I have just cut into a USB cable to get access to the +5V and ground wires. The cable is the normal big USB plug to a micro USB B plug, i.e. the 'normal type Android phone data and charge cable.
After cutting the +5V and ground wires I was able to add a variable-resistor/decade-box in the +5V wire and add a digital multi meter (DMM) to measure the current flowing through either the +5V wire or the ground wire.
I wasn't prepared to risk using either the drone'
s controller or my 'good' phone as Guinea Pigs so connected the cable between a powered USB hub ( connected to my computer) and an old Samsung smart phone.
With the resistance set to 0 the current in the +5V wire is around 450mA, and the phone is seen by the computer.
Once the resistance is increased to 3 or 4 ohms the analogue display of the DMM goes haywire and indicates a rapidly varying current, the value indicated on the DMM's digital display drops significantly but is unstable. The phone display switches between showing charging and showing not charging, the computer was still able to see the phone.
I did flick through a few higher resistances and the DMM's displays did not settle down.
As I didn't want to needlessly risk damaging the phone I returned the resistance to 0.
Disconnecting the ground wire did not disconnect the phone or from memory reduce the current in the +5V wire.
It seems to me that what ever goes on in these cables and connections is not simple................ho hum