Suggested summer cabin temperature to maximize MPG and battery life?

Discussion in 'Gen 3 Prius Care, Maintenance & Troubleshooting' started by StephenFromCT, Jul 26, 2019.

  1. StephenFromCT

    StephenFromCT Junior Member

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    Hello all,

    I appreciate that driving with the windows open, especially on the highway, reduces the fuel efficiency, because the added turbulence changes the aerodynamic shape of the car. Increasing drag lowers fuel efficiency, which is why people recommend against adding a luggage carrier on top or a bicycle carrier on the back. I suspect that keeping the windows closed may also keep both the cabin air filter and the HV high voltage traction battery fan cleaner.

    I also appreciate that that the HV high voltage traction battery is cooled using air which is sucked from the cabin (via the vent near the right rear passenger seat).

    I have a 2011 Prius Gen 3 (Model 3). I have not checked the HV fan to see if it needs cleaning, but that is on my "list of things I need to learn to do."

    Questions: On hot summer days what cabin temperature do you use as a compromise between maximizing high voltage battery life, HV and 12V battery charging (and maximizing fuel efficiency)? Does lowering the cabin temperature, say in 5 degree increments, have a noticeable effect on fuel efficiency? Or do you set the interior cabin temperature based on the outside temperature?

    It is helpful to keep the battery temperature low, but if the air conditioner is on full blast then less energy is available to charge the HV and 12V batteries.

    On the other extreme, if the cabin temperature is too hot, then the HV is not cooled adequately and I believe the lifespan of the HV battery (and probably the 12V battery) is reduced.

    Thanks for sharing your insights and experiences.
     
  2. Mendel Leisk

    Mendel Leisk Senior Member

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    I found little drop in hybrid battery temperature with AC, monitoring with ScanGauge. Next to none, a degree or two at most. This was starting out with a warmed-up car that had sat in full sun on hot day, driving for about 15 minutes with just windows rolled down, then closing everything up and running AC set pretty cold for about 20 minutes.

    I think Myth Busters found better mpg without AC, just running the windows right down? Maybe depends on the car too?

    Through summer we mostly just control cabin temp by rolling down windows, as much as needed. Our trips are mostly local, not long highway drives, and our climate is more temperate. We do run the AC periodically, which is a good idea to keep the oil in the system circulated.

    We probably use the AC more in winter, in rain, to prevent window fog up. I've noticed if vent mode is set to heat/defog, AC will sometimes run, even though the AC button is not lit. Again this is monitoring with ScanGuage.

    How many miles on your 2011? It's definitely worthwhile to dig down to the hybrid battery fan, especially that first time so you know the condition. Watch @NutzAboutBolts video, pinned at top of 3rd gen maintenance forum. One comment: you can avoid pulling off the rear seat (shown in the video); push apart the seat and back cushions, just below the intake grill, and reach through with maybe a 6" and 3" extension on a ratchet, and a 12 mm socket (IIRC). There's one bolt you need to remove. Be careful not to drop it.
     
    #2 Mendel Leisk, Jul 26, 2019
    Last edited: Jul 26, 2019
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  3. bisco

    bisco cookie crumbler

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    there an old priuschat saying, 'if you're comfortable, so is your battery'. however, this does not apply to those who like heat :cool:
     
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  4. Raytheeagle

    Raytheeagle Senior Member

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    I enjoy when old Prius Chat parables come out:).

    What other ones you got:whistle:?
     
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  5. fuzzy1

    fuzzy1 Senior Member

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    I regard driver-passenger comfort as the primary concern, heat stress has significant health and mental function downsides. Not being a hot climate person, I need to keep my own body functioning well. That should also take care of the big battery.
    The engine will keep those batteries charged up regardless of AC setting. (Aside from the brief time spent in Neutral in tunnel car washes. ;)) The only 'less energy' you will see is the remaining fuel in the gasoline tank, and that will be quite small compared to the benefits of AC.
     
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  6. Mendel Leisk

    Mendel Leisk Senior Member

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    Meh, do yourself a favour and test these maxims.
     
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  7. bisco

    bisco cookie crumbler

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    i agree with testing. i would expect different climates to yield different results
     
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  8. CR94

    CR94 Senior Member

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    Interesting question. The main compromise is between comfort and big-battery life on one side, versus fuel consumption and compressor life on the other. (12-v life is not much of a factor.). You need to run ac at least enough to keep battery temperature within safe limits.

    Open windows use more energy than ac only at highish speed, in general. How about partially open?

    The battery is slow to respond to changes in interior temperature, which might be why Mendel doesn't notice the correlation.
     
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  9. Grit

    Grit Senior Member

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    :cautious:
     
  10. Mendel Leisk

    Mendel Leisk Senior Member

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    No kidding: I've done a couple of dr Prius readings now, and even after overnight sit the battery is 4~5F above ambient in garage. Unless the sensors read high. Sensor 2, in middle of pack, is still higher than 1 and 3.
     
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  11. schmuber

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    Same here. I plan on replacing the shocks and cleaning traction battery fan next weekend (if weather permits), and since I'll have the battery accessible as a result, I plan on taking a few FLIR photos of it… just to see how accurate those sensors are.
     
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  12. Rebound

    Rebound Senior Member

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    Keeping the windows open should keep the cabin air filter cleaner, not dirtier. The cabin air filter cleans air coming from outside the vehicle into the vehicle. I don’t know if it filters recirculated cabin air, but it definitely filters the air from outside the cabin.

    Anyway, the thing that will really trash a cabin air filter is letting leaves or other plant material to fall and remain onto the cabin air intake vents along the edge of the windshield. That material will release sap that will just make a sticky gross coating through the air intake and onto the filter.
     
    #12 Rebound, Jul 27, 2019
    Last edited: Jul 27, 2019
  13. jerrymildred

    jerrymildred Senior Member

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    My comfort and the battery's comfort have about the same relationship as a pilot's safety and that of his passengers. :) I set the temperature to a level that keeps me comfortable. The battery will be happy with that and so will I. If it costs an extra $0.00001 per mile, oh well.
     
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  14. StephenFromCT

    StephenFromCT Junior Member

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    I am still enjoying the process of learning about how Prius cars work.

    I went on my first long drive yesterday. Over several hours I increased my fuel efficiency from 45 miles per gallon (at the start of the trip) to 50.1 miles per gallon (at end end of the trip). How does the fuel efficiency calculation work? To have a current fuel efficiency of 50 mpg, does it mean that yesterday I was averaging MORE than 50 miles per gallon?

    And yes, I was driving with the windows closed and the cabin temperature set to 80F.

    @Mendel Leisk, thank you for sharing that your HV battery temperature readings using Dr. Prius. If the car is not used for more time, say a full day, is the battery temperature still higher than the ambient temperature? For the ambient temperature does Dr. Prius report the same temperature as the car's display? Do other members who have Dr. Prius see the same elevated temperature?

    I did not know about the Myth Busters experiment. I just searched google using the following terms: myth buster air conditioner car windows

    I did not try to find the original show, but I read reviews of the season 3 episode where Myth Busters compared AC use (on/off) vs windows (up/down). In their comparison, which used only one vehicle, I assume they had the AC on full blast (and therefore continuously on and not related to the ambient temperature). In their first comparison they reported only three combinations, of the possible four, but they did not report the fuel efficiency for AC on and windows down. Probably not very informative, but I am curious.

    Apparently, they also tried a second test where they used two different vehicles. What they did NOT show in the second test is that both vehicles get the same fuel efficiency.

    One question I have is whether the vehicles used in the Myth Busters test have AC compressors which runs via an engine belt vs the electrical AC compressor in the Prius. Also, does belt tension matter? Belt tension makes a difference with non-hybrid alternator efficiency, since a loose belt makes an alternator spin less. However, it may not make any difference to the original question about windows up or down vs AC.
     
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  15. Mendel Leisk

    Mendel Leisk Senior Member

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    This was a good Google search string for me:

    mythbusters ac vs windows down

    And I see now, at least a couple of videos. In one I hadn't watched before they use a single vehicle, and monitor mpg electronically. In that test, at 55 mph, windows down is not as efficent as AC. In another (the one I watched before), they use a measured amount of gas and run each vehicle till it dies, one with AC and the other with windows down. However, the two vehicles look to be different, and they don't clarify. Comments on the video mention this too.
     
  16. bisco

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    ask mendel what the ambient temp was during his experiment, compared to what you're driving in with the interior set to 80.
    i suspect 80 is not going to be good for the battery in the long run, but i haven't done any experimentation
     
  17. Mendel Leisk

    Mendel Leisk Senior Member

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    The battery tends to run markedly higher than ambient temp, any time. I've given up on ScanGuage mostly (it was apparently the cause of occasional Check Hybrid System warnings), but I can can recall driving home along the Barnet Highway (coastal highway, speed limit 80 kmh, aka 50 mph) on a cool evening, ambient temp around 20C at most, and the battery temp was steady around 35C (fan speed maybe 1).
     
    #17 Mendel Leisk, Jul 28, 2019
    Last edited: Jul 28, 2019
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  18. Grit

    Grit Senior Member

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    In order to keep battery around 80 in summer, your ac would be on full blast with hv fan speed set to 6 during 30 minutes of driving. A lot of us are doomed.

    The air duct thing I did does keep it at 80F w/ac on lowest speed.
     
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  19. Grit

    Grit Senior Member

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    test not relevant to our vehicle. The AC is belt driven and ours is electric, the faster the speed, the more the rpms so AC wont draw take much gas, on top of everything else.
     
  20. Rebound

    Rebound Senior Member

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    First, driving with the windows closed and temp at 80F is unnecessary and you’ll probably sweat inside your car and make it stinky and who wants that? You should be comfortable.

    Second, to measure your mileage, you shouldn’t rely on the Prius MPG meter. It’s not accurate enough. I’ve been doing this since I got my new 2010 Prius, and here’s the method:
    1) Fill your tank with Regular has
    2) Reset your trip ODO
    3) When you next refill (doesn’t matter how much fuel is in the tank - Empty, Half, 3/4...) just fill the same way, to the first click off.
    4) Read out the number of gallons and the number of miles on your Trip ODO.
    5) Miles / Gallons = MPG. This is the accurate way.

    There’s a website and corresponding smartphone app called Fuelly.com which will help you track this. I highly recommend.

    In my experience... first, I think hypermiling is unnecessary and dangerous. But the main way you keep mileage high is you accelerate gently, you brake gently, and you keep your peak speed low.

    Braking gently serves many benefits:
    * Regenerative braking charges your battery, which boosts mileage
    * Regenerative braking uses no brake pads, so your brakes could last 200,000 miles
    * Gentle acceleration/deceleration is ideal for any vehicle. Every nut and bold is kept under lower stress if your drive your car gently.

    Drive with the HSI display. Brake within the left-most CHG zone as much as possible, but never to the point of endangering anyone. Accelerate within the ECO zone or without filling the PWR zone, as practical. This is the Prius self-learning indicator for high mileage driving. IMG_7231.jpg
     
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