As far as I can tell, the latest 1.1.7 BIOS update for the XPS 13 removes the ability to charge via USB Power Delivery at 5 volts. I didn’t do a lot of testing, but I know that when I first got the machine, it did charge from a Nexus 6P charger plugged into the Thunderbolt/USB Type C port. Slowly, and the BIOS griped at boot, but it did charge.
I’m pretty sure I lost the capability with the BIOS update. I’m going to try reverting to 1.0.5, but the machine won’t do a BIOS update unless it’s plugged in, and at the moment I don’t have a charger.
Dell, if you’re listening and I’m correct that the change was intentional, please reconsider this regression. Some of us bought the XPS 13 in part because of its ability to use USB Power Delivery (without qualification). And even if the charge rate is extremely slow, 5-volt charging can be a day-saver for someone who left a standard charger at home but happens to have a USB-C phone charger available (like me).