12V Battery charge when driving?

Discussion in 'Gen 3 Prius Main Forum' started by bobwaber, May 12, 2021.

  1. bobwaber

    bobwaber New Member

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    Does the gas engine have to be running to
    charge the 12V battery? Say you are driving
    but the gas engine is off, will the 12V still be
    charging?
    I have to charge the 12V if the car sits for 7 days
    but 3 or 4 it starts OK. I drive short distances
    once a week is all.
    I am measuring 14.9V on the plug in port when
    driving, which seems like excess voltage for a 12V
    battery even if it is charging
    The port goes dead if the car is turned off so
    I wonder if it is really the battery there.
    Thanks much
     
  2. ChapmanF

    ChapmanF Senior Member

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    Whenever the word READY is shown on the dash, the 12 volt battery is being charged.
     
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  3. sam spade 2

    sam spade 2 Senior Member

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    And his 14.9 measurement probably means that the battery is VERY low and the charging system is trying desperately to bring it back up. Not common to see the voltage that high but not necessarily a problem either.
    Unfortunately, the system doesn't have the capacity to do that over short runs.

    At the very least, his battery needs a manual charging and a load test (or 3).
    But most likely that his driving habits have caused the battery to age prematurely and a new one is needed.
    An occasional manual charge should keep the new one healthy a LOT longer.
     
  4. rjparker

    rjparker Senior Member

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    A well charged battery will often start charging in the high 14's when the car is first put in Ready and will drop to the mid 13's when it is charged up. The inverter's dc to dc converter does the charging.

    The 12v outlet is wired to the 12v buss but is electronically current limited and turned off with the car.

    Most likely you have an excessive parasitic amp draw on the battery when the car is off. It can be an aftermarket obd2 device, an aftermarket insurance tracker, third party stereo, lights left on like the hatch light or a number of other things. In some cases it has been a keyfob left too close but thats stretching the issue. Usually these cars draw 15-30ma in off state but we see cars with 90-500ma routinely. The higher the ma draw the faster the battery goes dead. A decent battery with an acceptable parasitic draw should last three or maybe four weeks and still start the Prius. Testing and diagnosis is an electrically advanced skill everyone does not automatically possess.

    Low amp battery maintainers are a stopgap solution for a car that won't last a week.
     
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  5. sam spade 2

    sam spade 2 Senior Member

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    Given the use pattern that he described. I don't think that is likely AT ALL.

     
  6. jzchen

    jzchen Senior Member

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    Yes, even when the engine is off but you are driving , as @ChapmanF notes. More specifically whenever the green “READY” on the dash is displayed, the 12 V is being charged, irrespective of the engine actually running or on electric propulsion.

    EDIT- 14.9 V is within spec, I believe the acceptable range is 13 - 15 V.
     
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