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cobbling together a ni-cad charger

Discussion in 'Electronic Basics' started by kell, Apr 29, 2005.

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  1. kell

    kell Guest

    I have a cordless floor sweeper that's missing the wall wart power
    supply it came with. Inside the sweeper there is a 7.2 volt nicad pack
    with just a power resistor to limit current to the battery when
    charging (very primitive). The resistor is something like an ohm, and
    the size is a watt or two. I don't know what voltage the wall wart
    was, but it was probably unregulated. The sticker on the sweeper says
    that when new, and charging the (empty) battery for the first time, to
    let it charge for 16 hours.
    When experimenting I hook the sweeper up to wall warts that have 15 or
    20 volts open circuit things get hot -- the resistor in the sweeper,
    and the wall wart.
    I have a 7808 8-volt regulator I could wire up to a 12 or 15 volt wall
    wart. My question, is 8 volts enough for charging a 7.2 volt nicad
    pack? A really slow charge is good, this thing will sit in the hall of
    the building where I live and probably just stay on the charger all the
    time. If 8 volts is too low, I can get whatever voltage I need by
    putting a diode or diodes in the ground terminal of the regulator to
    bump the voltage up, or use a voltage divider like on a 317... I'm
    really asking what fixed voltage would be best, with a 1-ohm resistor
    in series, to leave a 7.2 volt nicad pack on all the time and not
    overheat; it's okay if it takes a whole day to charge.
     
  2. mike

    mike Guest

    Use your 15V wall wart and put a light bulb in series. Size the light
    bulb to get the current you want. I expect 100mA is good depending on
    the size of the batteries. 12V automobile lamps are good for this.
    Don't GUESS, don't use the rated current of the light bulb, you won't be
    running it at rated voltage, MEASURE the current in your circuit.
    mike

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  3. spudnuty

    spudnuty Guest

    I have a circuit that uses a light bulb as above and a LM317 voltage
    regulator to limit charging current to 100ma and adjust the cutoff
    voltage as required.
    Richard
     
  4. mike

    mike Guest

    Tell us more.
    How did you configure the light bulb as a current limit without
    letting the voltage on the lm317 input collapse? I've never tried to
    run a LM317 with in/out voltage less than the min spec.
    What voltage do you use for cutoff and your rationale for choosing
    that particular voltage?
    Thanks, mike

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  5. kell

    kell Guest

    It's a little unclear whether spudnuty used the 317 in the
    constant-current config or in the usual voltage-regulating config and a
    bulb in series with the output.
    Mikes suggestion is the simplest but I want a setup where the charging
    current drops off as the nicads voltage rises. That's why I was
    thinking about using a comparatively low 8 or 9 volts, so that as the
    batteries rising voltage approaches that, the voltage across the
    current-limiting element (bulb or power resistor) will fall very low,
    causing the charging current to diminish. Actually I know this is not
    the best way to charge nicads; it is more how you would charge
    lead-acid batteries. Maybe I should tape a thermistor to the battery
    and use that to turn off the charging.
     
  6. mike

    mike Guest

    You can do that with lower differential voltage and lower voltage light
    bulb.

    Actually I know this is not
    Temperature cutoff is the best way I know to cook the life out of nicads.

    For my dustbuster, I use the supplied charger on a timer. Every night,
    it charges enough to put back the average daily use plus some efficiency
    factor. This works well even though my cells are old and have
    significant self-discharge.
    mike



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  7. kell

    kell Guest

    that timer sounds like a good idea.
     
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