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

DC buss drops to 13.4 volts

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by koolingit, Apr 10, 2010.

  1. koolingit

    koolingit Member

    Joined:
    Mar 17, 2009
    158
    50
    0
    Location:
    Mocksville NC, USA
    Vehicle:
    2009 Prius
    I've been coming here since I got my Prius in December of 09. Up till now, I've been getting my answers by searching. I've learned a lot. This time, however, I don't see my problem.
    For the most part, my dc buss stays at around 14 volts but lately, I've noticed the voltage at 13.4. I notice this after pulling into a parking space. The car is still ready and in drive and it stays at a solid 13.4 volts. If I shift to park, the voltage jumps back up to 14 -- put it back in drive (or reverse) and it drops to a solid 13.4. If I shut down and go back to ready, I have a solid 14 volts and can do nothing to get it to drop back down to 13.4. This bug shows up when it feels like it.
    The next time I noticed the 13.4 volts, I fooled around and this is what I was able to tell:
    o The car has to be in drive or reverse for the voltage to drop. o AC auto must be off. Switching it on brings the voltage up to 14 -- off and it drops back down.
    o Radio or headlights have no effect.
    o Rear window defogger brings voltage down to 13.2 volts.
    o Driving does not bring the voltage up. It stays at 13.4
    o Shifting to park or neutral brings the voltage back to 14.
    o Turning ac auto on brings the voltage back to 14.
    o Shut down and restart and I have a solid 14 volts no matter what I do.
    Voltages are via scan gauge, volt meter at jump terminal under hood and MFD (signal test). They all agree within .2 volts. Battery is at 12.2 volts after overnight rest and before powering up. I removed it and charged it for 36 hours at 2 amps. The battery terminals and body connection for negative cable are clean and tight. Sorry, I don't know what the voltage was right after charging but for a while, I'd see 12.4 volts before powering up.
    If I could reproduce this problem (?) at will, I'd bring it to the dealer but every time I'm in the neighborhood, the scan gauge says 14 volts! O yeah, I tried disconnecting the scan gauge and that did not help.
    Any suggestions or advice?
     
    #1 koolingit, Apr 10, 2010
    1 person likes this.
  2. qbee42

    qbee42 My other car is a boat

    Joined:
    Mar 2, 2006
    18,058
    3,073
    7
    Location:
    Northern Michigan
    Vehicle:
    2006 Prius
    12.2 volts at rest is low for a 12V battery, but not a complete red flag. The DC to DC converter should be able to maintain the 12V bus in the situations you describe. It sounds like something is causing the DC to DC converter to vary its output. When you increase the load on the HV system, the 12V bus voltage jumps back to where it should be.

    This one is a bit of a puzzle. I would be temped to disconnect the 12V battery and see how the bus voltages behave without it.

    Tom
     
    #2 qbee42, Apr 10, 2010
    1 person likes this.
  3. hschuck

    hschuck Member

    Joined:
    Mar 27, 2005
    385
    17
    0
    Location:
    bay city, ca
    Vehicle:
    2005 Prius
    I have experienced the same thing in my 2005. I first noticed it a couple of years ago but don't know when it started as i didn't and don't normally monitor battery voltage while driving. (I have the original scangage with the upgrade to make it compatible with hybrids.)
    I replaced the original battery with an Optima about a year ago which had no affect on the fluctuation of the 12v bus. I just learned to live with it and hadn't even thought about it recently until this thread. The next time I am out I will check to see if the bus still fluxuates.

    I had previously determined that the car had to have completed all initial warm up sequences and some significant load applied such as the air circulation fan, or auto a/c to cause the bus to jump back up to 14v. I will also try the defroster, I hadn't done that and had never seen a lower bus voltage beyond the single step below 14v.
     
    #3 hschuck, Apr 10, 2010
    1 person likes this.
  4. koolingit

    koolingit Member

    Joined:
    Mar 17, 2009
    158
    50
    0
    Location:
    Mocksville NC, USA
    Vehicle:
    2009 Prius
    I'm a little scared to disconnect the battery while the car's running. I won't see any sparks, will I?

    If I do disconnect the battery, I'd probably just undo the bolt from negative cable to body. Does that sound right?
     
    #4 koolingit, Apr 11, 2010
  5. koolingit

    koolingit Member

    Joined:
    Mar 17, 2009
    158
    50
    0
    Location:
    Mocksville NC, USA
    Vehicle:
    2009 Prius
    What you describe pretty much sounds like me. I didn't think to try running just the fan to see what effect it would have. I did try the front defroster but that turns on auto AC which brings the buss back to 14 volts.

    It won't be long till I have the AC on full time and this won't be a problem till fall. I really didn't notice this low voltage till about two weeks ago. Now I'm going to be paranoid about it. Maybe it will drop when just the right combination of conditions exist even with the AC on.
     
    #5 koolingit, Apr 11, 2010
  6. uart

    uart Senior Member

    Joined:
    Sep 7, 2009
    4,215
    1,200
    0
    Location:
    Australia
    Vehicle:
    2005 Prius
    Perhaps the car has simply determined that the 12V battery is close enough to a full state of charge to temporarily let it float. I don't think there is necessarily a problem here.
     
    #6 uart, Apr 11, 2010
(You must log in or sign up to post here.)