I've done this before, and never want to do it again. I used 2 mutlimeters. One was reading the amp draw, and the other the voltage. For my load, I used a variable resistor, and as the voltage dropped, I adjusted the resistance to keep the amp draw the same. A lot of work, and you have to babysit the batteries. Not easy or enjoyable. Much easier and simpler(and more accurate) to just get a charger/discharger that reads out the capacity of the pack.