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Transformer current and a NiMH battery pack

Discussion in 'Electronic Basics' started by [email protected], Feb 17, 2005.

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  1. Guest

    A while back I built an adjustable power supply with a 12V 1.2A power
    supply, a full wave bridge rectifier, and an LM117 adjustable
    regulator. I was thinking about using this adjustable power supply to
    quick charge a small 5-cell 6V NiMH battery pack I use in a R/C car.

    If I connect the battery pack to my adjustable supply (set @ 6V) what
    current will the power supply put out? Will the power supply put out
    the 1.2A rating of the transformer? I do plan to carefully time the
    charge so that I do not overcharge the pack.
     
  2. You built a power supply by using a power supply? Hmmm.
    Power supplies, unless designed with current limiting to protect
    themselves, will put out enough current to damage themselves.
    The question you want to ask is what current can be taken
    from the supply without degrading its reliability to a level
    worse than you would like or need.
    It is a rare power supply using linear regulation that can
    deliver (for very long) a DC current equal to the RMS
    current rating of the transformer. A typical ratio is half
    as much DC current as RMS tranformer rating, due to
    the cresting factor in the transformer current waveform.
    I doubt that will help. Your charge times are likely
    going to be longer than the thermal time constants
    of the components that would be over stressed.
     
  3. Guest

    You built a power supply by using a power supply? Hmmm

    I meant to say transformer.
     
  4. Well I guess you havent got too much to lose by connecting up to the battery
    with the volts set low and then increasing until the charge rate or the max
    output of the supply is reached (current). At that point you should be
    checking the heat generated on the heat sink of the supply and of the
    battery. This would be a check to see if it would be possible to do what you
    want. If successful then you will have to consider how to make it work for
    practical purposes, ie when the battery is discharged it would be easy to
    overload the supply if you set the volts too high. Probably some form of
    current limiting could be used and constructed easily if it's not in your
    supply anyway.
     
  5. Bill

    Bill Guest

    Well, you can't set it at 6V or else it will never charge because a 5
    cell NiMH has 1.45V/cell under charge.
    Therefore your pack voltage under charge will be
    5 x 1.45 = 7.25VDC
    You don't say what the capacity of your pack is but IIRC from my days
    in R/C racing they used 1.4AHr packs so that is what I'll base all the
    following calculations on.
    If you wanted a slow charge you could charge at C/10 rate which would
    be 140mA.
    If you set your power supply to 12VDC and then hooked a current limit
    resistor between the power supply and the battery you would calculate
    as follows
    R = (12-7.25)/.14
    which equals 33.9 ohms.
    Figuring your power dissipated in the current limit resistor
    P = .14x.14x33.9
    which equals .664 Watts
    so for a C/10 charge you would be fine with a 33 ohm, 1 watt resistor.
    To summarize set the power supply to 12VDC and put a 33 ohm, 1 watt
    between the power supply and the battery pack.
    It should take about 15 hours to get your pack charged completely at
    this rate but at C/10 you will not cause any damage if it goes longer.
    If you want a faster charge C/5 (.28 amps) then half the resistor
    value to 16 ohms and double the power rating to 2 watts. This should
    have you charged in about 7 to 8 hours.
    Not knowing anything about the internals of your power it's hard to
    say whether either of these current rates would cause damage to your
    supply but I doubt that .28 amps out would cause any grief unless you
    have some kind of low current fuses or really low power devices in
    your internal power supply circuity. Really it's not possible to know
    without more info.
    If you want a really fast charge, on the order of 2 and 1/2 hours you
    can put a 6.8 ohm, >= 3.5W between the 12VDC power supply and the
    battery and charge the pack at C/2. If you do this, I would get the
    battery off charge pretty soon after the 2.5 hours. I would also
    watch my power supply carefully to make sure it can take it.

    -wlb
     
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