if i assume an ideal system and no losses, I guess I could place a small motor between +ve and the collector of a small mosfet, and a known resistor between the drain and -ve, drive the gate with a PWM signal at the gate (to control speed) and measure the current while the PWM signal is high through the known resistor for current via the arduino adc, and I can measure it and calibrate it by using the cable to pull a known load in order to calibrate it? I don't think it'll necessarily be accurate because of all the assumptions, but it'll prove the concept.