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12 volt battery discharge solution

Discussion in 'Gen II Prius Care, Maintenance and Troubleshooting' started by epoch_time, Oct 25, 2011.

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  1. epoch_time

    epoch_time Member

    Joined:
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    Location:
    las vegas
    Your Vehicle Year:
    2009 Prius
    Model:
    N/A
    There is a common latch up problem with the gen II abs ecu whereby the 12 volt battery cannot bring the car to ready mode causing low voltage to the abs ecu and semi-permenantly energizing abs-1 relay and for ever after putting a .54 amp (540ma) load on the turned off gen II prius................................................Here are a couple of tests to see if your prius is affected after having a dead 12 volt battery..........After driving your prius and you shut it off exactly 2 minutes and 2 seconds later abs relays 1 and 2 should de-energize releasing the final .54 amp load...............................This audioable click can be heard from drivers seat or anywhere in a quiet garage.................You can also tell if the abs-1 relay has released by its temperature after the car has been shut off over night.......the abs-1 (green relay in fuse box in engine compartment on drivers side) will be warmer than the other relays.............If in the summer months the temperature difference can be felt with fingers in cold months an infared thermometer could be used to find warm energized relay..................................................................Another test for discharging load on gen II prius would be to monitor 12 volt battery voltage at cigarette lighter adapter or at under hood fuse box jump start terminal.....after the car has been off for 12 to 24 hours...........................At the above monitor points and just after bring car to ready mode if voltage is sustained for any more than 30 seconds above 13.8 volts you have something draining your 12 volt battery..........EXPLANATION a drained 12 volt prius battery will pull lots of current.... My prius sitting for 20 hours with latched up abs ecu (purchased from dealer this way) pulled over 50 amps (@14.3 volts) from the 13 volt dc-dc onverter for 5 minutes then 40 amps (@14.2 volts) for 5 minutes then 30 amps (@ 14.1 volts) for 5 minutes and reaching full charge after about one hour.............................................. The dc-dc converter/inverter has a remote sense lead The remote sense wire (small 20 guage wire pluged into positive 12 volt battery clamp) that samples the drained battery voltage, adjusts the dc-dc converter output.....The more drained the 12 volt battery the higher the charging voltage at the jumpstart terminal/cigarette lighter to keep the voltage at the battery terminals at 13.8 volts via the sense lead subtracting the voltage drop on the large 12 volt cable.......................................................................................................................................THe solution to the abs ecu latchup battery discharge problem is to disconncet the 12 volt battery overnight to reset all ecu's...................You will loose radio memory presets, fuel trim memory (no big deal), drivers window limit switch memory...Some people report having to cycle power ready start button a couple of times to remove computers errors before car will become ready after 12 volt battery has been disconnected.. To re initiaiize drivers window open half way then pull and hold up button for one second after window full closes and release........... The 12 volt battery can be disconnected by unpluging two connectors from the front side of the 12 volt positive battery clamp assembly...unplug the small wire/plug first then the large wire/plug second..... reinsert large wire/plug first small plug/wire second after leaving battery disconnected overnight to reset abs ecu.............Normal battery drain on properly operating gen II prius is .02 amps or 18 milliamps This is for a prius equiped with smart key / keyless entry system.....
  2. SeeingDouble

    SeeingDouble New Member

    Joined:
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    Location:
    Iowa
    Your Vehicle Year:
    2006 Prius
    Model:
    II
    Ironically, I went to start my '06 this afternoon and it wouldn't start. The 12 volt battery only had 7.86 volts, so it's on the trickle charger overnight. I guess it's had some somewhat unusual use the last few days. Last Friday I reset the 'Maint Req'd" light. Saturday I started it long enough to move it to the middle of the garage so I could wax it. The doors and trunk were open off and on to wax the inside of the frame, but certainly not long enough to drain the battery seriously. After I was finished, I started it long enough to move it back to its spot in the garage. It had been sitting until today. Could these very short starts and stops have freaked out something in there? What causes the ABS ecu issue? What is the permanent fix? I think my warranty is probably done, so I shudder to think about having to replace relays. I hope the car decides to behave tomorrow or that the worst case scenario is just that the battery decided to shoot craps. Wish me luck!
  3. epoch_time

    epoch_time Member

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    Listen for abs relays 1&2 to de-energize 2 minutes and 2 seconds after powering down the prius..you should hear a nice click...
  4. Patrick Wong

    Patrick Wong DIY Enthusiast

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    It is likely that you will need to replace the 12V battery now.
    1 person likes this.
  5. ericbeeliner

    ericbeeliner New Member

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    Location:
    Gold Beach, OR
    Your Vehicle Year:
    2005 Prius
    Model:
    Two


    Thank you, epoch_time. I left the hatch door ajar for two days and had a dead 12V batt. Charged it up and was able to go, but the window open with one button push wouldn't work. I followed your tip, and all is fine. Thanks, again!
  6. richard schumacher

    richard schumacher shortbus driver

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    Having been drained once the 12V battery now has very little life left in it. If it is drained again odds are that no workaround will get you going.
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