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does higher SOC indicate better MPG?

Discussion in 'Gen 2 Prius Technical Discussion' started by statultra, Feb 7, 2008.

  1. statultra

    statultra uber-Senior Member

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    if the prius battery has all blue bars, which is 7 i think, and your cruising 65 mph would you get more gas mileage then if you had 3 bars ?
     
    #1 statultra, Feb 7, 2008
  2. efusco

    efusco Moderator Emeritus
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    Highest blue is 6 bars.
    I'm going to answer your basic question with a tentative "yes"...briefly.

    Because the Prius strives to keep the battery SOC at 60% (5-6 bars on the Energy screen) a small amount of energy will be diverted to restore the battery SOC if you're at 3 bars and transiently your mpg will be very very slightly lower until the "optimal" SOC is reached. Realize that this will be quite quick at highway speed, so your lower mpg would probably last 30-60 seconds max.
     
    #2 efusco, Feb 7, 2008
  3. statultra

    statultra uber-Senior Member

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    lets say hypothetically that you have a battery that can supply the 6 blue bars ( 60 % ) constantly, then you would see a mileage benefit?

    also if its all green bars, ( 80 % i think ) the car would increase the use of the electric motors? resulting in better gas mileage?
     
    #3 statultra, Feb 8, 2008
  4. miscrms

    miscrms Plug Envious Member

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    1) I believe it would be somewhat improved, yes. The PHEV folks call this charge sustain mode. Basically, the larger capacity and lower resistance of the extra battery increases efficiency enough (maybe ~5-10%?) to at least offset carrying the several hundred pounds of added weight after the battery is run down.

    2) This is also yes. This is why the PHEV conversions can get 80-100mpg at highway speeds while the secondary battery is charged up. I think the best number is around 74%. Of course, this is less a big improvement in efficiency and more a shift of fuel from gasoline to the electric grid. As you approach 80% the car starts dumping charge in very inefficient ways to protect the batteries.

    Rob
     
    #4 miscrms, Feb 8, 2008
  5. efusco

    efusco Moderator Emeritus
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    I guess I don't really know what you mean. The SOC of the battery is exclusively maintained by the ICE via MG1 when cruising at hwy speed. The battery serves as a buffer only.

    If the car were programmed to maintain an 80% SOC you'd get the same MPG as if the car were programmed to maintain a 40% SOC. There would be no net increase or decrease in energy to/from the battery during steady state conditions like hwy cruising.

    Any increased energy from the battery to the electric motors due to a high SOC would be immediately and equally balanced by an increase in ICE running to maintain that 80% SOC.

    Now, if you had some secondary source of energy to maintain the battery SOC (let's say solar panels could do that just for a hypothetical example) AND the Prius was programmed as it currently is (to maintain a 60% SOC) then yes, you'd see a higher MPG due to increased draw from the battery...but you have to keep in mind the additional energy in equation from your solar panels.
     
    #5 efusco, Feb 8, 2008
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