Extreme Voltage drop in starting battery -2006 Prius

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by AaronFrom, May 9, 2016.

  1. AaronFrom

    AaronFrom New Member

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    Hello

    This is my first time posting to the forums... and I am doing so because I am trying to diagnose a 2006 Prius that keeps having a dead starting battery. I noticed that when I charge up the battery fully.... it doesn't have a voltage drop... but when I connect the MIDDLE (Large) white plastic connector to the starting battery... the voltage drops very rapidly. This is even when the safely plug is removed from the hybrid battery.

    My next option is to use my multimeter and check to see which wire in the harness/plug is causing the extreme voltage drop.

    I tried looking online for pictures or info (schematics) relating to the connectors that plug into the starting battery but to no avail.

    If anyone has any tips or tricks please feel free to comment.

    Thanks in advance!

    -Aaron
    Seattle, Wa
     
  2. JC91006

    JC91006 Senior Member

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    How old is your battery? If it's over 5 years old, it may no longer hold a charge. Time to replace it
     
  3. bisco

    bisco cookie crumbler

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    welcome! could it be that when you put a load on a damaged battery, you get the extreme voltage drop? or do you think something in the car is doing it?
     
  4. AaronFrom

    AaronFrom New Member

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    So the starting battery charges up nicely... but when I connect the middle longer white plug to the starting battery... instant voltage drop at the starting battery... so much the car won't even start. The starting battery is fairly new but I might go stick a new one in there just to see what happens. I'm just wondering the function of the middle white plug I am referring to... it seems be to the problem.
     
  5. AaronFrom

    AaronFrom New Member

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    And I just stuck a new Toyota quality Prius starting battery in... same thing... connect the middle long white connector and bamm.... loss of voltage bigtime...
     
  6. S Keith

    S Keith Senior Member

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    Please give us numbers and provide details on ANY non-factory installed items.

    EDIT: try checking voltage at the jump connection points under the hood with all doors closed and the car completely off.
     
  7. bisco

    bisco cookie crumbler

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    not sure what a middle connector would be...(n)
     
    #7 bisco, May 9, 2016
    Last edited: May 9, 2016
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  8. Former Member 68813

    Former Member 68813 Senior Member

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    you are clearly doing it wrong. there should be no middle connector to 12v battery. there are only 2: + and -.
     
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  9. edthefox5

    edthefox5 Senior Member

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    Oh boy I hope your not swapping the battery terminals....that will toast a Prius.

    On the 12 VOLT BATTERY:
    The positive + side of the battery under the red plastic cover is 2 wires. A thick black one and a smaller thin black wire. Both terminate into white plastic plugs that just click in and out of the assy on top of the positive post of the battery. Then the red plastic cover goes over that whole positive post assy. The thick black one goes directly into the main fuse box under the hood on the right in the black plastic box. Then inside that box that wire is bolted under the main positive + jump point stud in that box. The front convenience jump points. The thick black wire is the main 12 + to that fuse box. The smaller black wire is 12 volts to the metal box right next to the 12 volt battery. That is a bank of capacitors used as 12 volt back up power supply for the brakes.

    The negative side of the battery has a short jumper cable that bolts right to the back wall of the trunk. That wire is only about a foot long.

    There's really big + and - signs on the battery itself to identify the posts.

    If your not nuts and have the battery hooked up correctly and its killing the battery most likely the Inverter is shorted. Very surprised its not blowing the fuses.
    There's 2 in that positive assy under the red plastic cover. A 100 amp and a 5 amp.

    What got you to this point anyway? Is this a salvage title or auction buy?
     
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  10. Patrick Wong

    Patrick Wong DIY Enthusiast

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    How about posting a photo - I have no idea what you are referring to.
     
  11. S Keith

    S Keith Senior Member

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    I think he's referring to the harness. That harness has 3 plugs. The "middle" plug (big white one) is the main connection.
     
  12. AaronFrom

    AaronFrom New Member

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    Yes I am talking about the main harness at the starting battery itself... I ended up unplugging everything... and recharged the battery separately and reconnected the harness. It looks like the phantom voltage leak is gone... but when I plug in the battery connectors... at the starting battery... the car turns itself on... and won't turn off. Regardless of holding down the power button.

    And no I did not reverse or cross the leads. I have a different starting battery I could try just to see if low voltage is the problem.
    This new battery I put in doesn't seem to hold it's charge... so switching back to the old starting battery might be the next step.
     
  13. JC91006

    JC91006 Senior Member

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    Are you putting in a NEW battery or just a different battery that's not new?
     
  14. S Keith

    S Keith Senior Member

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    Check the hybrid battery safety plug. I experienced exactly what you describe when the interlock mechanism wasn't engaged as in step 3:

    [​IMG]

    The fix was a different safety plug. Oddly enough the problem safety plug worked without issue on a different pack. I didn't troubleshoot any further.
     
  15. AaronFrom

    AaronFrom New Member

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    Put in a brand new OEM starting battery... when I connect the white connectors at the starting battery... the voltage does seem to drop to 11.0 or so... and the dang battery drops voltage after taking it off the charger... so I'm thinking this is a bad NEW battery.

    Going to plop the old one (relatively clean and new) in... it's just sitting at 12.5 volts fully charged so I'll see what happens when I slap that puppy in.

    Thanks for the quick feedback.
    I'm hoping this car will stop turning itself on. Crazy hybreed!!!
     
  16. S Keith

    S Keith Senior Member

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    OEM = Toyota? If you got a yellow top Optima, there have been reports of issues with them. Personally, I don't buy the their hype.

    11.0V isn't that alarming as long as it holds that voltage. There's a pretty large demand right at power-up.

    12.5V fully charged isn't great. 24 hours after fully charging a battery, it should be at 12.65V or higher. 12.5V is about 80-85% SoC, It's more likely a weak cell with sagging voltage that will drop out under load. If it's been sitting awhile since charging, it's probably fine. If you charged it recently (within the last few days), it's highly suspect.
     
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