This circuit is for a temperature controlled constant current NICD charger.
G. Forrest Cook, September 1993, cook@stout.atd.ucar.edu
This circuit may be used and copied freely as long as credit is given to
the designer (Forrest Cook).  Commercial distribution is not permitted.

The Thermistors and most of the other parts in this circuit may be obtained
from DigiKey (1-800-DIGI-KEY).  Also see: http://www.digikey.com/
I have no connection to DigiKey other than being a satisfied customer.

Theory:
  This charger will help prevent overcharging of NICD cells.  Overcharging
is one of the main causes of low cell life because hot cells pop their internal
seals and vent electrolyte.  A useful side affect of cell heating is that
NICD cells on a charger stay cool until they are almost fully charged at which
point the cells start to warm up.  This charger senses that warming and flips
to a trickle charge mode.  The temperature is sensed by a differential
thermometer which compensates for changes in room temperature.  The charger
is started by pressing the reset button and will charge the pack with
a preset constant current.  When the pack becomes warmer than the reference
temperature plus a small temperature delta set by the potentiometer, the
charger flip-flops to a trickle charge mode which the pack can remain
connected to indefinitely without harm.

Construction Hints:
  Parts placement is non critical with the exception of the thermistors which
should be epoxied to separate aluminum plates and connected back to the main
circuitry via a cable with two wires and a shield.  The 7805 regulator should
be mounted on a heat sink so that it never gets too warm to hold.  The current
setting resistor may be a high power potentiometer, switch selectable
resistors, or a single fixed resistor.  Be sure to thermally separate the
battery and sensors from the other electronics so the heat from the circuitry
doesn't affect the sensors.  In my prototype, I glued the both thermistors
to separate aluminum sheets, approximately 2" X 2" and set the battery
sensor on top of the pack being charged, usually with a weight on top.
It is important to make sure the batteries have a decent thermal contact
to the sensor.

Calibration: 
  Allow the two temperature sensor plates to reach the same temperature,
place a volt meter between TP1(-) and TP2(+), adjust the sensor balance trimmer
for a reading of -0.02 volts on the meter.  Press the reset button and make
sure the "charging" LED lights.  Warm the battery temperature plate up and
observe that the "done" light lights.

Usage:
- Connect a NICD pack to the charger "NICD +" and "NICD -" connectors.
- Place the "battery temperature" sensor on top of the NICD pack and hold
  it in place with tape or a heavy object.
- Place the "reference temperature" sensor in a location that is not too
  close to the charger, the battery, or any other source of heat.
- Press the "reset" button, observe that the "charging" light lights.
  Note: if the battery was recently discharged at a high rate it may be
  warmer than the ambient temperature, let it cool for a while or place
  a warmer object on the reference sensor if you're in a hurry.
- Note that some cells in a series string will always be first to get warm.
  After several cycles it would be a good idea to leave the pack on the
  charger for a few hours to trickle charge the lower cells up to a full
  state of charge.  This process is called "equalizing" the pack.

Discharging NiCd Packs:
A good way to kill NiCd cells is to put a reverse voltage on them.
If you are in the habit of fully discharging your NiCd packs, the weak cells
will go to zero volts and then go negative as the stronger cells continue
discharging, thus damaging the reversed cells.
If want to discharge your cells, it is best to discharge them individually.
A silicon diode in series with a resistor on each cell will discharge the cell
to about 0.7 Volts, which is better for the cell than a full discharge to zero
volts.
For multi-cell packs, this circuit can be built with a multi-cell battery
holder and a handfull of cheap parts.
A typical discharge current is C/5 where C is the amp hour capacity of the
cell.
For a 500ma AA NiCd a 1N4001 or similar 1A diode in series with 5 ohm 1/2W
resistor should give a 100ma discharge rate assuming a 1.2V cell, and an 0.7V
diode drop.

Afterthought:
  I This circuit has been working quite nicely for several years now, I have
recharged many packs repeatedly with no problems.