Re: cobbling together a ni-cad charger

kell wrote: 
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.


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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