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12 Volt Charging

Discussion in 'Electronic Design' started by mikem, May 21, 2004.

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  1. mikem

    mikem Guest

    Actually, the VR controls the current to the field (it actually
    PWMs it) so as to keep the voltage at the "big" alternator terminal
    (the one connected to the battery Plus) as close to 14.25V as it can.
    I dont understand the reference to 11V, but the battery acts as
    the "filter" to smooth the alternator ripple. The alternator
    has a three-phase full-wave rectifier; its ripple content is
    quite low. For a three-phase full-rectified wave whose RMS value is
    14.25V, the min value of the ripple is 13V, the peak value is 15V.
    Thats the way it works, even within the ripple cycle. In the long
    term, every electron that flows through the "loads" comes from the
    alternator. Only temporarily does the battery supply any of the
    load current.
    You need to read up on the electrochemistry of Flooded Lead-Acid
    Batteries. You will find that if a FLAB is connected to fixed
    13.65V voltage source (float voltage), the FLAB will initially draw
    current (charging current). As the FLAB accumulates charge, the current
    will asymtotically decrease, settling to a final steady-state
    current which just equals the FLABs internal leakage (tendancy
    to self-discharge, typically under 100ma).

    At the nominal automotive VR setting of 14.25V, the charging
    current initially decreases over time but settles to higher
    final value. The higher steady-state current is due to
    electrolysis, which causes gas evolution (H & O) at the plates.
    At 14.25 the rate of gas evolution is still small; the steady state
    current is ~250ma.

    The 14.25V is a compromise setting; it keeps the battery nearly
    fully charged, while not "using" electrolyte at a prodigous rate.
    14.25V is too high to be used as a full-time float voltage, but it
    is ok for a car that is only being driven 2 to 4 hours out of every
    24 hours. 13.65V is used as a full-time float voltage where
    the goal is to keep the FLAB from self-discharging when it sits
    unused for months at a time.

  2. Tam/WB2TT

    Tam/WB2TT Guest

    For what it's worth department. We used a 2.2AH 12V sealed lead acid battery
    at work to power some test gear, and charged it off an HP lab power supply.
    To get it to draw 250 ma charging current, I had to crank the voltage up to
    about 14.3V.

    In my car, the battery is under the rear seat, which implies some resistance
    back to the alternator. With the engine running, both the instrument panel
    VM, and a DVM plugged into the cigarette lighter show about 15.2V. The
    car/battery will soon be 4 years old. So the batt is not being self

    I have an electric start riding lawn mower, which I trickle charge during
    the off season. The battery is a 30AH car type lead acid. The charger is an
    ATT 12VDC 300ma wall wart from a cordless telephone. The open circuit
    voltage is 16V, and current at 14V is less than 10 ma. This may not be
    enough to fully charge the battery, but it did crank over at full speed.
    Also, I did not keep it charging continuously,

  3. Randy Gross

    Randy Gross Guest

    Greetings Group,

    This is a hypothetical question concerning an automotive charging
    system. I know that the voltage regulator controls battery charging by
    regulating the field current in the alternator between 13.5 and 14.5
    volts. The question I pose is this:

    If the voltage from the alternator (unregulated) was held at a constant
    11 volts , that's 11vdc to a peak of 15.55vdc, full ripple, no filter,
    would this damage the lead-acid battery?

    Theoretically, in this model, the battery works in conjunction with
    alternator to supply the load instead of intermittent use. When the
    alternator voltage drops below 13.5, the battery starts conducting its
    supply to the load. When the alternator voltage exceeds battery voltage,
    the battery is charged.

    Where is the flaw in this line of thinking?

    Randy Gross
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