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Battery Volt Drop

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by red07gen2, Jan 8, 2016.

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  1. red07gen2

    red07gen2 Junior Member

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    I swear I looked on the forum and could not find an answer.
    the other day I was messing with something in the prius and I pushed the power button from the passanger side to turn on the accessory mode . So the engine is not powered , I have a radar detector plugged in to the power plug and it reads the battery volts going to it . 13 is what i see all the time but in a matter of 30 sec. or so it went to 11. something and the low battery alert started beeping ... Car starts up fine every morning and I have no problem with any thing except if left in accessory mode the 12v battery drops QUICKLY. Is this normal (i dont think so) but what is the experiance others have ...
    Thank you
     
    #1 red07gen2, Jan 8, 2016
  2. bisco

    bisco cookie crumbler

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    hard to say. why not put a volt meter on the battery, so you don't get stranded. how old is it?
    it doesn't take much to power up the computers, but when you put a load on it, that tells the real story.
     
    #2 bisco, Jan 8, 2016
  3. edthefox5

    edthefox5 Senior Member

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    You have a soft 12 volt battery so in ac mode no charging voltage your reading the real load voltage which is pretty low.

    Get a voltmeter and read the jump points under the hood after pushing the READY button twice no brake that's ignition. Its probably at 10.9 pretty dead. Right around that voltage the only thing the car does to warn you is throw a big red exclamation point on the dash at READY boot up. Only there for a second but hard to miss. Its the only thing that does not come on with bulb check READY boot so quite noticeable when it appears for a second. It does not log a dtc either. Its a courtesy alert hey get ready to be stranded lol.


    Time for a new battery. Its getting cold you know.
     
    #3 edthefox5, Jan 9, 2016
  4. wjtracy

    wjtracy Senior Member

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    ...sounds like a new 12v may be needed...keep in mind the 12v does not recharge all that quickly in a Prius really need some longer trips if you got low for some reason. The other thing I am going to do next time when I get a new 12v battery is make sure its full charged before I install.
     
    #4 wjtracy, Jan 9, 2016
  5. red07gen2

    red07gen2 Junior Member

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    well the voltage never dropped below 10.9 with all the load i could put on it ...accessory button, headlights, fog light bar, scanner, radio , dashcam, plugged in couple things into the cig. plug still holds at 10.9
    But everyone thinks the battery is getting dead SOOOOOOOOO
    ordered the optima yellow top and it arrived tonight, put it on the charger in the morning and install tomarrow afternoon...Will see if this makes a difference ...
     
    #5 red07gen2, Jan 19, 2016
  6. bisco

    bisco cookie crumbler

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    maybe you should have charged it first to see if it would hold.
     
    #6 bisco, Jan 19, 2016
  7. red07gen2

    red07gen2 Junior Member

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    well maybe but with 188thousand on the clock and this is the original battery ....time is about up for it ...and how can you go wrong with an optima?
    Plus always use the battery in other places for a stand along 12v power system ....
    But your right might have thought to put the OEM on the charger overnight
    but I would have thought that with the amount of running around I do (feel like a taxi driver sometimes) and yep I leave the prius running , heck I dont even turn it off at the gas station .....And sometimes I jump out and forget to turn it off till I am at my daughters practice and I hear the car fire up ...oh I forgot to turn off the prius.
     
    #7 red07gen2, Jan 19, 2016
  8. bisco

    bisco cookie crumbler

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    yeah, your probably right.
     
    #8 bisco, Jan 19, 2016
  9. sillylilwabbit

    sillylilwabbit Active Member

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    How about 11.1v under load: headlights, rear defroster, and full blast heater

    4 years old and 38k on the clock.

    Is 11.1v still ok?


    iPhone ?
     
    #9 sillylilwabbit, Jan 19, 2016
  10. S Keith

    S Keith Senior Member

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    Probably, but I would regard it as marginal. I'm jaded here in AZ. Regular batteries don't last longer than 2-3 years.
     
    #10 S Keith, Jan 19, 2016
  11. red07gen2

    red07gen2 Junior Member

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    New optima battery in . Been snowing for the last few days so the prius has been hiding in the garage . Been driving the 4wd to run fire calls.
    Will check it out on mon ...will run the prius to school to drop off the daughter .
     
    #11 red07gen2, Jan 23, 2016
  12. red07gen2

    red07gen2 Junior Member

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    Well damn what a difference. I drive about the same route every week at about the same time every week.
    before battery install last 4 fill ups : 41.1, 41.1, 38.7, 37.2...First fill after install 42.1, and the on-board is showing this tank is 100+ miles into it at 45mpg.....Guess the ICE was running longer to compensate for the battery, to keep it charged up.
    It least I hope, because this MPG I like.
     
    #12 red07gen2, Jan 29, 2016
  13. bisco

    bisco cookie crumbler

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    interesting. a lot of 'experts' here claim that it's impossible for a bad 12v to noticeably reduce mpg's.
     
    #13 bisco, Jan 29, 2016
  14. DRACO

    DRACO Member

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    Logically it should, shouldn't it, since the inverter has to constantly charge the weak battery, taking away power from the HV battery in turn ICE needing to be turned on resulting in lower MPG.
     
    #14 DRACO, Jan 29, 2016
  15. red07gen2

    red07gen2 Junior Member

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    the computer is showing me around 45-46 mpg and just under half a tank . will see the actual mpg when i fill and do miles to gallon math .
     
    #15 red07gen2, Jan 30, 2016
  16. red07gen2

    red07gen2 Junior Member

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    filled up today 51.39 MPG ...yep the 12v aux battery does nothing to the fuel mileage...
    Just hope this keeps up ....heck the 30 + miles I put in it so far still in the 46-47 mpg range
     
    #16 red07gen2, Feb 1, 2016
  17. DRACO

    DRACO Member

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    Still not bad at all, she's a sipper :)
     
    #17 DRACO, Feb 1, 2016
    Last edited: Feb 2, 2016
  18. TheNovice

    TheNovice New Member

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    Got my wife to check the voltage on her one just there. Had to give her the instructions over the phone! Pulling teeth would be easier!

    Anyways, loading the battery with dipped lights, a/c and rear demister on, the voltage read 10.5V. Reckon the battery is fairly goosed, so will go on the hunt for a new one. Still on the original battery (2008 car), so can't really complain!
     
    #18 TheNovice, Feb 2, 2016
  19. DRACO

    DRACO Member

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    +1 10.5v is not good, inverter should help keep up with the voltage requirements but what is it at resting after 30-60 minutes? If it recovers to 12.3+ volts, it may still be good.
     
    #19 DRACO, Feb 2, 2016
  20. sillylilwabbit

    sillylilwabbit Active Member

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    That is pretty good for an 8 year old batter, but it is time to look for a new battery.

    My load test was 11.1v with headlights on, rear defroster a and full blast heater on, on a 3 1/2 year old battery.


    iPhone ?
     
    #20 sillylilwabbit, Feb 2, 2016
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