Give up on the idea of a drone being able to lift the extra weight of a high current supply cable, itsnot going to happen.
The only hope for tetheredflight is to send the power at a higher voltage and buck-convert on the rotorcraft
Remind us how you are converting the 20V down to the LiPo voltage?
That behaviour sounds more like you are overheating the voltage converter.
with two very thin (0.2mm) cables to power on ground.
Did you take your multimeter and measure the total resistance of your supply and return wires? Eg...... how many metres of wire is it? And what's the measured resistance along those lengths.
How is that tiny 'copter supposed to lift that huge buck converter then?
If you want to run from external power you need high power high voltage low current motorson the 'copter to allow very fine wires - and no-one to my knowledge makes such things.
Just the drag and torque forces from lifting a long wire are likely to mess up the flight stability too - I reallydon't think its going to work.
At least 12.3g of additional cable should be fine for drone to lift tethered.
Want more flight time for indoor use? (which I think is the original point)Get more batteries.