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Why 13.8 V not 13.7 or 13.9?

Discussion in 'Electronic Design' started by Harry, Jun 14, 2007.

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  1. Harry

    Harry Guest

    What is the votage of the battery in a car? 12 V right?

    Then why is Cigar-Lighter Adaptor 13.8V ?

    Also, how the 13.8 (not 13.9 or 13.7) V is derived?

    -- Harry
  2. James Beck

    James Beck Guest

    Nominal voltage of the lead acid battery.
  3. Gibbo

    Gibbo Guest

    No it's the nominal *charge* voltage. The "at rest" voltage of a lead
    acid battery is nowhere near that high.
  4. James Beck

    James Beck Guest

    OK, it is the nominal "float" voltage of a lead acid battery.
    Feel better?
  5. Gibbo

    Gibbo Guest

    Not especially so. Just that your first answer was wrong. Dress it up
    with sarcasm however you like. Doesn't alter anything.
  6. To hold a 6 cell lead acid battery at full charge, you have
    to apply 13.8 volts to it. The alternator is set to hold
    about that voltage, to charge and maintain the battery at
    full charge.

    When it is not being charged but supplying a load (like when
    you turn your headlights on, but the motor is not running)
    then the output voltage stays near 12 volts for a
    significant part of the discharge time.
  7. Meat Plow

    Meat Plow Guest

    Charge voltage in an auto should be 14.4.
  8. Gibbo

    Gibbo Guest

    If only it was that simple.
  9. Eeyore

    Eeyore Guest

    13.8V / 6 cells = 2.3V / cell, the nominal 'on charge' voltage.

  10. The '12v' battery, has a lot of different voltages associated with it.
    For normal 'float' charging (in your car, with the engine running, and the
    battery already reasonably charged, assuming it has a modern alternator
    that doesn't keep trying to push too much voltage into the battery), the
    cells need about 2.3v/cell (13.8v).
    Fast charging, can use up to nearly 2.5v/cell (14.8v). Cars commonly used
    14.5v, and older electrics on a car often charged all the time at this
    higher voltage - which is why the electrolyte needed topping up so
    Idle, without significant load, typically about 12.9v.
    Then during starting, down to under 7v momentarily, when 'cold cranking'.
    '12v', is just a figure used to distinguish this system (with 6 cells),
    from systems using 12 cells (lorries - nominally 24v, but up to 29v in the
    real world), and ones using 3 cells (some older motor bikes). It bears
    little real resemblance to the real figure seen in the car...

    Best Wishes
  11. Meat Plow wrote:

    At what temp? (K, C, R, F) chose one, then define concentration of acid
    in ppm and purity of solvent, then give us a number(s).

    Do I hear anything???

    Have fun

  12. ian field

    ian field Guest

    One battery I had showed a label "charge voltage not to exceed 14.4V", IIRC
    the Optimate battery conditioner float charges at 13.6V to minimise
    electrolyte loss due to gassing.
  13. Rich Grise

    Rich Grise Guest

    But it _is_ the typical float voltage. Neither of them is "nominal" - that
    just means "by name", i.e., the nominal voltage of a 12V battery is 12V,
    even though the actual voltage is usually different from that.

    As to why it's 13.8 instead of some other value is because that's the way
    the chemistry works out.

    Hope This Helps!
  14. Gibbo

    Gibbo Guest

    Re-read his original reply.
  15. Rich Grise

    Rich Grise Guest

    That's a non-answer. Where does the 2.3 come from? ;-)

    Spoiler: It's the chemistry. ;-)

  16. Jamie

    Jamie Guest

    your vehicles battery is actually less than that,
    aprox 12 volts. The 13.8 is needed to put charge in the
    battery.. The alternator (Charging system) has it's own
    voltage regulator to maintain the 13.8 volts
    The voltage has been selected as the standard to use with
    common lead acid type batteries in this voltage range to put in a
    charge, but not so that it'll damage the battery.
  17. mc

    mc Guest

    12.6 fully charged, 12.0 about half discharged...
    When the car is running (and charging the battery), that is the normal
    voltage of the electrical system.
  18. JackShephard

    JackShephard Guest

    If you tried to charge a car battery with 12V, you would get NO charge.
    In order to charge a battery, one must overcome the internal resistance
    of the battery and actually FEED electrons into it.

    The 13.8 volt figure does that, and does so without too much charging
    current, as that must also be considered. One would not want one's
    battery to go BOOM!
  19. JackShephard

    JackShephard Guest

    The MeatTard strikes again.

    Wrong, Slug-o!

    When your IQ gets above that of a slug, come back and try again.
  20. I don't know about that.I guess it depends who you are.

    There are more than a few folk who would be pleasantly surprised if
    your battery went BOOM!
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