I know this is an ancient post, but wanted to report some findings I'm doing on my current loaner while my Model 3 is in the shop:
I had a rather janky setup but had the following equipment:
- Drok USB Power Meter
- Note 8
- iPhone 7
- Anker 10000 mah powerbank
- A variety of USB cables and adaptor
- 2018 Model S 75D
Plan on getting a proper USB3.0 extension cable so I can test the front USB ports properly. My existing set of adaptors/dongles causes too much voltage drop due to the increased resistance impacting the tests. I was unable to connect the evice directly to the front USB ports as well due to the shape of the tester. What I did find was the following:
- Front USB seems to have a greater voltage drop than the rear USB
- Rear USB can charge up to 1.5A on my powerbank, but all ports seem to limit to 1A on my both my phones
Front USB -
Vopen ~ 5.09
Vload ~ 4.5 to 4.75v (loose connection and or extension cable so...take these values with a grian of salt)
Max Current ~1 A with phone charging
Rear USB
Vopen ~5V
Vload ~4.9v
Max Current ~1 A with phone charging, ~1.5A charging powerbank directly. 1A with USB cord
Check out this dongle monstrosity

in my attempt to get an working extension cord. Current dropped to a mere 0.3A at 4.5v due to the poor connection and reduced resistance.
USB Y-Splitter -> Mini to Micro USB dongle -> Micro to USB A dongle -> USB power meter -> USB A to USB C -> Note 8

I had some other various USB extension cords but all them had some type of obstruction that prevented a proper connection.