
NiCad Battery Discharger
Russell Salerno - 2/25/93
73707,1105


An easy-to-build, inexpensive battery discharger.


HOW IT WORKS
------------
The pushbutton initiates the discharge cycle: once current flows through
the switch, and thus the relay coil, the relay will hold itself closed.
When the battery is drained to about 2-3 volts, the relay will open,
shutting off the circuit and keeping the battery from going completely
dead (a no-no for nicads).  It's not precise, but it works just fine.
I added the pilot lamp to show that the unit was in fact discharging
the battery, it also gives you a good idea of battery condition.


SCHEMATIC
---------

    |-------------------------|-------------------------|
    |                         |                         |
    |                         |                         |
    |                         |                         |                         
    |                      \                         \
    |       N.O. pushbuton  \                         \   N.O. relay contact
    |                        \                         \
connector                     |                         |
   to                         |------------|------------|
 battery                      |            |            |
    |                         |            |            |
    |                        +++           |           +++
    |                        +++          +++  6v      +++
    |        10 ohm, 10W     +++          +++ pilot    +++  5v relay coil
    |     wirewound resistor +++          +++ lamp     +++
    |                        +++           |           +++
    |                         |            |            |
    |                         |            |            |
    |-------------------------|------------|------------|


PARTS LIST
----------

Radio Shack parts:

relay             SPDT, 5V coil, 1A contacts      275-240          1.99
pilot lamp        ("grain-of-wheat")              272-1098      3/  .99
N.O. pushbutton                                   275-1547      4/ 2.69
resistor          10 ohm, 10W wirewound           271-132       2/  .99
connector female  5.5mm OD, 2.5mm ID              274-1576         1.59
connector male    5.5mm OD, 2.5mm ID              274-1568      2/ 1.29
enclosure         "Archer Perfbox"                270-291          4.99

misc parts:

12-2 Romex (NM) cable
some stranded hookup wire
soldering iron, solder, etc...


I assembled the unit in a Radio Shack "perfbox", it's size is ideal.
Because I wanted to use the unit with different types of batteries
(I have NP-22 and NP-77 batteries laying around) I went with the barrel
connectors (272-1568) rather than hard-wiring the battery clips.  My
biggest concern was getting good contact with the battery terminals.  The 
nicads packs I have were alligator-clip-resistant -  no commercially
available clips worked.  I finally made a clip out of a few inches
of 12ga solid copper wire yanked from a piece of Romex (NM) house 
wiring cable.  I took the black wire, stripped the ends, and fashioned it 
into a C-clamp shape that gripped the battery pretty nicely.  You'll need
a pair for each battery type you want use with the unit.  I've also 
seen battery connectors made using velcro, so feel free to experiment.

This circuit is non-polarized, that is, it doesn't care
which terminal is positive and which terminal is negative.

In use the resistor gets quite warm, this is normal.  Ohms law tells us that
a 6v battery will dissipate 3.6 watts through a 10 ohm resistor, well within
the 10 watt rating of the resistor.  10 ohms also provides a discharge
current of 600 ma, which gives a reasonably fast discharge without running
the risk of overheating the battery.



===========================================================================
Disclaimer:  This information is provided for informational purposes only.
The author assumes no responsibility for any damage, injury or any other 
occurrance, mishap or event that may directly or indirectly result from the 
use or misuse of any device described in this document.
===========================================================================

