Anyway, here it is:
This was posted by Mark Richards <akafoofighter@hotmail.com> on the newsgroup rec.models.rc.land awhile ago. His original posting (including the schematic and other files) can be retrieved here.
To change the current this discharger will drain your batteries at, change the value of R2 using the formula V=I*R. Since you know V (your battery voltage in volts) and I (the current you'd like to discharge at in amperes) then a more useable formula would be V/I=R. Make sure you use a resistor of sufficient wattage -- remember that P=IV where P is the power you need, I is the current you are drawing, and V is the voltage drop of the resistor.
A couple examples. Let's say you want to discharge your 7.2 V
battery at 20 amperes. This means you would need
7.2/20=0.36 ohms or thereabouts resistance. You would also need
a resistor capable of dissapating 20 Amperes * 7.2 volts = 144 watts
which is a whole heck of a lot of pwer. This can be accomplished by either
using something like light bulbs wired in parallel, or a whole bunch of
power resistors wired in parallel. Make sure your relay can handle the
extra current!
Anyway, here it is:
Parts List. ( codes at time of making for MAPLINS electronic stores )
--------------
Verostrip Board FL17
Zener Diode 5.6V QF47B
Transistor BC107 QB31J
Resistor 1 1k, M1KO
2 1Ohm 25W P1.0R
3 360R M360R
4 360R M360R
Relay 6v DPDT FJ42V
Push Button FH59P
LED's Red and Green WL27E, WL27F
Diode 1N4001 QL73Q
Case LF10L
Heatsink FL41U