I removed the insulation on the USB cable, cut the red cable and soldered a 100 ohm resistor in series. This allows enough voltage to pass through the cable to keep the USB connection alive, but reduces the charging amperage to the minimum. Although the Android battery icon shows the phone is charging, the charge percentage isn't growing. In this mode, the
Mavic 2 controller discharges at a rate of 1% per ~3 minutes.
For comparison, when I leave the controller on without any USB connection, it still discharges at a rate of 1% per ~3 minutes.
A simple resistor does the job quite well.
Tested on Samsung A50 and a
Mavic 2 controller, but I am expecting similar results on Mavic Mini.