Bad Gas mileage - 12v battery solution

Discussion in 'Gen 3 Prius Fuel Economy' started by karl.d, Jan 15, 2019.

  1. karl.d

    karl.d Junior Member

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    2011 Prius , with 172+K miles (95% Hwy miles), suddenly dropped gas mileage from a consistent 49mpg to a range of 38 to 43 mpg. There was no warning lights or codes on the dash.

    The other behavior was the HV battery display on the dash rarely fell below 6 bars and almost always at full 7 bars. ICE seemed to run more than what I remember. No apparent HV or 12v battery issues, even in 18F weather.

    Only items changed was 10K oil change at dealership and new tires. Replaced Michelin Defender XT (lasted for 90K) miles with Michelin Defender T + H. Selected Michelin Defender to avoid tire difference issues.

    Toyota dealership refused to investigate or troubleshoot low gas mileage unless there was a warning light on code illuminated on the dash.

    Thanks to a thread on PriusChat, I replaced the Aux 12v battery on a whim. 12v battery in car was the original one that came with car in 2011 and now had 172+K miles on it. I did not bother to check battery voltage as I do not have a meter

    Selected ACDelco ACDB24R at $150 from online giant.

    Mileage is now back up to 48mpg.

    Thank you.
     
  2. ASRDogman

    ASRDogman Senior Member

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    Original battery, 8 years old? Should have been your first suspect. :)
    Might be worth it to spend $20-30 and get a volt meter.
     
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  3. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    It is odd but true that some failure modes (certainly not all) of the 12 volt battery involve continuous, fruitless, charging. For a Prius, that is power that might have propelled the car down the road.
     
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  4. bisco

    bisco cookie crumbler

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    well done!(y)
     
  5. karl.d

    karl.d Junior Member

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    I do not know how to edit my post, but I also wanted to add a couple more observations of the 12V battery at issue, in this particular case (if it helps others).
    1. In trying to run in "EV mode" on a flat surface under 25mph, the car would start running in EV mode, but as soon as battery level dropped below level 6, it would cut out EV mode with "EV mode not available" and startup ICE.
    2. Also, sometimes the FOB would sometimes not lock the car from familiar distances, even though I replaced battery in FOB.
     
    #5 karl.d, Jan 15, 2019
    Last edited: Jan 15, 2019
  6. karl.d

    karl.d Junior Member

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    At 160K, toyota dealer checked 12v battery levels and did not find problem. Given that I got 70K miles out of the first set of Yokohamas and am still running on the original brakes, I had no reason to doubt the 12v battery. "If I knew then what I know now ..."
     
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  7. Brenden Truman

    Brenden Truman New Member

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    I have had similar problems that karl.d described with a sudden drop in mpg's, correlated with weird behavior of the hybrid system keeping the ICE on for a very long time (much longer than the normal heating up the engine time in the winter). I'm trying to decide if it's worth the $160 to buy a new 12V, or if that will fix the problem at all. Does anyone have any suggestions for me, or explanations as to why I could see a 10-15 mpg drop off with all other variables (tire pressure, oil level, etc) the same, as well as the weird behavior of the hybrid system?
     
  8. ASRDogman

    ASRDogman Senior Member

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    If you scan prius chat, you'll see approx. 5 years is the "average" age of the 12v battery.
    I am NOT saying replacing it with a NEW one will resolve your problems, but from what I've read from other with
    experience with certain problems, and OLD 12v batteries cause a lot of weird things to happen.
    Just because a check says the battery is good, doesn't actually mean it is good. HOW did they check it?
    Did they check it correctly for a Hybrid vehicle, or a standard vehicle? There seems to be a fine line of
    a good battery and a bad battery. It might of barely tested good, but since it's 8 years old, I would not trust it.
    If it was not a hybrid, I would say okay, just carry a jumper with you.

    But it's your car.
     
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  9. PosauneGuy

    PosauneGuy Member

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    So out of curiosity, why would the 12v battery have such an impact on MPG? I thought that the 12v battery was only used to get the car started up.
     
  10. Grit

    Grit Senior Member

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    It does need to be charged also when engine is ready. If 12v batt doesn’t hold charge anymore, hv battery and or regen will send power to keep charge 12v bat constantly thus lowering mpg, inverter won’t get optimum power to haul butt because 12v is some sucking regen power so ice stays on lingerie.
     
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  11. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    All the cars accessories are 12 volt, very little is 201 volt, but the engine starter is 201 volt. The Inverter charges the 12 volt battery between 13.8 and 14.5 volts.

    Imagine for a second that one of the 6 cells in the 12 volt battery short circuits, you now have a 10 volt battery. That is still enough to start the computers on a good day. (The Prius is never really off, the 12 battery drains when idle) The inverter is now pushing as much as 5 volts more than the battery will charge to. The max charge is about 10 amps, so you are using 50 watts to heat the battery instead of charging it. This may eventually run your battery out of water, but it is an electrical drain all the time you are in Ready.

    There are other ways for a 12 volt battery to fail, so it is not always going to make a difference,

    8 common car battery problems | Car batteries | The NRMA
    BatteryStuff Articles | Common Causes of Battery Failure (as an aside, if you read this, you realize why Toyota put the batteries where they were air conditioned)
    What Causes a Car Battery To Fail? – Auto Pro Auto Parts
     
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  12. karl.d

    karl.d Junior Member

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    I believe Amazon has a liberal return policy with return shipping.
     
  13. Mendel Leisk

    Mendel Leisk Senior Member

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    So after settling the battery issues, would you say the Defender T+H are delivering similar mpg as the older version Defender?

    I noticed the older Defenders used to have a Green-X badge, and the new T+H say Total Performance. Kinda thinking it's the same rolling resistance, just marketing smoke-and-mirrors, but wondering.
     
  14. karl.d

    karl.d Junior Member

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    As I recall, after replacing XT with T+H , mpg was similar, at least nothing significant that caught my attention. It was shortly after the 10k oil change, that I noticed sudden low mpg .

    A correction, I got 100+K (171K - 70K) out of the XT with legal tread to spare. Only replaced tires as an abundance of caution (in certain rural sections of NV, UT, NM, WA, ID, WY, MT cell phones do not work, even though I have Verizon & Tmobile). Also, when I went from original Yokohamas to the XT at 70K, I noted a permanent loss of 2 - 4 mpg.
     
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  15. eliotb

    eliotb Junior Member

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    I'm posting what may be the same question re a 2014 P-Five. In the past month, with temps at 100 or more for one week, my mileage dropped into the mid 40s. Next tank was roughly the same though temps dropped to upper 80s to low 90s. I have twice checked the 12v battery per guidance. My test today showed 12-12.1v when I activate the test program with the car cold, doors closed. I think the first one was showing about 11.7-11.9. Once I started the car today with the test program still on, the voltage popped up into the 14 range. Am I correct that most likely the issue is an old 12v battery? The build date on this vehicle is 2/2014, so in theory the battery is roughly 5.5 years old.

    Thanks to anyone who offers some concrete guidance.
     
  16. Mendel Leisk

    Mendel Leisk Senior Member

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    It's hard to evaluate battery life base on the car's displays. At the least, check with a digital volt meter, first thing in the morning, at the under hood jump point is fine. Preferably with the hood popped the the evening before (if practical), since just opening the car to pop the hood seems to "wake it", and drop voltage measurements by 0.1~0.2 volts.

    To do your own diagnostic, look into something like Solar BA9, an electronic load tester device. They're around $50.
     
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  17. Grit

    Grit Senior Member

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    5.5 years old + measured below 12v = replacement battery needed

    Anything below 12.6v is showing battery wear overnight w/no load, and your below 12.0v. I’d shop around...now in your shoes
     
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  18. karl.d

    karl.d Junior Member

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    I have not measured my battery output (replaced battery earlier in the year), but in the past 2 weeks, 98+ degree temps, AC has been running almost continously, freeway driving with some altitude (CA Hwy 395 Eastern Sierra Nevada ranges) and my mileage is in the very low 40s.
     
  19. eliotb

    eliotb Junior Member

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    Thanks. Mileage has popped back up to the 50-ish range now that temps are down and I have no other symptoms of battery issues. However, I have a service coming up shortly and I’ll just ask my mechanic to replace the thing. I appreciate the input.
     
  20. eliotb

    eliotb Junior Member

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    Temps are now down into the low 90s at most and the mileage is back up in the 50-ish range. I did turn up the AC set-point to about 80 to reduce the load. That said, I have a service coming up and I’ll just get my mechanic to replaced the battery while he’s at it. Better safe than .... etc.
     
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