Connect with us

12 Volt Charging

Discussion in 'Electronic Basics' started by John Popelish, May 21, 2004.

Scroll to continue with content
  1. I don't understand your Dc versus peak voltage numbers. Automotive
    alternators are 3 phase generators with full wave rectifiers. The
    ripple voltage is very much lower than what you specify (about 5%, I
    think). See:
    This part makes sense.
  2. Yes. This is true for a single phase AC sine wave.

    But an automotive alternator is a 3 phase source of AC, with 3 sine
    waves overlapping and shifted by 120 degrees, each. When full wave
    rectified, this produces 6 half sine waves (one arriving every 60
    degrees) overlapping per cycle. So there is no time when all of the
    half sine waves are zero volts, at the same time. By the time one of
    the peaks has gone down by 5%, the next one rises above that, and
    takes over. So the peak to peak ripple voltage is only about 5% of
    the peak voltage. if the peak is assumed to be about 14.5 volts, the
    minimum the alternator puts out is 95% of that or 13.77 volts. And
    this does not take into account the flat topping effect of the current
    on the not zero impedance of the alternator.
    Quite probably. For this reason, single phase battery charging
    systems do not supply load current, full time, unless they contain
    filtering components to reduce the ripple voltage.
  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
  4. Randy Gross

    Randy Gross Guest

    This is a gray area for me. Rectified AC is still AC to me, it's just
    all positive, so scratch the dc reference. OK. The AC is rectified and
    all positive with no smoothing. From 0 degrees up until the alternator
    voltage exceeds that of the battery, the battery supplies the load. What
    concerned me in this example is the voltage at the peak causing damage
    to the battery during the degrees of charge. I've read that voltages
    over 14.5v can cause the battery to overcharge and lessen its life. To
    achieve a 12v rectified output, you need 13.5vrms from the alternator
    minus the 1.4v drop across the rectifier. That gives a peak voltage of
    19v. Will this damage the battery?
  5. (single phase, I assume)
    If the peak is 14.5 volts, then the load must see 14.5 volta during
    those peaks.

    Are you saying that the supply averages 12 but peaks at 14.5?
    I am not sure what you are asking. Are you asking if there is some
    way to clip the peaks off so that the load never sees anything higher
    than 12 volts? That is what regulators do.

    Or are you asking if the increased current that the load draws during
    those peaks helps to sag them down a bit? It does, since no power
    source is truly a zero resistance source.

    Or am I missing your question, entirely?
  6. What is the source of power turning the alternator?
  7. John Miller

    John Miller Guest

    As written, it sounds as though there is nothing in the system but a 12v
    motor, an alternator and an automotive battery. If that is the case,
    you're wasting your time, for reasons that countless persons here will be
    happy to explain.

    If that is not the case, please clarify the configuration, particularly the
    external energy source driving the alternator.
    John Miller
    Email address: domain,; username, jsm

    Reportedly, in Blythe, California, a city ordinance declares that a person
    must own at least two cows before he can wear cowboy boots in public.
  8. John Miller

    John Miller Guest

    No, it's based on your attempt -- whether you realize it or not -- to build
    a perpetual motion machine. The energy consumed by the motor must be
    replentished by a source external to what you have described so far.
  9. You are attempting to build a perpetual motion machine - it won't
    work!! You will end up with a complex battery discharger.

    Neither the motor, battery nor alternator are 100% efficient. If, for
    example, you put 100 watts into the motor, you may only get 70 watts
    (a wild guess) out. Putting that 70 watts into the alternator might
    give you 50 watts out - which is not enough to drive the motor, much
    less allow the overall system to do any useful work.

    Peter Bennett, VE7CEI
    peterbb4 (at)
    new newsgroup users info :
    GPS and NMEA info:
    Vancouver Power Squadron:
  10. Randy Day

    Randy Day Guest

    Just to clarify; you're saying the
    alternator runs the motor which does
    .... what? Charges the battery? Powers
    the alternator?

    If you answer 'yes' to either of these
    questions, look up 'Law of Entropy' and
    'perpetual motion machine' on the
  11. John Miller

    John Miller Guest

    Well, there's the error. There two fewer sources of power than you think.

    The battery is a device that stores power.
    The alternator is a device that converts power.
    Neither is a source of power.

    John Miller
    Email address: domain,; username, jsm

    In case of fire, stand in the hall and shout "Fire!"
    -- The Kidner Report
  12. You have a big problem. Both alternator and motor are energy
    conversion devices. Neither makes energy. It takes more than 1
    mechanical watt into the alternator before 1 watt electrical comes out
    (it is not 100% efficient). The motor takes more than a watt
    electrical in to produce 1 watt mechanical out (it is also not 100%
    efficient). All you get by adding the alternator to the motor battery
    combination is that the alternator losses have to come out of the
    battery power. If the alternator was used only during braking, you
    could, theoretically put some of the energy that would have gone into
    heat production in the brakes into the battery, but that is the best
    you can do.
  13. Ratch

    Ratch Guest

    You all mean energy, not power, right? Ratch
  14. Yes, the system will reach a state of equilibrium - that state will be
    with the battery totally discharged, and the motor and alternator

    You have no "source of energy" in the system - the motor, alternator,
    and battery all _convert_ energy, with less than 100 % efficiency. (I
    suppose the battery is initially a source of energy, but it will not
    last for long.)

    It does not matter how you design the alternator - you can't get more
    power out of it than the motor puts into it.

    (perhaps you should have a talk with feerguy, who posts about
    impossible energy storage on one of the other sci.electronics

    Peter Bennett, VE7CEI
    peterbb4 (at)
    new newsgroup users info :
    GPS and NMEA info:
    Vancouver Power Squadron:
  15. Randy Gross

    Randy Gross Guest

    John Popelish wrote:
    Understood! There is one more question I may as well address while we
    are on the subject. The main focus was based on alternators. Assume for
    this question that the source of power is 60 HZ, rectified and smoothed
    with a peak of approx. 14.5v. The load is 12v with a maximum current
    draw of 15 amps. Can I drop the 2.5v without shunting a lot of current
    from the main circuit or, would losses make this unnecessary?

  16. Randy Gross

    Randy Gross Guest

    Single phase
    I want to use an automotive system in a Electric Vehicle I'm attempting.
    I want to use a 12v Lead-acid battery to start a 12v, 10 amp motor. Once
    the motor is engaged, I want the alternator to float recharge the
    battery and supply the 12v motor. I'm not even sure it will work as
    intended. I have one shot, currently, because of the expense and serious
    mistakes will hurt. I'm attempting to take the concept one step further
    and eliminate the need to plug the vehicle in every night for a
    recharge, charge-as-you-go so to speak.

    At the present time, I'm experimenting with a 60 hz source, single
    phase. If I can get the alternator to drive the motor with minimal
    assist from the battery, I will have crossed the first hurdle.
  17. Randy Gross

    Randy Gross Guest

    Initially, the battery starts a 12v motor that drives the alternator.
    One phase(?) is dedicated to driving the motor with assistance from the
    battery when needed. This is the gray area. Theoretically, the
    alternator should power the motor once the operating speed is achieved
    but, (not being an electrical engineer) I still know there's a flaw
    there. So, I included a battery not to drive the motor, but to assist,
    which is why I chose single phase. This would allow the battery to
    discharge and recharge intermittently and assist the alternator in
    driving the motor.
  18. Randy Gross

    Randy Gross Guest

    That's the reason I'm here John. The question that keeps reoccurring is:
    "How do I keep this thing running without draining the battery?" But,
    before I go on, is the "no go" from the group based on the battery
    supplying current to the field?
  19. Randy Gross

    Randy Gross Guest

    I was thinking that this was the common thought. No. Perpetual motion is
    a dream. What I am attempting deals with or remains within the law. I'm
    trying to arrange the law to work in this situation. I was so involved
    in describing the whole, I neglected to describe the alternator.

    This alternator is hand built with two banks of 3 phase windings. The
    rotor has 16 permanent magnets separated on a two 8 pole disk. This is
    the power source and, along with the battery, are consumable. I am
    simply trying to arrange it to where they work together to produce the
    result I want and stay within the law. Each one of these devices
    produces power, one has to be charged, the other rotated to produce. My
    problem is the handshake.
  20. Randy Gross

    Randy Gross Guest

    This is a start Peter. Like other advances in the past, they had to have
    a beginning. It may not work properly to begin with but, there are great
    minds out there that may be able to find the pieces to the puzzle,
    within the law, to make this a reality. It is not Perpetual Motion. It
    is applying the law in ways thought not possible. Entropy tells me this
    device will try to find a state of equilibrium. OK! Entropy also tells
    me I must keep it in "disorder" to work. I haven't circumvented the law.
    The law told me what to do.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day