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 think I've seen a picture of this somewhere in this forum. It looks like your basic energy screen but with no arrows. It's that point where you're about 40mph or less and your foot is slightly pressing the accelerator. Not enough for yellow arrows, but not so little that blue arrows appear. There are no arrows displayed at all, thus "black". Sometimes I see it at greater speeds but it's terribly difficult to find this point and keep it.
     
  2. rusty houndog

    rusty houndog mountain rider

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    You are correct; Toyota ECMs are programmed to turn off fuel delivery during B mode if the accelerator pedal is not depressed, just like a Jacobs brake does. You can drive completely in B mode if you so desire.

    If the definition of efficiency only includes fuel use the definition is incomplete. We do not have a magic perpetual motion machine in the Prius. There is efficiency of inertia expenditure and recovery in travel under varied conditions. There is efficiency of lower maintenance costs such as not replacing worn brake parts so early in the life cycle. At my age, there is the efficiency of fewer massage visits.

    As I said, I keep my foot off the brake pedal as much as possible. In D mode, pressing the brake pedal initially engages regenerative braking, but hit a bump in the road, almost any bump, and that disappears to be replaced by friction braking; it eats brakes. Recovery time of D mode regenerative braking is ECM controlled and not predictable. There is no loss of regenerative braking except for full charge in B mode; no brakes eaten, bumps or no bumps.

    On an inflexible income impacted by rampant inflation, I'm more concerned with eliminating gross cash expenditures than with complex, and marginally safe, fuel saving maneuvers. I'm making a sufficiently brazen green statement simply driving a Prius.

    Exactly what is "glide and pulse"?
    Doesn't that engender road rage in others during commutes?
     
  3. nooaah

    nooaah New Member

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    Yup, I figured it out. I can only maintain that sweet spot for a minute or so. I'm sure I'll get better as time goes on. I'm only about 70 miles in now.
     
  4. JimboK

    JimboK One owner, low mileage

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    As I said previously, reaching maximum state of charge eliminates further regeneration, whether in B or D. But that's really sort of a moot point. The car seems to put itself in B mode -- or at least exhibits B-like behavior -- on a downhill at a maximum SOC, regardless of the gear shift lever's position. EDIT: That should be, "regardless of whether the gear shift is in B or D."

    Certainly your choice, and my goal too is to keep total cost of ownership as low as possible. That, BTW, is one of the things that led me to drive a Prius. I'm not making a statement of any sort or any color. Well, maybe it is a green statement -- green for cash.

    Please don't judge others and their legitimate fuel saving maneuvers -- which, by the way, are not complex and, if done right, are perfectly safe and will reduce outlays; babying the car won't increase repair costs.

    Do a search on "pulse and glide" or "P&G" and you'll find several threads on the subject, and how fuel-saving techniques can be done safely and courteously.

    My overriding principle in applying P&G and other techniques is to keep things safe, legal, and courteous. If someone uses such techniques without regard for other motorists, it indeed could provoke road rage and possibly could be unsafe and illegal. With that goal in mind and in practice, I still routinely manage drives in the 75-85 MPG range through suburban and urban areas.

    Of course, driving normally can incite road rage too. A jerk will be a jerk. So to that end, I refuse to capitulate to tailgaters who try to insist through their actions that I should drive 20 MPH over the speed limit in the right lane or wait until the absolute last minute to brake as I approach a red light.
     
  5. rusty houndog

    rusty houndog mountain rider

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    After following your suggestion I reread much of what Hobbit and others have to say and made some adjustments in my ordinary running around on the flat. Adjusting tire pressure, feathering for zero ICE and regenerative activity, and even outright N mode have lead me to a further understanding of the ease with which this Prius coasts and even accelerates on very gentle slopes. I've never experienced a car with such small drag.

    Hobbit talks much about the sweet spot. His descriptions correspond with my short experience with the Prius and high speed running to Sheridan on the old roads the Interstate "replaced." Right.

    I've found the sweet spot at this altitude will run as high as 65 mph which makes it ideal for the "old" roads; much less traffic on them. Most runs from Buffalo to Billings are on the old routes. Those old routes run mostly on valley floors and only climb hills to avoid tight right of way next to the rail lines.

    This Prius and efforts to discover what it really is has become a wonder filled exercise.
     
  6. jps000

    jps000 No Exit

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  7. JimboK

    JimboK One owner, low mileage

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    Amen! Glad you're making some headway.
     
  8. richard schumacher

    richard schumacher shortbus driver

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    Summarizing: to maximize fuel economy
    - Gliding (no arrows) is better than coasting,
    - Coasting is better than braking,
    - Braking is better than stopping.
    - Don't use "B" (it's there only to avoid riding the brakes on long downgrades).

    Why the above? Because energy conversions such as charging and discharging a battery are inherently inefficient and thus should be avoided.
     
  9. ronhowell

    ronhowell Active Member

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    Doesn't your final comment Richard beg the question: "then why is it there, along with all of the associated hardware, in the first place?". To which the answer is, I suspect, the following:

    1. The ability to move from a standing start under electrical power only, with the ICE stopped.

    2. The ability to obtain reasonable acceleration using both motive power modes up to highway speeds, taking advantage of the electric motor's high torque at low speed, and an ICE operating in a very efficient Atkinson cycle mode.

    3. The ability to regenerate energy provided by gasoline into stored electrical form, which, despite conversion losses, is unique to the Prius design; while at the same time stopping gasoline consumption yet maintaining forward motion.

    No doubt others could expand on these points. Have I missed any?
     
  10. Celtic Blue

    Celtic Blue New Member

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    ronhowell,You missed idling with the engine off when stopped and in other low load conditions to power the lights, AC and accessories such as brakes and power steering. You don't want to use the battery for traction any more than you must, but it is great at allowing the car to perform basic functions wth no engine input until it is really needed for traction. It is certainly counterintuitive, but you really need the traction battery system to make the other feasible. (I still rely on the battery far too much for traction, but I'm still a novice.)
     
  11. Danny Hamilton

    Danny Hamilton Active Member

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    The high voltage battery (and all the associated hardware) are there to make the car drivable in real-world conditions, and to make it marketable to a reasonable number of potential customers.

    The battery isn't "necessary" for starting the vehicle. There was a time once when cars had a handle to turn the engine over by hand when starting.

    A/C isn't "necessary", the car will move without it.

    Most 12V accessories aren't "necessary" and it isn't usually the 12V battery that is being referred to when people recommend avoiding using it for better MPG anyhow.

    While under certain circumstances it can be more efficient to move at very slow speeds for short distances under electrical power only, the fact that the vehicle will eventually need to put that electricity back in the battery reduces the benefit a bit and makes those occasions rare.

    You mention the ability to "regenerate energy" and the ability to "obtain reasonable acceleration". The vehicle would be far more efficient if the brakes only functioned at speed below 7 MPH, and the driver was required to glide down to this speed with no regen or brake pads. It would also be more efficient if the battery wasn't used when accelerating up to freeway speeds while using the same amount of ICE power as it currently does.

    Using the battery doesn't improve efficiency over not using it in any of these matters. (stopping the ICE, braking, accelerating) On the other hand, it drastically improves drivability and marketability.

    The battery makes it easier to create a marketable vehicle that is drastically more efficient than the average vehicle. However in situations where it is possible, it is usually more efficient to avoid using the battery.
     
  12. bushface

    bushface Junior Member

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    Maybe I should not worry about it, but everyday my battery fully charges half way down the mountain with the car in "B" mode. Then it brake, brake, brake with the car in "B" mode with the engine humming somewhat loudly the rest of the way down. I wonder how long my brake pad and shoes will last. I get 75,000 in my manual Jeep Wrangler front pads and my back shoes have never been replace even with 186,000 miles on them, yes the back shoes were just checked, broke spring was making a noise. I bought this jeep new in 93.

    I noted someone wrote that the battery charges faster in the “B†mode, but I notice that not true for me. If I not paying attention and forget the “B†mode my battery fully charges before the truck brake cool down pull off. With the “B†mode it is fully charged at the sand pit.

    Back to the original concern, I worry about the overcharging the traction battery.

    My lifetime is now at 50.03 mpg at just over 37,000 miles. I know no big deal for some of you. I have noted if a am away on vacation and away from this gas killing mountain my mileage would problay be about 2 to 3 mpg higher. If anybody would like the numbers with the brand purchased, just let me know. I general buy 1 brand for a while to get better idea if it makes any differents with the brands. most of my driving is open road and interstate, 1% city driving.
     
  13. richard schumacher

    richard schumacher shortbus driver

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    The battery system computer will not let it overcharge.
     
  14. JimboK

    JimboK One owner, low mileage

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    As stated early in this thread, the car goes to great lengths to avoid overcharging. On your downhills the battery will charge to the maximum allowable (again, about 80%), then any further energy that would otherwise go to charging instead is spent spinning up the engine -- hence your "humming."

    The battery will charge faster in "B" than in "D" if you're not braking. If that's not your situation, I would guess it's because you're braking frequently or even steadily as you descend. Instead of gaining regeneration (and controlling speed) with B, it's happening through regenerative braking. That's fine if the incline still allows you to control your speed and avoid hard braking.

    Friction braking kicks in only with hard braking, and it's where hard braking potentially is needed that using "B" is most desirable as a brake-saver. If you're avoiding regular hard braking, your brake pads and shoes should live a long life.
     
  15. bushface

    bushface Junior Member

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    If you go down Coxes Creek Road in WNC between Marion and Spruce Pine you will brake often. The average safe speed is about 33 mph and there are many curves that most take about 25 mph. Many truckers aviod this 3 miles of road.

    I do not believe that it charges faster in "B" becasue the engine is holding back some even before the battery the Max allowable charge. I hear it spinning although not as loud.

    Oh Yeah, JimboK, thank you for the reply.
     
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