The Samsung Galaxy S9+ has the advantage of supporting USB Power Delivery 2.0 on top of Samsung's own brand of Adaptive Fast Charge technology which allows a wide range of external chargers to be used with the phone. Here we can see how the Samsung Galaxy S9+ gets power supplied over Type-C by certain USB Power Delivery 2.0 based power banks like the Anker PowerCore 13400mAh Nintendo Switch Edition PD Portable Charger.
Plugging in the Samsung phone to the Type-C port of the Anker power bank, the phone at 0% battery quickly pulled current at the beginning of a 5 volts rail startup voltage to initially charge up to around 7.8 watts (5.2 volts @ 1.5 amps). When the battery has charged for about 65 seconds, current was pulled much higher resulting in about 13.3 watts (5.3 volts @ 2.5 amps) charging. When the phone was actively used it pulled lower current to charge at mostly around 11.7 watts (5.3 volts @ 2.2 amps). While in sleep mode the phone drew higher power to max out charging at around 13.3 watts on average. Click here to see more details by downloading the trace captured by the GRL-A1 power analyzer.
The Anker PowerCore 13400mAh Nintendo Switch Edition PD Portable Charger is able to provide decent charging at close to max 15 watts the Samsung Galaxy S9+ can get from its own shipped Adaptive Fast Charger.