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Note: this is not about a specific device but the general aspects of charging a USB device using accessories with different USB standards so I hope this fits here.

I've got a Samsung Galaxy Smartphone with a USB 2.0 Type C charging port.

Using the stock Samsung wall plug (9 V, 1.7 A output or 5 V, 2 A fall-back) and charging cable (USB A to C) the device gets charged utilizing Samsung's "adaptive fast charging" technology.

Using the included charging cable with a generic USB A wall plug without Power Delivery (5 V, 2 A output) nets a very slow charging speed of approx. 900 mA. Incidentally the same current as specified for USB 3.0 ports.

The same exact charger and a regular Micro USB cable would charge any old smartphone with a maximum of 2A sans utilizing USB-PD. Why doesn't it work with the OEM Samsung USB A-to-C cable? The USB 2.0 specification also states a maximum of 500 mA and every charger in the last years exceeded that. The Samsung device doesn't even have a USB 3.0 port, it's just USB 2.0 with a Type C connector.

I'm trying to understand this from a technical viewpoint. Can you point me into the right direction? USB 3.0+ and Type C is honestly pretty confusing.

USB Type C is simply the connector - the protocol in use is USB 3.X. Of course, it affects the connector, but the limitation is in the phones support of 2.0 and not because its a type C connector.

Also, AFAIK Samsungs "adaptive fast charge" actually uses Qualcomms solution of delivering a 9V load instead - this is then converted at the phones side. VOOC (or oneplus Dash) charging works with current only, this is where you may get confused. I think the device should still take 2 Amps, but it may be device limited to work only with samsung fast chargers - for safety or for exclusivity, maybe both.

If you see Oneplus cables, they are comparably quite thick - this is to support the thicker wires, which of course are needed for the 4 amps sent to the phone. Your cable might not be thick enough to support the larger currents - try another USB-C cable if you can, one that's preferably rated at a larger current. The included cable is good for 9V at a lower current, but may not support currents larger than the fallback of 2.0A. That's why its recommended to try different chargers. cables, etc. Also, to make it a fair test, try and let the battery discharge to somehting like 20% - so that the device isn't software limited the charge! (It reduces it to scale of milliamps at 80% to protect the battery)