The problem there is you need to build a constant-current load (one that does not vary with voltage). A resistive load varies greatly with voltage, ie. a 1-ohm resistor will draw 1.2A at 1.2V and by the time the battery drops to 0.9V, it will only be drawing 0.9V. You can kinda fudge things by taking the average, but you're going to lose real accuracy if you do that.