The Google Pixel XL supports standard USB Power Delivery 2.0 technology for fast charging over USB Type-C which means the phone should be able to charge with compatible chargers like the Anker PowerCore 13400mAh Nintendo Switch Edition PD Portable Charger. Let's see how the Anker power bank delivers power over USB Power Delivery to the Google phone when directly connected using a USB Type-C to Type-C cable.
The Google Pixel XL on a dead battery at plugin was observed to quickly pull current along a 5 volts rail startup voltage to briefly draw up to 5.1 watts (5.1 volts @ 1 amps) from the Anker power bank. Charging then got terminated for few seconds when the phone suddenly stopped pulling any current. After the voltage rose up to a 9 volts rail only then the phone began pulling current again to resume charging at about 4.3 watts (9.2 volts @ .47 amps). The phone eventually pulled higher current to settle at around 10 watts (9.1 volts @ 1.1 amps) of steady state charging with the power bank. Click here to download the trace captured by the GRL-A1 power analyzer to view more details.
By pulling only about 1 amps current, the Google Pixel XL kept charging at around 10 watts thus not taking full advantage of the 15 watts minimum that is typically possible with USB Power Delivery technology.