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car battery recharging question

Discussion in 'Electronic Design' started by Kooner, Nov 1, 2003.

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  1. Kooner

    Kooner Guest

    My car battery measures 12.4V when the engine is off. When the engine
    is running the battery measures 14.4V. Why is there a 2V difference?
    Even if the battery internal resistance is 0.01ohms and the trickle
    current is 0.1A that would give a voltage drop of 0.001V. So why don't
    I measure 12.401V when the car is running? What is the schematic
    representation of a battery? Can anyone recommend a good book on
    rechargeable batteries? Thanks,
  2. Jim Thompson

    Jim Thompson Guest

    Lead acid cells have a tremendous TC.

    Cold cells can require 15.1V to charge, nominal temperatures require
    typically 14.6V, hot is 13.3V.

    Look up my alternator regulator patents for more information.
    (Numbers give on my website.)

    ...Jim Thompson
  3. I read in that Jim Thompson
    Yes, but the overvoltage when charging is not primarily a thermal
    effect. The battery doesn't only have its linear resistance and off-
    charge e.m.f; it also has 'partial' or 'contact' voltages (AKA 'redox
    potentials') at each plate when being charged. These are the
    consequences of the chemical reactions that are taking place at the
    plates, even at trickle charge rates.
    The equation is a negative exponential (Nernst equation, IIRC), like the
    Vbe equation of a bipolar transistor.
  4. No battery is 100% efficient. More power goes in than comes out.

    14.4V is a typical charge voltage for cycle-use lead-acid.
  5. Tim Shoppa

    Tim Shoppa Guest

    The representation you are using - a "perfect battery" + internal resistance -
    is certainly the most common. But it's not perfect, and nobody will
    pretend it's perfect. I think your model for the internal resistance
    too low for anything but the larger truck/marine batteries - a 0.01
    ohm internal resistance would imply that the battery would deliver 1200 Amps
    to a dead short, and that's too high by a factor of several for a small
    car battery. That's not to say that dropping a wrench on the battery
    terminal is a safe activity !

    At 14.4V the battery is not being trickle-charged. The alternator
    is probably putting more than 10 Amps into it at that point. To further
    complicate things the alternator doesn't produce flat DC but instead a
    very highly pulsing DC.

    Some modern alternator voltage regulators will fall down to a
    "trickle charge" after a start, but the classic alternator VR designs
    know no such concept. As the electrolyte boils away, that's why you have
    to add water... It's not the kindest thing to do to a storage battery
    but they're built to take this kind of abuse.

  6. Jim Thompson

    Jim Thompson Guest

    On 1 Nov 2003 14:53:44 -0800, (Tim Shoppa)

    The biggest problem is that the auto manufacturers, particularly
    Detroit are CHEAP. Not a single alternator regulator actually senses
    the battery voltage directly. So wiring harness drops determine
    whether you have an over- or under-charged battery.

    ...Jim Thompson
  7. GPG

    GPG Guest

    Alternators are generally wired direct to the battery with a (non adjustable)
    internal regulator. Old systems had an external mechanical reg, similar to dc
    generator but no cutout relay (not needed)and could be adjusted.
  8. Jeff

    Jeff Guest

    My car battery is rated for 1000CCA (garanteed amperage after 10 seconds and
    not dropping below 7.2V at something like 0 deg C) and 1250 CA (garanteed
    amperage after 10 seconds and not dropping below 7.2V at about room temp) ,
    granted it's slightly bigger then some of those motor cycle sized batteries
    used on some imports. Short circuit current would be much higher. With my
    battery at room temp, 0.0038 ohms of resistamce is calculated, however this
    would be somewhat higher then the intial resistance. 3000 - 5000 A would be
    the intial SC current.
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