12.4v at battery, 11.8v in car, no ready

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by LKasdorf, Aug 2, 2025 at 1:40 PM.

  1. LKasdorf

    LKasdorf Junior Member

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    My son's 2005 Prius is doing something odd.
    It is acting just like a low 12v battery, and the maintenence console as well as a voltmeter in the cig lighter show 11.8 - 11.9 v.
    The car will not enter the ready state- i.e. lots of lights on, and if you try to put it in D, you get red triangle.
    The strange part is that the 12v battery is in excellent condition. Reads 12.4v at the terminals (not 11.9). I checked it on my bench with a load tester and an Ancel battery tester. Execellent condition.
    So- how can the battery show 12.4, but only 11.8 be getting to the guts of the car? I cleaned the battery terminals very well and used contact cleaner on the 2 white plugs on the battery harness.
    I am puzzled.
    BTW, it doesn't show any battery or engine error codes in Dr Prius. I will connect Techstream and see what it shows.
     
    #1 LKasdorf, Aug 2, 2025 at 1:40 PM
  2. JC91006

    JC91006 Senior Member

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    how old is the battery? You need to read the codes, there should be a reason to all the issues and lights.
     
    #2 JC91006, Aug 2, 2025 at 1:48 PM
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  3. bisco

    bisco cookie crumbler

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    might be worth trying another battery, or a jump with that one disconnected
     
    #3 bisco, Aug 2, 2025 at 1:51 PM
  4. pasadena_commut

    pasadena_commut Senior Member

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    What state is the car in when 12.4 V is measured at the battery?
    I'm assuming "off".

    What state is the car in when 11.8 V is measured at the power outlet (it is not a lighter) under the dash?
    Must be "IG-ON" or "ACC", otherwise the outlet has no connection to the battery.

    How much current is pulled out of the 12V battery at off?
    Roughly 25 mA. Is that enough to reduce the open circuit voltage? Not significantly.

    How much current is pulled out of the 12V battery at "IG-ON" or "ACC"?
    Several Amps with "IG-ON" pulling less than "ACC". With "ACC" if the cabin blower is on drains over 20A are possible. Is that enough to reduce the open circuit voltage significantly? Yes.

    Ever look at the voltage on a regular car when the starter is used? Down to nearly 9V is OK on a good battery. That is a 3V drop due to a very large current. Here it is only 0.6V.

    The 12v battery voltage is not the problem (although a loose or missing ground somewhere could be). The OP needs to get either techstream or a Prius compatible code reader like the Autel AP200 and read the codes. (There is a thread somewhere discussing these devices.)
    Edit: Gen2 OBD2 app review | PriusChat

    It wouldn't hurt to put the 12V on a charger in the meantime. Since the car isn't running it cannot charge it up itself, and the OP is likely to run the battery down further diagnosing this issue.
     
    #4 pasadena_commut, Aug 2, 2025 at 2:06 PM
    Last edited: Aug 2, 2025 at 2:14 PM
  5. Mendel Leisk

    Mendel Leisk MMX GEN III

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    Did the Ancel show it's as-tested CCA?
     
    #5 Mendel Leisk, Aug 2, 2025 at 2:20 PM
  6. PriusCamper

    PriusCamper Senior Member

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    How old is the battery? It is NOT in excellent or even good condition!

    Anything below 12.7volts with car off is either a discharged battery that needs to be charged with a legitimate charger or a battery that needs to be replaced.

    Basically this 12v doesn't have enough power to turn on the computers, nor turn on the high voltage relays.

    If you were to connect this battery to a regular car and try to start the engine the battery wouldn't be able to even turn over the starter motor.
     
    #6 PriusCamper, Aug 2, 2025 at 4:21 PM
  7. LKasdorf

    LKasdorf Junior Member

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    I work on batteries- this one is perfect. I took it out, put it on the bench, load tested it, checked with battery tester. All good.
    Like I said, 12.4 at the battery and 11.8 everywhere else!
    The voltage at the fuse box is the same as at the battery- a solid 12.4v, which is plenty. But the cig lighter outlet reads 11.8 and the service mode display says 11.8.
    I will try to get my techstream computer going.
     
    #7 LKasdorf, Aug 2, 2025 at 5:37 PM
  8. LKasdorf

    LKasdorf Junior Member

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    The ancel tester showed 330 CCA on a 325 CCA battery.
     
    #8 LKasdorf, Aug 2, 2025 at 5:38 PM
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