, that regen could not tame.
     
  13. DITB

    DITB Charged.hk co-founder

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    I am quite content with that answer, thank you. I have not yet really found anything I DON'T like about this car, apart from a bit of a worry about all the gadgetry - will it last the test of time? I remember when all the main brands of premium cars went very electronic (including Mercedes, Audi and such) - from pure and simple turn the key and start to computers taking over. Stories of how an almost new car would just refuse to operate, because of computer problems.

    Simple and pure is often easier to maintain, so I hope the Tesla S is NOT so over-engineered that it will cost a fortune to maintain ...
     
  14. VolkerP

    VolkerP EU Model S P-37

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    OK, it's time to put all fears to a stop :smile:

    Going downhill "frees" potential energy (m*g*h), where m is vehicle mass, g is gravitational acceleration, h is height (elevation). The energy conversion rate (called P for power) depends on speed of elevation loss dh/dt, which in turn depends on vehicle speed and grade:

    P = m*g* dh/dt

    To solve for elevation loss

    dh/dt= P/m*g

    Using metric system here for easiness of conversion: m=2100kg, g=9.81m/s², P=60kW=60.000kg m / s³, result is dh/dt = 2.9m/s.

    What does that mean? You have to go downhill at a rate in excess of 10ft or 3m of elevation loss per second to max out regen.
    • on a 10% grade, that's 30m/s or 108km/h or 67 mph. At that speed, drag losses are ~18kW, slowing you down, too.
    • on a 20% grade, that's 54km/h or 33mph.
    • on a 30% grade (Filbert Street, SF), that's 36km/h or 22mph. I'd say you can do that if you're bold enough not to touch the brake pedal. :biggrin:

    Here is a picture of the world's steepest residential street, Baldwin street in NZ, at 35%. You should get the idea of really steep from that.
    My conclusion: your will not go down such a grade and be surprised that regen is maxed out. Your gut will scare you way before that happens.

    320px-DunedinBaldwinStreet_Parked_Car.jpg
     
  15. DITB

    DITB Charged.hk co-founder

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    Great information, VolkerP, thank you for taking your time to clarify it.

    And nice picture! Had I parked my car there, I would have turned the wheels all the way toward the curb though ... !!!
     
  16. eledille

    eledille TMS 85 owner :)

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    #16 eledille, Nov 15, 2012
    Last edited: Nov 15, 2012
    Except that if that in almost every manual transmission car, there will be some slack in the drivetrain, so whenever you let go of the gas pedal, there will be a nasty jolt. This does not happen in the Roadster, it transitions completely smoothly between acceleration and regen.

    It feels almost as if the road was conveyor belt, and you control its speed with the accelerator. We're all used to the car responding immediately when you move the steering wheel. The Tesla regen adds the same feeling to the forward movement as well - it's always "connected to the road" in that direction too. Don't know if that made any sense, but that's how I would describe it.

    I was fortunate enough to get the opportunity to drive one for an hour some time ago. I found only one major flaw - you look like a complete idiot when you try to stop smiling and discover that you can't because your front teeth have dried... :biggrin:
     
  17. smorgasbord

    smorgasbord Active Member

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    Reports are the the Fisker Karma does a really good job at blending regen and friction braking and at providing good braking feel throughout.

    Regen on Roadster does work really well. You need to relearn how to drive smoothly, though. With ICE and automatic transmissions, we're used to completely lifting off the accelerator to coast. On Roadster and Model S, you don't press down as far. On Roadster 2.x and Model S, you have a power gauge (kW) that you can use to help you find that coasting spot. You can always spot new Tesla drivers because their brake lights keep coming on even though there's no reason for them to be braking - and they're not, they're just regening when they should be coasting. Tesla should put a brake light indicator on the dash to help you learn when you're regening too much.

    The downside to regen on our rear wheel drive cars is that typically you want braking on the front wheels, due to weight redistribution. I think that's why Tesla both limited and feathered the regen on Model S. Roadster gets away with more and more instant regen because there's so much weight on its rear wheels (you may have seen Roadster falling off 4-point lifts not properly equipped) that it's OK to brake heavily on them. But, Model S has a better for driving 53/47 weight distribution, so braking hard on the rear wheels only would not be safe. I'm expecting that future AWD Teslas will again have heavy regen since they can do so on the front wheels, which is the safe way to brake.

    So, if you can figure out how to combine regen and friction brakes, you do get some advantages. You could use front friction and rear regen to get balanced braking at all times. You can have an easy coast mode that doesn't require the driver to find the sweet spot on the accelerator. You can get better track performance, too (at Laguna Seca, the trick the Roadster lap time record holder used was to 2-foot drive the car, which enabled him to keep the accelerator at coasting while the other foot nailed the brakes).

    But, I agree that on Roadster on the street, the regen/brake setup is wonderful. The only downside is that I don't use the brakes enough and so when I do need them they squeak embarrassingly. Even after a day of hard braking at Laguna Seca, they were back to squeaking within a week.

    Back to hills, I live at an elevation over 2000' and drive into Silicon Valley every day. It's about 5-6 miles down and I gain about 3 miles of range going to work, even with a full Standard charge. The road is twisty, but if I choose the right speeds I don't need to brake at all unless there are other cars on the road. I do, however, have to press the accelerator some since full regen on Roadster would make the trip too slow for my tastes (but made for an interesting regen experiment I did a year ago and posted about on TMC).
     
  18. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    I've found a good compromise that gets most of the regen energy, gets me places a little faster, and keeps the brake disks wiped.

    The energy that is recovered on flat ground is [SUP]1[/SUP]/[SUB]2[/SUB] * m * v[SUP]2[/SUP]. Because the remaining energy to be recovered when stopping is proportional to v[SUP]2[/SUP], if you use friction braking for all of the last 1/2 of the speed reduction, then you have only given up 1/4 of the regen available, 1/3 of the speed gives up 1/9, 1/4 of the speed gives up 1/16, etc. Also the time to cover the total distance is disproportionately used when dwelling at lower speeds; that's why you always wait until the last second to brake on the track.

    So, my method is to try to let up on the accelerator pedal so that I have to start applying friction brakes during the last 1/2 to 1/4 of the speed in deceleration to a stop. This loses little regen, saves me time, and wipes the brakes. Your mileage may vary.

    Coming back to this thread, variations of this work on hills as well.
     
  19. strider

    strider Active Member

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    Switch to the Carbotech 1521 pads. No squeaking and nearly no dust while still providing great stopping power when you need it.
     
  20. smorgasbord

    smorgasbord Active Member

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    I've thought about that, but am waiting to see if Tesla does a brake system upgrade and how well that gets reviewed. Ideally, someone will do the kind of test that was done for aftermarket HIDs versus Tesla's HIG upgrade, but for Carbotech's versus Tesla's upgrade, if and when they actually have one.
     

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