I use a 50 ft long extension cord to run off of my car while it runs outside to do some testing with so I get the same current and output that I would on the car. Just clip the ends and add gator clips at one end and a block of 2x4 and two 16 penny nails about 6 in apart at the other end then wrap whatever wires you want around the nails. I use 12-2 for mine.
You can expect different results with different methods of powering your unit, so try to stick to one that you will be duplicating when you use your cell on your car(the same car you will be using it on should be the one you test with as well). A charger, on or off the battery, on or off the car, will give you different results so try to keep the battery charged with out using the charger with the cell on.
I actually tried this two ways; Running the cell with a analog amp-meter and with a digital current sensing meter, and the analog meter made my cell perform 30 times better! It causes a break of sorts to pulse the current where as the digital one only reads off the current it senses through insulation. The only thing that changed was the presence of or not of the amp-meter.
Its done right or its not done !
Hail HHO.