1. Attachments are working again! Check out this thread for more details and to report any other bugs.

Decreasing MPG checked HV battery? Help

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Cregg101, Jul 5, 2014.

  1. Cregg101

    Cregg101 New Member

    Joined:
    Jul 5, 2014
    6
    0
    0
    Location:
    Raleigh, NC
    Vehicle:
    2007 Prius
    Model:
    ----USA----
    Hi Friends! I have read a LOT of what you have posted about the HV battery, and it is great info. This is where I am and I would like your advice. I have an 07 Prius great car I drive 92 miles/day round trip to work and back home everyday. I bought my Prius with 98-k miles and it would get 42 to 45 MPG all the time. I now have 172-k on it and it is still running great, but about 4 or 5 months it dropped to 36 to 38 mpg. I did the normal things, changed the plugs, air filter, checked tire pressure. No change in mpg. I decided it must be the HV battery. Because sometimes when I start it in the mornings the MFD only has the 2 pink or red bars showing but builds up to green within the first about 20 miles of driving. I was a mechanic (many years ago before hybrids) so I felt if so many other people can do this I can as well. So I went to You Tube (I guess that was my first mistake, wish I had found your site first). Everything I saw showed how to do the job and the main thing I was counting on, most everyone said you could easily tell which cells were bad as they would be much below the others in voltage). I have tested the 12v battery by the info I found here on your site with the MFD (that is GREAT info) it is good and my charging system is good.

    Here is my issue, when I separated the connections and checked each cell, all were between 7.35 and 7.38, so not much difference between any of them. So now I am wondering if all the cells are bad and I need to purchase a complete battery because all of them have a low voltage reading and there is 172-k on it. Or is there something else I could do to this one other than purchasing a new battery. Or is there something else I am missing?

    Your advice is much appreciated, Thanks!
     
    #1 Cregg101, Jul 5, 2014
  2. JimboPalmer

    JimboPalmer Tsar of all the Rushers

    Joined:
    Apr 14, 2009
    12,470
    6,871
    2
    Location:
    Greenwood MS USA
    Vehicle:
    2012 Prius v wagon
    Model:
    Three
    There are some failure modes for the 12 volt battery in which it constantly attempts to charge. This draws a lot of energy, and so lowers mileage. After the car has set for several hours (overnight is great) check the voltage of the 12 volt battery before starting the car.

    Details on how to do this vary between different models.

    Weird stuff happening? MPGs dropping? Test The Battery | PriusChat
     
    #2 JimboPalmer, Jul 5, 2014
    Last edited: Jul 5, 2014
  3. Cregg101

    Cregg101 New Member

    Joined:
    Jul 5, 2014
    6
    0
    0
    Location:
    Raleigh, NC
    Vehicle:
    2007 Prius
    Model:
    ----USA----
    I will check it in the a.m. thanks for your reply.(y)
     
    #3 Cregg101, Jul 5, 2014
  4. nh7o

    nh7o Off grid since 1980

    Joined:
    Dec 10, 2011
    1,686
    340
    0
    Location:
    Hawaii
    Vehicle:
    2009 Prius
    Model:
    II
    I would suggest that you note the instantaneous MPG that you see on the MFD while going at a constant speed of ~50mph on straight level road, after the car has warmed up completely, hopefully with no wind. In such a state there should be no arrows going to or from the battery on the energy display, or if there are, they may switch to and from the battery quickly (which means that the actual current flow is fairly small in that situation.) You should see something well above 40 mpg. 50mpg would be typical for a well running car.

    That test essentially takes the HV battery out of the equation, but tests the power train. If you see less MPG than expected, then there is something causing excess drag: tires, alignment, brake drag, or 12V battery failure as noted above. Or there is a problem with the engine showing reduced efficiency, which causes heavier demands on the HV battery to compensate.

    This is a guessing game with the growing reports of high mileage cars shedding MPG's. The important thing to remember is that the car is not powered by the battery. The only power source is the gas engine. If there is not varying speed or sloping terrain, then the car is no longer operating as a hybrid. So try to determine if the loss you see is due to poor power from the ICE, or poor energy recovery from the hybrid system while braking and accelerating.

    Without DTC's set, it is hard to know which. And yes, the car can produce real loss of performance before it will set an error condition. Things need to be pretty advanced for that to happen.
     
    #4 nh7o, Jul 5, 2014
    Last edited: Jul 6, 2014
  5. Cregg101

    Cregg101 New Member

    Joined:
    Jul 5, 2014
    6
    0
    0
    Location:
    Raleigh, NC
    Vehicle:
    2007 Prius
    Model:
    ----USA----
    Thanks for your reply, and coming from a person that has been "Off The Grid Since 1980" I'm sure you know something about alternative power. I will try these this week to see if I can narrow the issues which are causing my problem. Thanks for your time!
     
    #5 Cregg101, Jul 6, 2014
(You must log in or sign up to post here.)