Seems that should work, not clear on the charging speed though.
Regardless of your solution you're aware that you'll get less than 2 charges from your powerbank, right? Maybe just buying another battery or 2 is actually better than buying and...
As I mentioned with 15V it will work if the powebank allows going above the official 3A max of the 15V profile. If he showed it working it's probably fine with this specific powerbank, but it's not a given for others (or possibly revisions of the...
He wants to charge from a PD power bank, not a vehicle, the quote was an attempt to have said power bank "simulate" a car for the car charger to be connected to but yeah, won't work.
They don't care, but the charging regulation is NOT internal to the battery so whatever you connect must be a proper CC/CV supply adjusted accordingly.
Problem is the DJI car charger stops early to prevent overdischarging your car battery, at...