12 volt battery charging voltage when fully charged?

Discussion in 'Prime Main Forum (2017-Current)' started by schja01, Jun 14, 2019.

  1. schja01

    schja01 One of very few in Chicagoland

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    Is my 12v battery fully charged or is it defective?

    When I start my Prime my voltage monitor says 14.5v.
    After driving the car a few miles the voltage begins to step down to about 12.7v and
    stays there.
    I drive infrequently.
    Is this just a sign the 12v battery is fully charged?
    I saw the same thing when I trickle charged it but I don't do that any more.
    I did have one incident of the car not starting due to low 12v battery so I am trying to stay on top of things.
    That incident was about 6mos ago give or take and hasn't happened again.
    Any insight appreciated.
    J
    PS. When I brought it in for the sensor campaign I mentioned the failure to start and they checked the battery at that time and said it was OK fwiw. That was months ago.
     
  2. jb in NE

    jb in NE Senior Member

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    What voltage monitor are you using, and where is it monitoring the voltage?
     
  3. schja01

    schja01 One of very few in Chicagoland

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    I'm using a monitor I plug into the front 12 volt accessory jack.
    I verified that it reads within 0.1 volt of what I get using a
    AstroAI Digital Multimeter and also verified with a Tacklife Ampmeter/
    Multimeter. Those readings were taken at the 12v battery posts.
    upload_2019-6-14_12-2-24.png
    upload_2019-6-14_12-4-59.png
    upload_2019-6-14_12-3-12.png
     
  4. bisco

    bisco cookie crumbler

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    your battery is fine. the car has to be designed to maintain the charge after it is full, not keep charging at a constant rate.
    otherwise, all the 12volts would die prematurely
     
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  5. schja01

    schja01 One of very few in Chicagoland

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    I like that answer.
     
  6. jb in NE

    jb in NE Senior Member

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    Yes. If the converter were to constantly ouput 14.5 V, the battery would overcharge.
     
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  7. sam spade 2

    sam spade 2 Senior Member

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    SIGH. Here you go again making statements that you have no good basis for.
    The "keep charging at a constant rate" is called "float voltage" and most systems absolutely DO that.

    In most cases, any reading while the vehicle is running that is below about 13.2 is reason to suspect the charging system is not working right. 12.7 indicates that no (useful) charging is taking place and the battery likely is discharging instead.

    Having said that, it is possible that some or all of the Prius models don't "float" the battery but turn the charging off and on as needed......but I doubt it.

    Any reading taken while the vehicle is running, regardless of the vehicle, tells you NOTHING about the health of the battery itself.
    For that the vehicle needs to be OFF.......or the battery disconnected.
     
  8. sam spade 2

    sam spade 2 Senior Member

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    Too bad it probably isn't right.
     
  9. bisco

    bisco cookie crumbler

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  10. Rob43

    Rob43 Senior Member

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    OP,

    I've been monitoring my 12v battery with my voltmeter virtually since day one, my battery & your battery are operating almost exactly the same way. I believe you're good to go...


    Rob43
     
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  11. audiodave

    audiodave Active Member

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  12. jb in NE

    jb in NE Senior Member

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    I believe the response was correct.

    The Prime charges the 12VDC battery from the DC-DC converter under the hood. The DC-DC converter is the major source of power to 12VDC loads when the system is READY. While the car was idle, and during computer start up before the car was READY, some capacity was taken out of the 12VDC battery. Once the system is in READY, then the converter provides all the power, including recharging the 12VDC battery.

    When the car is OFF or not READY, the state of charge of the 12VDC battery determines the bus voltage. Once the DC-DC converter comes online, it controls the bus voltage.

    A typical 12VDC lead acid battery is fully charged about 12.6 to 12.7 volts at rest. During operation, the DC converter not only supplies the operating DC loads, but recharges the 12VDC battery, by maintaining the bus voltage above the voltage of the 12VDC battery and forcing current into the 12VDC. As the charge in the 12VDC battery rises, the DC converter reduces the charge current (by reducing the voltage difference between the DC bus and the voltage of the 12VDC battery) so as to not overcharge the 12VDC battery. Once the 12VDC battery is fully charged, it floats on the DC bus at whatever voltage the converter is maintaining.

    If the 12VDC battery were significantly discharged and had a voltage down around 12 VDC (which would correspond to perhaps a 25% charge), then a bus voltage of as low as 13 VDC can put a significant charge in the 12VDC.
     
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  13. PriusV17

    PriusV17 Active Member

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    This chart should help understand what's happening. For my v charging is at about 14.6v and float charge is about 13.6v. When I'm not in Ready mode, but 12v (by hitting the Power button twice), I see 12.7v and dropping. The battery in Ready mode is either charging or floating to maintain charge. In 12v mode it's just drawing power and not charging. So your battery if in 12v mode appears normal. If it falls below 12v then that's near falling off the 'cliff' and not healthy to the battery's long term life. And in reverse when charging should be not pushed into the parabolic curve area and can overheat and shorten battery life. Either way you don't want to enter the zone of the 'sharp drops or the sharp rises'. I've highlighted the operating range it should be in longer life.

    I'm using the same 12v meter. So far a solid device.

    12v.png
     
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  14. sam spade 2

    sam spade 2 Senior Member

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    Yes but.......12.7 is NOT the proper "float" voltage for the battery.

    And it certainly is NOT proper if it really does "stay that way" for the majority of time that you are running the vehicle.
    When you measure that, it indicates that there is absolutely NO charging happening at the battery.......assuming that it's nominal resting voltage really is 12.8-12.9 volts.

    If that really is the way the system is designed, it explains a lot about why so many people have trouble with their 12 V batteries.
    The proper float voltage is something around 13.4 volts or a bit more.
     
  15. jb in NE

    jb in NE Senior Member

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    Why would it need to be charged if it is already charged and there is no load on it? The DC-DC converter is carrying the 12V loads.

    Assuming every time the OP gets into their car, and the battery is showing the proper voltage for a fully charged battery, then it is reasonable to assume that the DC converter is properly charging the battery each time the car is driven. I went to my garage and the 12V bus voltage on my Prime is 12.56 VDC. Turn on the interior lights, it drops as expected to 12.47. Ignition ON, some lights on including headlights, voltage drops to 11.98. Turn all that off and the voltage returns to 12.52 VDC. Still nearly a full charge.

    How do you know what the proper float voltage is for this battery in this application? Why would this need this high float voltage if the 12VDC is not handling any of the electrical loads going down the highway? Those are being handled by the DC converter. There should be little to no current being exchanged between the 12VDC battery and the rest of the DC bus system.
     
    #15 jb in NE, Jun 14, 2019
    Last edited: Jun 14, 2019
  16. lextoy

    lextoy Active Member

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    i have similar plug for cig lighter outlet. mine consistently starts off at 13.9, and within a few minutes drops to 13.1
    that is with few accessories running. that 12.7 while cruising along seems low to me...
    i've never seen it that low while running. are your headlights on off auto ??? radio on off hvac on off auto ? anything else weird plugged in or turned on ?
     
  17. schja01

    schja01 One of very few in Chicagoland

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    Headlights ON
    HVAC AUTO (but wasn’t running due to mild outside temp)
    Radio ON
    Nothing weird plugged in.
     
  18. sam spade 2

    sam spade 2 Senior Member

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    The application is irrelevant.
    The float voltage can NOT be lower than the nominal resting voltage or it really isn't a float voltage at all.

    The whole point of a float voltage is to keep the battery FULLY charged.
    If it is low, it won't do that.
     
  19. jb in NE

    jb in NE Senior Member

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    What is the nominal resting voltage of the OP's battery? You are certain the float voltage is low, without that information?
     
  20. schja01

    schja01 One of very few in Chicagoland

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    As the OP I took a short trip this AM.
    In IG ON voltage monitor was 12.1/12.4v Headlights ON fan 2 bars Radio on BT.
    After starting the voltage immediately jumped to 14.5 plus or minus 0.1v.
    After 1 minute of driving voltage dropped in two steps.
    12.9 then 12.6 with 0.1v fluctuations.
    I was taking observations at about 15 sec intervals. When my trip ended voltage was still 12.6.
    When I got into the garage I turned the car off, waited one minute and started it again. It briefly went to 14.5 then quickly settled to 12.6v when I shut it off once again. I never moved the car.
    J
     
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