Current draw from aux. battery when car is off?

Discussion in 'Gen 3 Prius Technical Discussion' started by derevis, Aug 26, 2015.

  1. derevis

    derevis Junior Member

    Joined:
    Dec 29, 2004
    5
    2
    0
    Hi!
    Does anyone know what the current draw should be from the 12V aux battery when the car is off? I am measuring 1.1A, with all interior lights off. This seems excessively high to me. Thanks in advance.
     
  2. mrbigh

    mrbigh Prius Absolutum Dominium

    Joined:
    Sep 6, 2005
    3,686
    690
    2
    Location:
    Long Island, NY
    Vehicle:
    Other Electric Vehicle
    Houston, we have a problem........
    At rest it should be in the order of milliamps, 15-20 if I'm not wrong
     
    derevis and Mendel Leisk like this.
  3. gdanner

    gdanner Member

    Joined:
    Jul 29, 2007
    67
    31
    0
    Location:
    Central Illinois
    Vehicle:
    2005 Prius
    Model:
    N/A
    I'll check the "off state" 12V current drain on my 2005 Prius and post the result.

    I've measured the 12V rail voltage numerous times but not the standby current drain (or charging current) for the 12V battery.

    I replaced my 12V aux battery yesterday. It read only 11.0V during "IGN-ON." Worse yet it dropped to 10.5 V when I turned on the headlights. My new 12V battery measures >12.0V under the same test conditions (IGN-ON and headlights on). I expect this may get a little higher after the new 12V battery charges fully (I didn't put a charger on it before I installed it). My old 12V battery was 5 years old. I'm glad I replaced it before it prevented my car from powering up. I used the O'Reilly "Sure Start" exact replacement 12V battery. It was in-stock at my local O'Reilly store. Price was $ 175.99.

    Super Start Platinum PRIUS - Battery | O'Reilly Auto Parts
     
    derevis likes this.
  4. Mendel Leisk

    Mendel Leisk Sunday driver DIY’r

    Joined:
    Oct 17, 2010
    42,093
    30,156
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    I checked mine, read around 16~18 milliamperes, with occasional upticks to around 40. You want to have the car as "sleepy" as possible: no fobs nearby, doors closed (obviously) and so forth. Good to get your wiring set up, then just let everything sit for about 1/2 hour minimum.
     
    derevis likes this.
  5. David Beale

    David Beale Senior Member

    Joined:
    Jul 24, 2006
    5,963
    1,954
    0
    Location:
    Edmonton Alberta
    Vehicle:
    2012 Prius
    If you have the hatch open to measure, the "door lights" (the ones down low) are on and the ones under the dash (floor or foot lights I quess). That's why you are seeing the high current.
     
    derevis likes this.
  6. Mendel Leisk

    Mendel Leisk Sunday driver DIY’r

    Joined:
    Oct 17, 2010
    42,093
    30,156
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    Yup, you can run the leads out the hatch threshold and drop the hatch on them, they hardly pinch with the thick gasket.
     
    derevis likes this.
  7. CR94

    CR94 Senior Member

    Joined:
    Dec 2, 2014
    1,752
    673
    0
    Location:
    Northwestern S.C.
    Vehicle:
    2011 Prius
    Model:
    Two
    Yup, or better yet, with the hatch still open for convenient access to the battery, shove the latch mechanism for the hatch into its fully locked position (two clicks), in order to "fool" the switch into believing the hatch is closed. Then you should see the much lower current Mendel listed, if all other doors are closed and if you wait for things to settle down. Otherwise, you'll see about 0.7 amperes plus current to power whatever combination of interior lights are on. That 0.7A alone will thoroughly deflate the battery within three days.
     
    derevis likes this.
  8. derevis

    derevis Junior Member

    Joined:
    Dec 29, 2004
    5
    2
    0
    Man, you guys are great. Thank you. This is my 4th Prius in 11 years now. I purchased, (I should say rescued), my '12 from a super shady dealership recently. They had installed a KARR immobilizer system.
    When I bought the car, I swapped the original (3 year old) factory 12V aux battery with a brand new Optima yellow top I had had in my '08 Prius. A week later, my car wouldn't start. I brought it in the house, charged it up, and a week later, same thing. Suspecting it was the immobilizer, I brought it into my Toyota dealership this morning and had them remove it. They just called and told me they removed the immobilizer, and are now measuring 150mA, vs. the 1.1A I was seeing. So when I get home, I'm going to follow your advice and run the ammeter leads out the trunk, make sure all doors are closed and keep the FOB away. I'll let you know what I measure later, but it looks like I found the phantom load! Thanks again.
    PS- I feel a bit guilty; I've been a member of this forum since '04 and I think I have a total of 2 posts. If anything, that's a testament to how damn reliable these cars are. Thanks again. I'll let you know what I find.
     
    kc5dlo likes this.
  9. Mendel Leisk

    Mendel Leisk Sunday driver DIY’r

    Joined:
    Oct 17, 2010
    42,093
    30,156
    80
    Location:
    Greater Vancouver, British Columbia, Canada
    Vehicle:
    2010 Prius
    Model:
    Touring
    Sorry if obvious:

    You put the meter IN the circuit, ie: any current moving has to go THROUGH the meter.

    In practice: get yourself a couple of leads about 3~4 feet, preferably with alligator clips. Disconnect battery negative clamp at the post and connect securely to one of the leads. Connect the other lead to neg battery post. Run both leads out across the hatch threshold and close the hatch on them.

    Then wait say 1/2 hour, and then hook up your meter leads, one to each of the previous leads. If your meter's NOT auto ranging, start on amperage scale, then milliamperes, just in case.
     
  10. derevis

    derevis Junior Member

    Joined:
    Dec 29, 2004
    5
    2
    0
    So, a little anticlimactic... I was excited when I got home, after the immobilizer was removed. I hooked up my ammeter in line with the positive post, all doors closed, double-latched the hatch back, fob distant, and the meter read exactly what you thought- about 18mA when everything got "sleepy" after about 5 minutes. Periodic spikes up to about 30mA, but she settled on about 18mA. "Great", I'm thinking. But then I wanted to reproduce the original conditions, where I didn't double latch the hatch back. As soon as my hatch was seen open, the current jumped back up to 1.1A. So now I'm wondering, did removing the immobilizer have any effect? The data doesn't suggest it did.
     
  11. derevis

    derevis Junior Member

    Joined:
    Dec 29, 2004
    5
    2
    0
    Anyway, the important thing is that I'm within spec when meauring a true car off condition. If the battery dies again, I have to think the battery is suspect. Thanks for all your input. What a great community! Peace.
     
    Mendel Leisk likes this.
Loading...