cobbling together a ni-cad charger

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

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

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

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

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.

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

that timer sounds like a good idea.