The circuits suggested so far are constant current sources, correct?
The load characteristics have to be such that a constant number of amps is being drawn from the battery (20A is my initial design point).
Accuracy doesn't have to be spectacular, within half an amp or so. (also, adjustable amperage between battery drain tests would be a nice additional feature)
The project is basically life cycle testing to figure out the change in capacity of a li-ion battery over multiple (probably hundreds) of charge/discharge cycles. The arduino comes in handy for automated data logging, alternating between charging and discharging, monitoring temperature, etc.
So the first part I am tackling is the circuit design for getting a constant current (20A) from the battery.