I have observed many many times during conventional menthod of current calibration of the 3DR Power module using arming the copter and motors that the current displayed by the power anlyzer is not stable and fluctuating by 1-2Amps or even higher. And calibration done via this old conventional is not proper and current reported in mission planner while the copter is in air, is in error by 1-2amps and that makes lot of error.
Initially I thaught of using two 100Watt 1Ohm resistors in parallel and energizing them via 30Amp brushed motor ESC connected to the output of the 3DR Power module (power tapped from same points where 4 ESC's are soldered. To control the brushed 30Amp ESC, I used standalone cheap servo tester. I mounted the two 100W 1Ohm wirewound resistors which were already in aluminium heat sinkable cases, on an additional large aluminium heat sink with a fan. I set the servotester knob to display 10Amps of current in the Turnigy Power Analyzer, but very soon the current dropped around an ampere lower. I again adjusted for 10 Amps and same thing again observed. Reason for this was the ohmic value of the resistor depend on the temperature and on increasing temperature the net resistance was increasing and hence the current drawn was decreasing. I then rejected this method and thought something else.
I then thought of using car head light halogen 100W lamp in place of two wirewound resistors, this was rather simpler and required no heat sink and the current drawn value whatever I set to was almost very stable. Rest you can watch the video.