MPG in Hybrid Mode

Discussion in 'Prime Technical Discussion' started by CEJ, May 13, 2019.

  1. CEJ

    CEJ Member

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    I am not sure I am following you completely. Let me summarize my results and conclusions and then you can poke holes or expand my understanding. The last two trips were very similar and produced about the same mileage (32.1 and 32.4). Using the wall recharge values (6.67 and 6.62 kwh) leads to a straightforward value for the total cost of EV driving ($0.0166 and $0.0164 per mile, using $0.08/kwh--our summer rate). The values for these last two trips are almost the same. The first trip had longer segments in it and that is reflected in a somewhat higher mileage (34.5) but about the somewhat similar wall recharge of 6.47 kwh. Cost in this case is $0.015/mi, a bit less. I think probably my interest here mainly was to determine the actual cost to me of EV driving and, of course, this can vary from trip to trip. I expected recharge kwh's to be similar under roughly the same conditions of weather and temperature--the variation here is about 3% which may be reasonable.
    We now come to the DM2 (or Eco Diary) values for mi/kwh which are considerably different from the values determined from the wall charge. Working backwards they can also be used together with the mileage to determine other kwh values (5.84, 5.732, and 5.8). I expected them to be close and they are, but what do they represent? My conjecture was they represent the battery charge (after it is fully charged) available to energize the wheels. There may still be some energy losses as the energy moves through the motor.

    I think you are suggesting there is energy going back into the battery during EV drive, but where is it coming from apart from regen in coasting and braking? What am I missing?
     
  2. Salamander_King

    Salamander_King Senior Member

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    Your first part of the calculation is fine. If all you want is "to determine the actual cost to me of EV driving" then you have the number for those trips. Of course, EV ranges will change from a trip to a trip especially for seasonal changes in winter will diminish the range substantially so that this cost is not constant.

    The problem in using this number and DM2 (or Eco Diary) miles/kWh in calculating full capacity stored in the traction battery for EV drive is that it assumes the amount of electric energy used for that distance is coming entirely from a fixed amount the stored energy in the battery alone. This assumption is wrong. The energy (kWh) in the battery is in flux even during EV drive. The energy going into the battery is exactly as you suggested. They are regenerated energy from braking and coasting. This amount is not constant at all, and it depends highly on your driving.

    Here is an example of kWh out and in during my 18 miles of commute all done in EV MODE. The actual amount of energy used for this 18 miles was 2.341kWh, which is a difference between 3.926kWh used and 1.586kWh regenerated. The graph on the bottom is showing the elevation changes along the 18 miles I drove and cumulative kWh of energy used (or generated) along the trip. Wherever I have downhill, energy is regenerated and put back into the battery indicated by the downward slope red line.

    Screenshot 2019-07-15 at 6.28.58 PM.png
    Screenshot 2019-07-15 at 6.40.53 PM.png
     
    #302 Salamander_King, Jul 15, 2019
    Last edited: Jul 15, 2019
  3. Tideland Prius

    Tideland Prius Moderator of the North
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    To further on @Salamander_King's post above, @CEJ , it might be worth checking the Energy Monitor for the total trip regen amount and see if that helps factor into your calculation. It's a bit of a hassle to get to every time.

    On the 11.6" screen, go to Energy Monitor (expand to full screen), then click the top right button that looks like bar graphs, then ensure you're on the "Battery" tab. It'll show you trip total regen.

    Regen: the AER booster | PriusChat


    I forgot to capture the trip when I came down the Cascades; that must've been in the 3kWh range. (It was at least 2kWh when I checked it out but I forgot to take a photo at the end.. again, it's not a one-button accessible page).
     
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  4. Salamander_King

    Salamander_King Senior Member

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    Thanks for mentioning this feature. Yes, it is hidden in the tab so that I don't often look at them. For comparison, I checked this morning after ~18 miles of commute to see if the numbers match between Battery tab and Hybrid Assistant App report.

    The result. They did not match. The Hybrid Assistant value is about 26% larger. I have other numbers from the Hybrid Assistant Report that are inconsistent with various numbers reported by the car. I don't know if that is the app's or car's bug or they are simply showing different data.

    Report_2019-07-16_06-23-03 Energy.png
    Report_2019-07-16_06-23-03 regeneration.png
     
  5. Tideland Prius

    Tideland Prius Moderator of the North
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    Were you in hybrid mode? I wonder if the car’s number is strictly regen via braking while hybrid assistant includes any that comes from the engine.
     
  6. Salamander_King

    Salamander_King Senior Member

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    No, those number comparison was from 100% EV MODE. No petro energy was used. That said, yes if I mix HV mode into the equation, the number presented by the Hybrid Assistant App gets very confusing, at least to me.:(:(:(

    Here is the excerpt from the Hybrid Assistant App report for today's run.

    Report_2019-07-16_06-23-03 trip summary-vert.jpg

    Report_2019-07-16_06-23-03 Trip.png
     
  7. CEJ

    CEJ Member

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    Great suggestion. Thanks! Didn't even know it was there. I am assuming the trip total resets each time you stop/start. Is that your understanding?
     
  8. Salamander_King

    Salamander_King Senior Member

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    Yap, it is only for one trip from Start to Off, then reset. Same as Hybrid Assistant App.
     
  9. CEJ

    CEJ Member

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    In thinking about driving in EV, starting with a full charge, (say 5.8 or so kwh allocated for EV) in terms of physics and energy conservation, it seems to me that no new energy enters the battery-motor-wheels system even though regen causes fluctuations in the battery charge. The battery delivers energy to and drives the motor which then passes this energy along to the wheels giving them rotational kinetic energy. During braking or coasting, the wheels lose kinetic energy which is passed back to the motor causing it to act as a generator which then puts charge back to the battery. The wheel energy came from the battery in the first place, via the motor, so I see no new energy entering the bmw system; it is just shuttling around. The total energy of course decreases as the system expends energy to propel the car against air resistance, friction, etc. So after EV range is exhausted, the original 5.8kwh (minus some internal inefficiencies) was presumably used to drive the metered mileage.
     
  10. Salamander_King

    Salamander_King Senior Member

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    Yeah... sure, it's "regenerated" energy or better term would be "recycled" energy. Recalling my rudimentary understanding of the first law of thermodynamics which I learned (or maybe slept through?) in the Physics 101 lectures, you maybe correct in all accounts. But the question you asked was the amount of the energy in the battery that is used for EV drive. That number, I still think you can't calculate it from DM2 miles/kWh and EV miles you have driven from a full charge. That's like if a gasoline engine car somehow "recycled" otherwise wasted energy and put it back into the tank as gasoline, you will not be able to calculate the amount of gasoline that was in the tank from given mpg display on the car and miles you have driven.
     
    #310 Salamander_King, Jul 17, 2019
    Last edited: Jul 17, 2019
  11. CEJ

    CEJ Member

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    Not sure I understand your gasoline analogy. Energy in a system is normally conserved or constant although it can be converted from one form into another within the system, like electrical energy into kinetic energy of motion and sometimes back again (as happens here) without being wasted, although the conversion may not be 100% efficient. Energy of a system is lost for useful purposes if it is dissipated through friction or overcoming air resistance (which happens with a car). I am still feeling that no energy is being added to the system beyond the initial allocation of charge for EV ( when the battery is fully charged) even though it changes its forms during EV driving and that it is that amount which is then finally dissipated when EV range is exhausted.

    I guess nobody knows for sure what the DM2 miles/kwh represents. But using that number in several different driving situations (as I have reported), together with the mileage (M) for each of these trips with the corresponding DM2 reading to calculate an average kwh value M/DM2 for each trip, it is about the same (5.8kwh) in each case. One would expect the allocated kwh for EV to be pretty much the same independent of trip and driving conditions and this is about the right number consistent with other estimates. So if this isn't the allocated kwh, what is it? Life would be a lot simpler if someone, somewhere, in Toyota-land would tell us what that allocation is (presumably somewhere around 60-70% of the full 8.8kwh); then we could check the numbers and might be able to determine once and for all the real meaning of the DM2 mi/kwh reading.
     
  12. Salamander_King

    Salamander_King Senior Member

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    Agreed. Without knowing exactly how and what DM2's miles/kWh represents, it is difficult, if not impossible, to determine the real meaning.

    BTW, I have discovered the way to repeat your experiment without using DM2 reset or actually running out of EV range to deplete SOC all the way to zero. Since we have established DM2 number of miles/kWh reflects the same value as in Eco Diary miles/kWh, I can do your experiment by using 100% SOC in morning and driving my morning commute of ~18 miles all in EV MODE. At the end of this morning commute, I have remaining SOC indicated on MID, however, the calculation is the same, miles driven (~18 miles) for amount of used SOC (100% minus amount of SOC% remaining at the end of my morning commute) and Eco Diary miles/kWh value at the end of the trip. With this method, as long as entire trip is done on EV MODE, my miles driven EV is already known. Assuming SOC % measurement on MID is accurate, I can calculate the kWh vale for 100% EV SOC. So, using this method, I came up with those numbers for three trials.

    Trial 1: EV SOC change 100%-31%, miles driven 18.48 miles, Eco Diary number 4.5miles/kWh, calculated 100% EV SOC value 5.951690821kWh
    Trial 2: EV SOC change 100%-45%, miles driven 17.82 miles, Eco Diary number 5.4miles/kWh, calculated 100% EV SOC value 6.000000000kWh
    Trial 3: EV SOC change 100%-41%, miles driven 17.81 miles, Eco Diary number 5.2miles/kWh, calculated 100% EV SOC value 5.805084746kWh

    Average of three value is 5.918925189kWh.

    The number is close to your average number of 5.790666kWh. But I do think it is too high to be a true value, for there are people who have shown full charge from 0% to 100% EV SOC using L2 EVSE taking only 5.75kWh from the wall. Meaning that the true amount of 100% EV SOC in the battery must be smaller than 5.75kWh.

    I also have used Hybrid Assistant App for each of those trips. The App generates detailed reports of each trip with miles driven, numerical data for real SOC% change (not EV SOC %), amount of total spent energy from the battery (delta kWh). From those data, I can do the same calculation for the trips above with extrapolated EV SOC 0%-100% to be corresponding to 15%-83% SOC value reported in the Hybrid Assistant App.

    Trial 1: SOC change 83.92%-35.69%, miles driven 18.48 miles, spent energy 3.749kWh, calculated 100% EV SOC value
    5.363487456kWh
    Trial 2: SOC change 83.53%-45.49%, miles driven 17.82 miles, spent energy 3.027kWh, calculated 100% EV SOC value
    5.490615142kWh
    Trial 3: SOC change 82.35%-42.75%, miles driven 17.81 miles, spent energy 3.128kWh, calculated 100% EV SOC value
    5.45030303kWh

    Average of three value is 5.434801876 kWh. This number does not match DM2/EcoDiary method, but close. And it is generally in agreement with observed or speculated values posted by others in PC. However, I have to note that calculated full capacity of the battery from the Hybrid Assistant App's reported SOC changes and kWh spent does not match the published full capacity of the battery's 8.8kWh. If I use the raw numbers from the app, the full capacity of the battery is more like 7.8kWh. I don't have a good explanation for this discrepancy. I checked the video by @john1701a from other thread Prime Prime - understanding kWh | PriusChat , and his Hybrid Assistant number is showing SOC change 86%-12% for full EV range with 5.221kWh spent. That means his battery now have only 6.96kWh full capacity instead of 8.8kWh. Unless those numbers indicate battery degradation, I maybe missing something or I don't understand what those numbers are really representing.

    Finally, there have been a notion speculated by a PRIME driver @mr88cet that allocated % energy in the battery for EV drive may change suddenly or gradually depending on various unknown conditions. If that is true, it will toss our results out of a window. Even though we know the full capacity of the battery is 8.8kWh from the spec, if the amount allocated to EV drive can change by some internal mechanisms, we are talking about non-constant value that calculated values from a few empirical data points have less meanings.
     
    #312 Salamander_King, Jul 18, 2019
    Last edited: Jul 18, 2019
  13. CEJ

    CEJ Member

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    I calculated 5.82 for my average over 3 trips. The variation between the 3 numbers was something like 2-3%. I would not be surprised if the charge allocated for EV did vary somewhat from trip to trip depending on temperature, etc. Your 5.91 is within 2% of my 5.82, so these all seem pretty much in agreement, especially when I realize my wall recharge values also varies (recently) about 3-4%. So the variations in the wall recharge and calculated M/DM2 values have similar spreads.

    I am eager to dig into your cool new approach, results, and findings, and to try them myself but am about to depart for a bit of vacation where I won't have the car, so it will have to wait until my return. But don't go away!
     
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  14. mr88cet

    mr88cet Senior Member

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    I personally haven’t seen mile/KWh numbers vary much since we bought our 2017 P.Prime. I have, however, seen considerable variation I how much “top margin” the charge management allocates. That is, how full it has historically let me charge the battery.

    It really wants to be kept fully charged (or nearly so) for as short a time as possible. If the battery is kept charged for long spans of time, the charging algorithm starts worrying about battery-capacity degrade over time, and prevents us from charging as full.

    It did this after I spent a couple months habitually charging it the night before (taking advantage of all-but-free charging) and leaving it almost fully-charged overnight. At least one other Prime owner, and I think others too, have observed this as well.

    FWIW, I haven’t seen, so far, much evidence that it “worries” about being kept charged to, for example, 15 miles on the GoM for long spans of time; just full or nearly so.
     
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  15. john1701a

    john1701a Prius Guru

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    Since I did the identical drive 2 years later and got the very same results, there's obviously something else at play. It could be that we discovered there's unreported buffer that doesn't get tapped until degradation starts to become measurable. That way, you'd retain "rated" distance for many years beyond what ordinary calculations would elude to. Not sure how such built in aging adjustment could be programmed to be so accurate. But we do know that Toyota collects & studies a massive amount of real-world data.
     
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  16. Salamander_King

    Salamander_King Senior Member

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    If your hypothesis is correct, I think the temperature also plays into this. At least for our local, where night temperature is 50F in summer, I did not see any degradation in EV range like you saw with my car even though I have kept my car fully charged overnight almost every day for most of my first year of my ownership. After I purchased the car, I habitually charged my car as soon as I came home from work without setting charge schedule, thus the car was sitting on my driveway fully charged over night. I continued this practice at least for the first year of my ownership. I now use schedule to make sure the the car finish charge shortly before my morning departure.

    That is assuring. I have no Hybrid Assistant date from earlier time point on my car, so I can't compare. If yours had the same results two years ago when the car was new, then it could not be a result of degradation. FWIW, by Hybrid Assistant, my PRIME shows 84% as full EV SOC, and 15% as 0% EV SOC, slightly narrower than your video result. Furthermore, I am still puzzled why the amount energy in kWh used for the full range of EV based on Hybrid Assistant data and car's miles/kWh would differ. Additionally, I have discovered the date Hybrid Assistant report for the "regenerated" energy does not match the amount reported by the car. See comments in #304-306 above. Don't know why...
     
  17. mr88cet

    mr88cet Senior Member

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    Interesting data point there!

    I can neither confirm or rule out that temperature was a factor, but that’s certainly not impossible. Generally speaking temperatures in Austin TX don’t vary a huge amount year-around. Certainly all in all warmer, and it can get pretty hot in August, but only occasionally particularly cold. I keep, and mostly charge, it in a garage — either our house’s garage or a parking garage, so that further moderates temperatures, and especially reduces heat.

    I gather, however, that Bob Wilson here saw a somewhat similar reduction in EV range around the same time frame, and he lives in cooler climes as I recall.

    In my case, best I can tell at least, what “brought it back” (partly — from 25ish back up to 29-30 now) seems to have been simply time spent not leaving it charged overnight. Nothing else I can think of changed particularly substantially. I have been putting more HV miles on it recently (helping out my mom in Houston), but it had already gotten back up to 29ish before %HV went up particularly much.
     
    #317 mr88cet, Jul 19, 2019
    Last edited: Jul 19, 2019
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