Glide or Battery Charge? Can the battery be charged too much?

Discussion in 'Gen 2 Prius Fuel Economy' started by Courtney, Jul 27, 2008.

  1. Courtney

    Courtney New Member

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    I am looking for a couple answers to my Prius driving questions! I bought my #2 '08 back in April and for the past few months I have been completely delighted to get 50 mpg. Now I am ready to improve that number!

    1. I have read in several posts that the glide is achieved during a black screen (no arrows). I had assumed that arrows to the battery were a good thing. Doesn't this charge the battery for better performance? Why strive to keep it in the black on a glide?

    2. There are a couple massive hills in my commute where I LOVE to coast and watch my battery charge up. Sometimes the pack is totally green. Is it possible to charge the battery too much? I have read other posts where drivers put their car into "B" while coasting down steep hills. Is a little braking better than putting it into "B"?

    Thanks a bunch for any input!
     
  2. doubleg2005

    doubleg2005 Member

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    i think you'll find all you need here: here
     
  3. Sufferin' Prius Envy

    Sufferin' Prius Envy Platinum Member

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    You can't overcharge the traction battery. Nor can you discharge it too much . . . unless you run out of gas and drive on battery (Don't do it!).
    The Prius takes care of the battery despite what the driver does.

    Is it better to glide or recharge?
    If you are approaching a stop, you may as well recharge rather than glide and hit the brakes hard at the end. If you are going down a large hill, you may as well recharge if it means preventing the car from gaining too much speed.

    If it is a long and steep downhill grade, I will use the "B" mode . . . otherwise, I will just lightly push the brake pedal.

    If it is a downhill followed by an uphill, I will risk a speeding ticket by letting the car gain speed to help with the climb ahead.
     
  4. Courtney

    Courtney New Member

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    Thanks for the info on the charge. I read somewhere on another thread that a full green battery wasn't necessarily a good thing. I feel a bit better now since reading your response. I have also learned in the last couple minutes from my first post that regenerative braking occurs when the blue arrows to the battery are seen so I am assuming that is why a driver would want to stay in the black on a glide.
     
  5. qbee42

    qbee42 My other car is a boat

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    Right. You stay in the black to avoid making or using electricity. Just using the momentum of the car to store energy is more efficient than converting it to electricity, then converting the electricity to chemical energy in the battery, just to convert it back to electricity and the electricity back to momentum. You want to avoid all of those conversions if you can.

    Tom
     
  6. V8Cobrakid

    V8Cobrakid Green Handyman

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    if you fill up the battery, you'll lose regen, and go faster.

    If you always go too fast down a hill and fill up the battery, use B next time.

    Regen is there to take the work off the brakes. B is there to take work off the brakes. you choose the balance.

    gliding is used to coast down small hills where you might usually keep the engine on with just about 0 assist anyways.

    so.. if you're battery is charged up on a steep hill, and you need to brake for a light, you'll be using manual brakes only. if you have a lot more down hill to go, they'll heat up. so using B will help maintain that lower speed while going down hill. then you can just brake a little in the corners.
     
  7. rusty houndog

    rusty houndog mountain rider

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    I may be wrong but I assume "glide" means putting the selector to the neutral position. That seems a somewhat dangerous thing to do, depend exclusively on friction braking to control the speed of the car.
    Having the brakes heat up is a very bad thing. The brakes on the Prius are smaller than needed for a same weight vehicle without dynamic braking.
    It does not take very much time to boil the brakes when they are used alone to control the vehicle.
    Heat builds up in the rotors and drums and ultimately boils the brake fluid in the brake cylinders converting it into gas, usually a fatal thing, as in you die when you hit something because the brakes do not work at all!
    The normal thing done by the Toyota Prius computer program will get you optimum use for your driving economy and safety.
    If one wishes to endanger one's life taking stupid chances do it OFF the road, and not where others will be fatally involved.
    You can "feather" the throttle, out of cruise, to "glide" if you wish no regeneration of electricity, but why would you want that?
     
  8. Courtney

    Courtney New Member

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    Hi Rusty,

    By "glide" I mean still be in D (not neutral), but have an absence of arrows on the display - not yellow, not orange, not blue, just that small moment of blackness that hopefully, if I am able to extend it, will lengthen the time I can glide. This shouldn't be a dangerous thing at all!
     
  9. F8L

    F8L Protecting Habitat & AG Lands

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    This is generally incorrect. By feathering the pedal to opt out of regen mode you can glide, arrow free, for much farther than you could if you simplyly lifted off the gas pedal a coasted to a stop.

    For the same reasons Tom stated above and I will quote:

    Because you already have the needed enegy to get your car to the next stop sign there is no need to try and capture any energy (except for the very last hundred yards or so). Think of it this way:

    Option #1: You see a stop sign 1/2 mile away and you choose to start a glide (no arrows) about 1/4 mile out and glide the entire way with no regen except at the very end of the glide as you come to a stop.

    Option #2: You see a stop sign 1/2 mile away and you decide to let off the pedal and coast (regen arrows going) about 1/4 mile out. You quickly realize you cannot coast that far because of the extra drag from the regenerative braking. I'll stop here because you can see where this is going.

    In option #1 you have stopped using fuel roughly 1/4 mile before the stop sign. You did not regen much energy to the battery but as we all know, using fuel to charge the battery is not productive due to the laws fo thermodynamics.

    In option #2 You would have had to start coasting much later, say about 1/8th mile out, to be able to reach the stop sign before coming to a stop thus you has to stay on the throttle longer and used more fuel to do so.

    I could be wrong in this assumption but it seems to me that gliding as far as you can before a stop is more efficient than shorter coasting just to regen the battery.
     
  10. Sufferin' Prius Envy

    Sufferin' Prius Envy Platinum Member

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    DON'T ASSUME!

    Nobody is talking about shifting the car into neutral.

    No, it won't.

    The car knows how to best take care of itself, but it doesn't know jack about traffic conditions, road conditions, if there is an uphill or downhill ahead, or even if it is about to go for a swim in a lake.

    How the driver controls the car has everything to do with getting the best miles per gallon out of a Prius. It doesn't magically do it all on its own.

    Because that is how you obtain the best gas mileage. And hopefully you aren't in 'cruise control' and trying to 'feather' the throttle for better MPGs. :rolleyes:
     
  11. rusty houndog

    rusty houndog mountain rider

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    Living in a town with two stop lights, in a county the size of the state of Massachusetts with under 10K population, spending much of my travel time on mountain roads, my perspective and experience is different from urban commuting.

    My average trip time is two hours one way.
    Constant changing from B position for control to D position is my sole attempt at mileage improvement on these rocky, dirt roads at altitudes from 6K to 9K feet.
    I use B position as much as possible to keep the car on the roads and out of the ravines.
    The result of trying for no regeneration would be cramps in my calf muscles and frequent new brakes, to say nothing about boiling the brake fluid out of the brake cylinders and ending up at the bottom of a ravine.

    Still, I get between 50 and 52 MPG on these excursions. Possibly, it's the Slick50?
    Maybe it's something Toyota did to start out with coupled with a common sense approach to safety and sociability.
    At least I don't cause road rage by being slower than whale sh*t.
     
  12. a priori

    a priori Canonus Curiosus

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    Your experience will be quite different from mine, I grant. I have lived in the mountains and GREATLY appreciated having a manual transmission for the control it affords. I believe your heavy reliance on "B" mode for this control may have some grounding, but it doesn't really work that way for most of us!

    Hmm. I think I see what you are saying, but I'm not certain your statements are internally consistent. Changing from B to D will help your mileage, but mainly because it will be a bit easier to "glide" from D than from B. I believe you can glide in B just like D, but there is a quicker deceleration in B after you have released pressure on the accelerator. To me, it seems it would be easier on your calf muscles to be in D and get more glide.

    I don't see why there is any danger to your brakes by trying for "no regeneration" -- if by "no regeneration" you mean "gliding." The danger of fade on your brakes is real if you are on a long downgrade. This is where the B mode shines, and I'm certain you see it more often than most of us.

    If you are driving a constant motocross, then B mode is for you. If you are looking to increase mileage, I think you can do it in either D or B mode, so long as you are traveling at highway speed. Around town, though, you may be asking for lower mileage, because in B mode the engine will be spinning more often and using either electrical or gasoline energy to maintain that spin.

    One great place to go for more information on B mode -- what it is and how to use it -- is Hobbit's description of B mode.

    By the way -- do you have any pictures of your corner of the world?
     
  13. F8L

    F8L Protecting Habitat & AG Lands

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    I think you need to modify the attitude a bit here.... It's fine to try and defend your stance on B mode use but now you are making veiled accusatory remarks. Not cool.

    You have an odd commute which seems completely different than 90% of the Prius owners in this forum. What works for you to maintain decent MPG and safety is not what the rest of us experience on a daily commute. If this works for you then that is great. Safety should come first as always. :)

    I drive 16miles on a very winding canyon road with a 1,400ft. elevation gain during the last 8miles or so. I never have to use B mode, my brakes are not catching on fire and I have yet to end up in a ravine. Maybe I need to see a 3,000ft. elevation gain to match your situation but again, that is not the most common situation therefore your advice doesn't seem applicable to the main PC membership.
     
  14. rusty houndog

    rusty houndog mountain rider

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    It would seem to me obtaining the EV activation accessory not supplied with US delivered vehicles would be an infinitely useful addition to the mpg race; zero fuel use while activated and possible up to 55kph, a good reason for regeneration.
    Sigma Automotive – Toyota Hybrid Prius 2004 - 2009 OEM & JDM auto accessories
    then do a page search for EV; it's the fourth or fifth find from the top.
    That source from Florida doesn't seem to answer the telephone.

    As for local pictures; mine are almost all of local medicinal plants and not indicative of road conditions. I'll keep in mind some shots of terrain with special attention to drop-offs and extra steep inclines. Unfortunately, the rolls of film I shot recently which included some terrain and altitude shots were underdeveloped by the local (Buffalo, WY) development service; they are all completely washed out.

    I have since found a more reliable service in Sheridan that waits for their system to warm up; demand for film development is rather low what with all the electronic cameras being used. I'll stick with my Nikon F2s and Leica CL; the resolution is so much higher even considering grainy stuff like TriX.

    Do a Google images search for Big Horn Mountains. Most of those images are tourist or camping related and do not show the dirt roads but do show some mountains and pack trails.

    As to whale sh*t, I'm just repeating others on Prius fora. There isn't an awful lot of road rage around Buffalo, Wyoming.
     
  15. JimboK

    JimboK One owner, low mileage

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    Rusty, your fact-checking needs a little work. This is the latest of several examples in this thread. The EV switch won't work above 34 MPH. And when it works, using it for propulsion generally will hurt fuel economy rather than help it.

    Where it helps is in limited circumstances where traffic, terrain, and road conditions allow gliding but where the ICE hasn't fully warmed, as I describe here. But this won't help someone who drives only rocky dirt roads at altitudes from 6K to 9K, who is convinced that the only way to help his mileage is to shift between D and B, and who is convinced that working the pedal to glide would result in cramps in his calf muscles, frequent new brakes, boiling the brake fluid out of the brake cylinders, and ending up at the bottom of a ravine.
     
  16. rusty houndog

    rusty houndog mountain rider

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    To err is human. The m has been changed to k. Life is, after all, an ongoing learning experience. It appears that to exaggerate is also human, Jimbo.

    During mountain trips I get over 35mph very infrequently. My battery gauge turns full green six or seven times each way and stays over the mid point more than under. The EV switch will definitely help me. As you say in your descriptive post, an ICE off switch.

    It seems the B position engages the large traction motor as a generator, thereby increasing braking power and increasing delta-I. It charges faster. When faced with rapidly changing hill accelerations and decelerations, faster charging stores more of the energy otherwise wasted eating brakes. Keeping my foot off the brake pedal is of paramount importance on these "washboard" roads. The EV switch will help convert that saved energy into zero fuel consumption miles where possible.
     
  17. lucky1

    lucky1 Member

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    After reading this thread I now have a better understanding of why you want to see a black screen and not be filling the pack.
    I have a couple of Prius taxi's and have 10 hours a day to practice pulse and glide. I get the screen to go black but for very short distances and don't seem to be able to hold it for long. I paid attention to trying to keep the screen black for about a week and gave up. Since then i have worked hard at trying to glide 4 to 5 times or more distance than my acceleration distance.
    I have gotten pretty good at keeping the consumption at 0.0 for the length of the glide. I end up anywhere from 4.3 to 4.7L/100 km. Have been as low as 4.0L/100 km. Works out to be around 60 to 65 mpg (imperial gallon)
    Considering that I work a job where time is money I am happy with that.
    I can't be crawling around the city to save money on fuel as it would cost me much more by doing a few calls less each day.
    I travel at anywhere from 40 to 75 kmh for most of my day.
    Any tips on why I don't seem to keep the screen black for very long distances would be appreciated.
    Thanks
     
  18. JimboK

    JimboK One owner, low mileage

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    Correction noted, thanks. Unless fairly obvious, exaggeration costs credibility.

    Certainly on your mountainous roads B mode is appropriate. I and others here are just trying to make sure you understand that it is less fuel-efficient than D. Its purpose is safety and braking, as you clearly do understand. But that comes at a cost of efficiency. I highly recommend Hobbit's paper that A Priori linked for you if you haven't read it already.

    As that article states, there is indeed more regeneration with B, so your statement that the battery charges faster is true. But it doesn't charge faster for long. The battery will charge to no more than ~80% (8 green bars). (FWIW, those of us with add-on instrumentation have seen it as high as 84%.) As it approaches 80%, to protect from overcharging, the car reduces and then eliminates further regeneration and instead uses that power in other ways -- namely, by spinning up the engine to a higher RPM, increasing engine drag (and decreasing overall efficiency). This, by the way, can happen with or without B mode; see this for a recent discussion.

    I've used B descending the Appalachian mountains, but the car is fully warmed by the time I get there (they're about a two hour drive), so I have no need for EV switch use there. Therefore, I don't know how the car would behave using the two modes simultaneously. It is my understanding -- and here's where I could be wrong -- that there is no fuel flow during the B mode ICE spin-up. (This discussion got me wondering about this. I might look into this further.) The exception presumably would be during the car's warmup stages, so the EV switch might have some use for you there as it does for me.
     
  19. nooaah

    nooaah New Member

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    What does a black screen look like? Can someone take a pic?
     
  20. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    sure you can... the Pri will simply bleed off the extra charge to protect the battery

    if you know the light or traffic will not clear, you should also in this situation lightly hit the brakes to slow down faster. that will increse regen current. hitting the brakes hard at the end increases the use of friction brakes and not recommended for boosting mileage

    all in all, you dont want arrows to the battery unless you are gliding downhill or slowing down simply because it still takes gas to charge the batteries and you lose energy in efficiency losses whenever you do that.

    another thing to increase your mileage is to know your route, experiment with alternate routes looking for less lights, less traffic or lights that are predictable. there is nothing wrong with starting to glide to a stop on a multi-lane road several blocks from the light especially if there is already a large line of cars waiting there.
     
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