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What the 12v battery powers on exactly?

Discussion in 'Gen 3 Prius Technical Discussion' started by MeGuinness, Dec 30, 2016.

  1. MeGuinness

    MeGuinness Member

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    What the 12v battery actually powers on other the the computer.. instrument panel and switches?who runs the waterpump ...Air-conditioning...The heater... Headlights? And why a 12v battery that is on the way out would create problems like random faulty codes or less mpgs?

    How long does take to pressurise the system after the car asn't been moving all night? In the morning once I open the driver door the pressurise starts and it takes about 10 to 12 seconds until it stops? Don't remember in the past that would take that long?

    Thanx

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  2. jdcollins5

    jdcollins5 Senior Member

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    The 12V battery powers up the computers, after performing a self check, which in turn energizes the main relays that connect the 220V hybrid battery to the Inverter/Converter, which connects to the rest of the electrical system and turns on the Ready light.

    The Converter contains a subsystem called the DC/DC converter that steps the 220VDC down to about 13.5VDC to charge the 12V battery and provide power to the 12V electronics.

    All of the engine components, water pumps, air conditioning, etc. is powered from the 220V battery and Inverter circuits.

    The computers use the 12V circuitry for its power. A marginal 12V battery can cause the computers to generate false diagnostic codes due to low voltage to the computers on start.

    10 to 12 seconds does sound like a long time for the brake accumulator to pressurize. You may want to have the dealer check this out. Have you had the Brake Booster Pump Recall performed on your car?
     
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  3. bisco

    bisco cookie crumbler

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    are you having an issue? the ready light should come on in a second or two, and you should be able to drive right away.
     
  4. MeGuinness

    MeGuinness Member

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    No recall about the booster yet ....Just airbags and fuel carbon canister in UK!
    Is the brake booster powered by the 12v battery?Would the battery effect the time that takes to pressurise?
    Why low mpgs then if the 12v battery is on is way out?

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  5. bisco

    bisco cookie crumbler

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    yes, when you open the door and hear the buzzing sound, only the 12v is active.

    low mpg's are caused by the engine running more to keep the hybrid battery charged, which is continually trying to charge a bad 12v.

    but this is observed only, with no real data to back it up.
     
  6. MeGuinness

    MeGuinness Member

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    On other posts I been discussing the state of my current 12v ,yes it is 7years old and the voltage looks low...., at same time lately I do get errors codes exspecially if I been keeping all headlights and fog lights on + heater.... P0a7f and P261B....Then I had a error about the powerstearing too then the one pictured...It seams to much of a coincidence that all those faults keep showing .. 1483109931684.jpg

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  7. bisco

    bisco cookie crumbler

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    might as well change it at this point, and hope for the best. at 7 years, there's not much to lose. if you'd rather not spend the money unless absolutely necessary, you're going to have to start with a voltage and load test.
     
  8. jdcollins5

    jdcollins5 Senior Member

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    P0A7F is Hybrid Battery Deterioration
    P261B is Coolant Pump B Control Malfunction (Inverter Cooling Pump)

    I would try replacing the 12V battery first.
     
  9. MeGuinness

    MeGuinness Member

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    Battery ordered...Should arrive on Tuesday.
    If the EV battery is trying hard to charge a bad 12v battery does mean that with a new good 12v the constant 14.5v charging stops and only restart to top it up again? And so on and on and on?
    Because what I see at moment is that 14.5v is constantly there no matter how many miles I drive. 1483180490971.png

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  10. jdcollins5

    jdcollins5 Senior Member

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    From what I remember, the charging voltage remains at 14.5V unless the 12V battery temperature gets above a certain temperature and then will drop to about 13.5V.

    When I check mine it is typically at 14.5V.

    You would need to look at charging current to determine if charging circuit was having to constantly charge a weak battery, not voltage. Once the battery is fully charged the current will drop to zero but the voltage should remain at 14.5V.
     
  11. MeGuinness

    MeGuinness Member

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  12. MeGuinness

    MeGuinness Member

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    Update...
    Received the new battery...Fully charged just in case before fitting....and then I still get this from the onboard diagnostics....but from terminals is over 12v ??? 1483521551993.jpg

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  13. jdcollins5

    jdcollins5 Senior Member

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    I had the same error on my Diagnostic display. The display reading was 0.7V lower than the voltage read at the battery terminals with the voltmeter.

    I haven't used the Diagnostic display since. I typically read my battery voltage at the jump point in the engine fuse box.
     
  14. MeGuinness

    MeGuinness Member

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    A couple of friends with same gen 3 Prius the diagnostic shows 12.4v ,I have a feeling there is a connection somewhere that isn't proper.

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  15. bisco

    bisco cookie crumbler

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    sounds likely. i use a dvm at the jump point.
     
  16. ChapmanF

    ChapmanF Senior Member

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    Shouldn't be surprising at all. The voltage displayed on the MFD is the voltage at the MFD. It's separated from the battery by several feet of fat wire, more feet of skinny wire, at least one high-ICR fuse and one branch fuse, and several connectors. All of those items have a certain voltage drop proportional to current flowing through them. And you have to be in ACC at least, if not IG ON, to see the display, so other electrical loads are active, and the voltage reaching the MFD will be lower than what you might measure at the battery terminals at the same time.

    Now, it'll be pretty consistent ... if you do measure at the battery terminals once at the same time and make note of the difference (and remember whether that was ACC or IG ON), you can certainly go on and use the MFD measurement for convenience later, just by doing the math. For mine the difference is a good fraction of a volt.

    -Chap