Effect of 12-volt battery on mpg: 6mpg difference? Why?

Discussion in 'Gen 3 Prius Fuel Economy' started by Robert Holt, Jun 10, 2017.

  1. Robert Holt

    Robert Holt Senior Member

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    Recently replaced the original 12-volt battery on our 2012 Prius hatchback with another Toyota one from the dealer (68,000 miles).
    Old battery was indicating morning before-start voltages of 11.7-12.3 volts over a 2-week period according to the Scangauge II readout, so failing but not completely dead. For that 500-mile interval before replacement I was having trouble maintaining the MFD displayed mpg at 57 mpg, which turned into an actual 54 mpg at refueling, which is for me a typical 3 mpg difference between displayed MFD mpg and actual refueling mpg.
    Néw battery has been in for 2 weeks and 120 miles now, and the MFD displayed mpg is 63. So for the MFD displayed mpg I am getting a surprising 6 mpg difference.

    Question 1: Is that amount of a difference caused just by a deteriorated versus new 12-volt battery plausible? Have other Prius owners found that much of a difference in indicated mpg?
    To the best of my judgment, fuel and driving factors were as constant as possible in this comparison. All driving was without AC and the temp differences were small, from the 50-70 range on the last 6 weeks on the old battery to the 55-75 range for the last 120 miles on the new battery. Mix of driving was identical in that we have our normal weekly routine of 5-15 mile shopping and errand trips in suburbia, which has remained a constant routine for last 3 months.

    Question 2: if true, What exactly causes this amount of difference in mpg?
    My guess is that the low voltage spikes in the12-volt battery voltage readouts triggers Motor Gemerator 1 to be used more often as a generator, and thus the HSD system can only use MG 2 for propulsion , and the knock-on effect of that is increased use of the ICE in its less efficient regimes. Has anyone with Techstream verified the real causes of this mpg difference?

    I would volunteer to swap the old and new batteries in and out to do an ABAB type of test, but based on the other posts on this topic I was expecting a much smaller, barely noticeable 2-3 mpg difference, so I turned in the old battery at the dealership shortly after completing the swap.
    So do any of you have definitive answers to Questions 1 or 2?
    Cheers!
     
  2. ChapmanF

    ChapmanF Senior Member

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    Well, it's not going to have anything to do with "low-voltage spikes in the 12-volt battery voltage readouts" because in READY mode, that voltage is under strict control of the DC/DC converter and it doesn't change much.

    Now, it could have to do with the battery accepting a constant, unusually large charging current at that converter-controlled voltage. It would take somebody driving around with a Hall-effect clamp meter on the 12 volt battery lead and recording the current going into it. Multiply that by about 14 and you've got watts. Divide by 746 and you've got horsepower. It's simple physics and straightforward math, but it requires that current measurement with the bad battery. We can measure our good batteries all day long and we'll only see what the normal charging current is. The only measurement that will tell the story that needs to be told here is of the current into the bad battery of someone having reduced mileage.

    That, unfortunately, is the measurement that I have so far never, ever seen anyone make in connection with this mileage observation, and as you've turned in your bad battery, you won't be supplying it either. :)

    -Chap
     
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  3. Prodigyplace

    Prodigyplace Senior Member

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    To replace the 12 volt battery the old one was disconnected losing all custom settings. Perhaps the computer is relearning your car.
    After a few tanks it may settle back down.
     
  4. Mendel Leisk

    Mendel Leisk Sunday driver DIY’r

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    That's a good idea, but maybe too good to be true? I've completely disconnected a couple of times in recent months, once for brakes, once to install a charger quick connect.

    Subsequent to each episode I did notice some apparent recalibration behaviour: principally the engine doing odd revving up, just the first drive or two. Mpg seemed about the same though.
     
  5. bisco

    bisco cookie crumbler

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    interesting that you were getting morning readings that fluctuated by as much as .6 volts.
     
  6. 4md

    4md Member

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    Just before changing mine I got around 45 mpg on a 340 miles highway trip. After I replaced the 12v battery on the same route I got 55 mpg. All these numbers are calculated when topped at the gas station, not from the trip computer. Both trips were on cruise control set to 70mph, with AC on and in "ECO" mode.

    When replacing the battery, I was too lazy to set all the settings on the navigation again so I swapped them as fast as I could and it didn't reset anything.
     
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  7. Robert Holt

    Robert Holt Senior Member

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    So for you for a 340 mile trip under as-similar-as-possible conditions, a change from old to new 12-volt battery created a 10 mpg difference as measured by refueling mpg. Interesting!
    I also did the disconnect-to-reconnect part of the swap in under 10 minutes as the total time elapsed from "clearing all junk from the storage area" to "restoring all junk to the storage area" was a tad under 30 minutes.

    I am trying to understand this phenomenon (if it is real) better and have kept one field of the Scangauge monitoring voltag. Since installing the new battery, the voyages whilst driving seem to settle on two modal points, one being 13.6 (+ or - 0.1 volt) and the other being 14.4 (+ or - 0.1 volt). The shift from one modal point to the other seems to depend on total draw of accessories used at the moment, and possibly the SOC of the 12-volt battery. Does anyone know if there are any control rules or logic for the output from the DC-DC inverter? Or is it simply the balance of supplied electrical power versus current draw?
     
  8. CR94

    CR94 Senior Member

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    I believe that depends on state of charge and temperature.
     
  9. Robert Holt

    Robert Holt Senior Member

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    Thank you!
    Ok, I removed and reconnected that temperature probe on top of the 12-volt when I replaced the battery, so that info is clearly available to the computers. But would you have any knowledge, or best guess, about how the the SOC and battery-area temperature combine to determine the 14.4 versus 13.6 shift from the DC-DC converter?
    Could it be something as simple as :
    "SOC < 80% or temp < 10 C set output voltage at 14.4 , otherwise 13.6."?
    Still trying to understand this vehicle!
     
  10. CR94

    CR94 Senior Member

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    That might be part of it. I see 14.4 or so the first few minutes after every cold start, even in warm weather, but for a shorter time.
     
  11. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    There are failure modes of the 12 volt battery that result in huge current draw and therefore power usage. More power is less MPG. There are 6 cells in a 12 volt battery, if one short circuits, the battery can only charge to 10 volts, but the DC to DC converter will try for 12 forever.
     
  12. ChapmanF

    ChapmanF Senior Member

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    While I don't question the existence of such failure modes, I still think it would benefit the PriusChat community if, one day, some member or members who believe that to be the problem would also take the current measurement that would confirm or disconfirm it. So far in all my years of PriusChat lurking, I haven't seen that step taken.

    -Chap
     
  13. ChapmanF

    ChapmanF Senior Member

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    It turns out at least one person has made such a measurement, three years before I posted that, even; I just hadn't stumbled on it until now.

    Measured 60 to 70 amps initially into a 60-to-70-percent discharged battery (the post doesn't explain how that determination of percent discharged was made). Slightly under a kilowatt (at around 14 volts), or about a horsepower and a third.

    Now, with anything like a sustained current of that magnitude into a battery (such as with a shorted cell), I'd expect substantial and noticeable amounts of heat and outgassing. That's nearly like having a regular household space heater tucked in the corner of the trunk.

    -Chap
     
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  14. csk47

    csk47 Junior Member

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    My 2016 Prius II has about 23,000 miles. I live in south Florida and the car has always been driven within a 100 miles of home so no cold weather driving. My driving is always within our community or nearby roads with almost no interstate driving. Since we bought the car new in July 2016 I had been getting 52 mpg as verified when we filled up the tank...until about March of this year when the mileage dropped to about 48 mpg. The original equipment tires had to be replaced in mid-April so I switched to Continental True Contact tires that are supposed to be excellent for mileage. After that my mileage dropped to 42 mpg. Having had a 2010 Prius I know changing tires can make a mileage difference. The dealer supposedly scanned for error codes when the car was taken in for an oil change on June 29 and nothing was found..so they are at a loss for the drop in mileage. Can it be the 3 year-old 12 volt battery is weak and is bringing down my mileage? The dealer doesn't seem to put much credence in that theory. He talks about cleaning the fuel injection, etc. Any advice is greatly appreciated.
     
  15. ASRDogman

    ASRDogman Senior Member

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    The 12v might be lower if you don't drive long enough to put a full charge on it.
    But probably not low enough to effect fuel mileage. You could buy an AGM battery charger and
    charge the battery overnight, then you'll know it is fully charged.

    As tires wear out, fuel mileage will drop, which seems to have happened with you because you
    said they needed to be replaced just after the mileage dropped.
    I have the same tires on my 2010. They are a little noisey, but are great tires! At least for me!
    I average 47mpg but have gotten 55mpg for mostly highway driving. I run 36psi front and rear.
    My average city driving is usually about 40-43mpg. There are a lot of stop signs and red lights.
    And other drivers so I can "ease" off the line using only electric very often. But it's a lot better
    than the 16mpg I get in my van!
    They really gripped on the drive across the state in pouring rain and flooded roads!!!
    According to others here, their mpg is lower than other tires, which cost twice as much, and don't
    last as long. I have just over 55,000 miles on the tires and they are just showing signs of wear.

    I use Marvel Mystery Oil in my fuel tank every 10 fill ups or so. It does a great job, and I've used
    it on other vehicle. You can also buy a bottle of fuel injector cleaner and put it in the fuel tank the
    next time you get fuel. What kind of fuel are you using? 7-11? Cumberline Farms???? I've been using
    Sunoco in all my vehicles and have never had a problem. I try to fill up at the same pump each time.


    Have your driving habits changed? Even slightly will effect fuel mileage. With only 23,000 miles, nothing
    should need to be cleaned. Maybe throw a bottle of Marvel Mystery Oil or Injector cleaner in it the next time
    you fill up and drive it on the high for a while. But the car a chance to fully charge the 12v battery and clean
    out the injectors, if they are dirty.
     
  16. Vman455

    Vman455 Senior Member

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    As tires wear, their rolling resistance decreases and fuel economy will usually improve. That's why replacing tires often comes with a hit to MPG.
     
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  17. ASRDogman

    ASRDogman Senior Member

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    Less tread, less grip, worst on wet roads. More energy needed....
     
  18. Mendel Leisk

    Mendel Leisk Sunday driver DIY’r

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    I've had tires changes that made no difference. Then again, having just put on new Michelin Primacy MXM4 (17 inchers) this spring on our 2010, there was a solid 10% hit. Very depressing. Each tank it's picked up a bit; this latest tank is somewhat promising.

    Bottom line: it could just be the tires; I'd give it time.
     
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  19. ASRDogman

    ASRDogman Senior Member

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    I will be buying the Continental True Contact again! Mileage is good, wear length is great.
    Dry road grip is great, wet road grip is great. Price was great!
    The radio is loud enough to drown out any tire noise, so I'm happy! :)
     
  20. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    csk47,

    Try rolling down the windows without being in Ready. (Don't press brake while hitting Power) time them then go to Ready and time them again. If there is a marked difference, you may have a weak battery.

    Get a Multimeter and get numbers at the battery.
    Off:
    Acc:
    Ready:

    www.amazon.com/AstroAI-Digital-Multimeter-Voltage-Tester/dp/B01ISAMUA6
     
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