HV Mode: Does a Fuller Battery Improve MPG?

Discussion in 'Prime Fuel Economy & EV Range' started by mr88cet, Jul 5, 2019.

  1. mr88cet

    mr88cet Senior Member

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    On a recent “road trip” (a little less than 200 miles each way), I saw considerably higher MPG numbers on the trip “to,” when I had GoM reading of ~16 miles, whereas it read only 3-4 miles on the way back.

    So, I’m debating whether I’m seeing evidence that having a fuller battery might give you better MPG even when you’re in HV (eco) mode?

    There certainly were other differences between the two trips, but they would, if anything, seem to favor the return trip, especially it being night, so conceptually less A/C power consumption; also less gasoline weight in the tank.

    Although I don’t have any evidence to support this hypothesis, it’s *conceptually* possible that having more battery capacity to work with, even while operating in HV mode, could give it more leeway to use the MGs to supplement the ICE, and thus run the ICE at closer to optimum RPM, especially on hills.

    Here are the approximate MPG numbers I saw:

    On the way “to,” with fuller battery, in daytime:
    77MPH = 54MPG
    68MPH = 67MPG
    66MPH = 69MPG
    61MPH = 73MPG

    On the way back, GoM reading 3-4 miles, at night:
    62MPH = 65MPG
    77MPH = 45MPG

    Although driving in opposite directions on the respective roads, 65ish MPH readings were taken on the same, pretty-flat terrain. Similarly, the two 77MPH readings were taken on the same, somewhat-but-not-extremely hilly terrain.

    Tire-inflation = ~40PSI all around on Bridgestone ECOPIA Plus EP422 tires.

    The daytime (outdoor) temperature averaged somewhere around 8 Fahrenheit degrees higher than the nighttime temperatures. Air-conditioner usage was pretty comparable, and overall conservative in both cases.

    Now, granted, my destination was at an altitude a few-hundred feet lower than where I live, so conceptually speaking, the trip to was “downhill,” and the return trip was “uphill,” but a few hundred feet spread over a couple hundred horizontal miles is pretty much flat (perhaps a 2000:1 gradient), so I really doubt that could have made much difference.
     
    #1 mr88cet, Jul 5, 2019
    Last edited: Jul 5, 2019
  2. schja01

    schja01 One of just a few in Chicagoland

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    So the question to me is can HV mode invade the imaginary EV portion of the traction battery for other than using it as a regen container.
     
  3. mr88cet

    mr88cet Senior Member

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    Perhaps a better way to put it would be, “operating in HV eco mode, if it has more battery capacity with which to assist the ICE, so that it can run the ICE at closer to optimal-efficiency RPMs, will it use that extra capacity — extra leeway — to do so?”

    So, for example, upon climbing a hill, will it “dip deeper” into the greater battery capacity available to let the MGs do more work so that it doesn’t have to gun up the ICE as much?

    FWIW, all in all, the trip-back numbers seem overall more believable than the trip-to numbers, from what I’ve seen anyway.
     
    #3 mr88cet, Jul 5, 2019
    Last edited: Jul 5, 2019
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  4. schja01

    schja01 One of just a few in Chicagoland

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    Assuming an 80/20 split for the EV/HV battery use I believe the assisting ICE in HV mode comes out of the 20%.
    If not it wouldn't be the first time I've been wrong. :)
     
  5. Prius from Dad

    Prius from Dad Senior Member

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    HV and EV modes use top down availability. Meaning EV "portion" gets used first. When that goes to 0, then HV will dip into the HV "portion" but EV mode will not.
     
  6. bisco

    bisco cookie crumbler

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    wouldn't the weight of the fully charged battery actually reduce mpg's?
     
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  7. Prius from Dad

    Prius from Dad Senior Member

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    I'm not very good with electricity or batteries, but electrons don't weigh very much. ;)
     
  8. mr88cet

    mr88cet Senior Member

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    That reminds me of the Dilbert cartoon where the pointy-haired boss complained about his laptop being too heavy, and Wally suggested he delete some files...
     
  9. bisco

    bisco cookie crumbler

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    me neither :unsure:
     
  10. Salamander_King

    Salamander_King Senior Member

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    I've done quite a bit of ~300 miles (one way) round trips in the past few months. I always save full EV charge going down to use it in the city. Having no way to charge, I am always near 0% EV on my way back. For those drives, I have seen mpg on my way back being lower but also have seen the opposite. To make any comparison, you must do the same route going the same direction with full charge vs no charge in very similar driving condition. On the way to and back from are two different driving conditions in most rout you take. There also are so many other variables that come into play affecting mpg. My feeling is that any differences in mpg are just coincidence. I would buy the hypothesis if many more controlled testing shows the same results, but until then, its a hogwash, IMHO.
     
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  11. Tideland Prius

    Tideland Prius Moderator of the North
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    No idea but my recent trip in HV mode with EV reserve and even going over the Cascades and highway driving of 70mph, the net trip was 3.5L/100km (67mpg). Now that included 3 full charges. The trip through the mountains was 3.4L/100km (69mpg).
     
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  12. fotomoto

    fotomoto Senior Member

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    In the Honda Clarity community, we are seeing a mpg benefit to saving a few Ev miles before moving over to HV mode. So, I think your hypothesis has merit.
     
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  13. mr88cet

    mr88cet Senior Member

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    I seem to recall that Honda worked with GM on the drive Clarity PHEV train, meaning that it’s somewhat similar with the Volt’s (mostly) series-hybrid architecture?
     
  14. fotomoto

    fotomoto Senior Member

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    #14 fotomoto, Jul 5, 2019
    Last edited: Jul 5, 2019
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  15. mr88cet

    mr88cet Senior Member

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    Ah, maybe that was it...
     
  16. jerrymildred

    jerrymildred Senior Member

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    So far, I do not see the word, "wind" on this page. If you went out with a 10 mph tail wind that's almost like reducing your speed by 10 mph at highway speeds. If the wind didn't change, then you came home facing a 10 mph headwind, which is almost like increasing your speed by 10 mph.

    It takes very little wind or elevation change to make a major impact.
     
    #16 jerrymildred, Jul 6, 2019
    Last edited: Jul 6, 2019
  17. mr88cet

    mr88cet Senior Member

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    Yes that’s a very reasonable point, and it had occurred to me, but I didn’t mention winds because I don’t know. However, I didn’t notice any signs of particularly strong winds (trees bending or swaying, etc.), but I don’t know what the wind speeds or directions were.

    However, two things along those lines are worth mentioning:

    First, I’ve seen pretty much the same (lower) numbers returning when I’ve taken this trip before. What’s new this time is that I hadn’t really monitored MPG on the way “to” before. So, if wind is a factor, it’s been pretty much the same every time I’ve looked on the return trips, which is certainly not impossible although *maybe* unlikely.

    Second, the trip consists of two legs: I-10 which runs pretty much east-west, and US-71 which runs more or less northwest-southeast. The 77MPH numbers I took on the US-71 leg between the towns of La Grange and Columbus, TX, and the 62MPH numbers I took on the I-10 leg, Northwest-Houston-Katy-Sealy.
     
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  18. mr88cet

    mr88cet Senior Member

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    Oh yeah, I just remembered another experiment I did on the trip back, and it, at least up to a point, contradicts the idea that a fuller battery improves HV mileage:

    As I was driving back, it became apparent that I’d end up with too much gas in the tank. “Too much gas” in that I pretty much always drive on EV around town other than my comparatively-rare road trips, and I don’t want a large cold of gas in the tank to go stale over months of non-use.

    Anyway, I therefore decided to go into Charge Mode for a while, trading off leftover gas for a shorter first charge after the road trip. I charged it up from pretty much zero on the Guesstimatron, to about 9 miles. I then switched back to ordinary HV Eco mode, and took another 77MPH reading. The 77MPH mileage reading was pretty much the same as when the GoM was reading all-but-zero — about 45MPG.

    Just today my wife and I took ... not exactly a road trip, but something longer than would fit in he battery, and I was able to take a 76ish MPH reading. It ended up very close to 50MPG: pretty much in the middle between the two readings from my road trip Austin->Houston->Austin earlier. So, maybe it was just wind, or some other factor that evens out between the two trips.

    So, what do you folks typically see at 75MPH in HV Eco?
     
    #18 mr88cet, Jul 6, 2019
    Last edited: Jul 6, 2019
  19. greenakina

    greenakina New Member

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    The alternator charges the battery and continues to try to charge the battery while the car is running. If the voltage level does not reach an acceptable amount, normally around 12 volts, the alternator may run non-stop during your entire drive. Since the alternator is linked directly to the drive or accessory belt that is turned by your engine, the added resistance of the alternator working can cause a slight decrease in fuel economy. In reality, this decrease would be too small for most people to realize.

    However, the life of the alternator could be shortened.
     
  20. Salamander_King

    Salamander_King Senior Member

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    Only thing is that there is no alternator in Prius (including PRIME).
     
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