DROPPING MPG AND SELF-TEST BATTERY READINGS

Discussion in 'Gen 3 Prius Main Forum' started by Anonymous!!, Nov 23, 2022.

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  1. Anonymous!!

    Anonymous!! New Member

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    2010 with 126,000 miles. MPG seems to be steadily decreasing:
    • July 2019 to Feb 2020 - 5,667 miles - MPG 48 (ending odometer 105,628)
    • 20 July 2022 to current - 4,636 miles - MPG 44
    • 26 Sep 2022 to current - 1,229 miles - MPG 38
    Put car through battery self-tests today:
    • ignition and all systems off -- shows 11.5
    • after starting car -- immediately drops as low as 10.7 but within maybe 10 seconds rises to 14.1
    • driving around for an hour or so after that -- goes to 14.1 but varies and sometimes gradually drifts down to 13.2
    • turning car off after that -- drops to 12
    What do you guys think -- do I need a new battery or batteries?
     
    #1 Anonymous!!, Nov 23, 2022
    Last edited: Nov 23, 2022
  2. ASRDogman

    ASRDogman Senior Member

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    How old is the 12v battery? Do you have a 4amp or less battery charger?
    You could put it on the charger overnight and give it a full charge.
    The 14.1 is what the inverter puts out. 12v is very low for a Prius battery.
    If it is the original battery, you probably should replace it. And make sure you charge
    the battery before putting it in. NONE of the stores keeps them charged.
     
    #2 ASRDogman, Nov 23, 2022
  3. Anonymous!!

    Anonymous!! New Member

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    Thanks. 12V is very low for a 12V Battery? So I guess 11.5 means it's just about dead? What is a 12V Battery supposed to put out when the car is shut down?
     
    #3 Anonymous!!, Nov 23, 2022
  4. bisco

    bisco cookie crumbler

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    how old is it?

    you can get a free load test at an auto parts store
     
    #4 bisco, Nov 23, 2022
  5. Anonymous!!

    Anonymous!! New Member

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    Battery was new in February 2016 at about 65,000 miles. So it's about 6 3/4 years and about 61,000 miles old.
     
    #5 Anonymous!!, Nov 23, 2022
    Last edited: Nov 23, 2022
  6. bisco

    bisco cookie crumbler

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    yeah, just replace it. it was a good run
     
    #6 bisco, Nov 23, 2022
  7. Anonymous!!

    Anonymous!! New Member

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    Note correction: it's actually only 6+3/4 years and 61,000 miles old. Still junk it? If I do, do you expect that to increase my MPG?
     
    #7 Anonymous!!, Nov 23, 2022
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  8. bisco

    bisco cookie crumbler

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    i would have it tested. you might get a few more years.

    can't guarantee mpg's, there are so may things on a prius that affect gas mileage
     
    #8 bisco, Nov 23, 2022
  9. ASRDogman

    ASRDogman Senior Member

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    Yes, for a Prius battery. 12.3 or 12.4 seems to be boarderline.
    Seeing that it is over 6 years old, replace it. Toyota gives me a 10% veterans discount.
    It's probably the BEST one for the Prius, since that is what it's designed for.
    Just make sure you charge it for at least 6-8 hours before installing it so it's got a full charge.

     
    #9 ASRDogman, Nov 23, 2022
  10. Anonymous!!

    Anonymous!! New Member

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    Thanks -- but about the charging: I'd have a shop do it, along with the other work I'll probably be doing. So how would I be certain that they charged it first?
     
    #10 Anonymous!!, Nov 23, 2022
  11. ASRDogman

    ASRDogman Senior Member

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    Well, watch them put the charger on it? Or trust them. When I got mine, It was at 12v.
    Maybe have then charge it for 4 hours at 4 or less amps. Do not let them FAST charge it!
    AGM batteries don't like higher amps.

    I had brought my charger because I already knew it wouldn't be charged.
    I just installed it, put everything back together. Just for gp's, I tried to start, but nope!
    So I put the charge in the fuze box under the hood, bout 3.5 hours, because I had to go somewhere.
    But it was enough to start the car. And I had my jumper, just case.
    I put the charger on it over night when I got home.

    Walfart has a 4amp charger for about $25-30. I forget the cost. Do not charge the Prius battery
    with more than 4 amps. You can damage it.
     
    #11 ASRDogman, Nov 23, 2022
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