I think with new bios firmware it might even work while turned on (though slowly). Did you try charging over one of the thunderbolt 3 ports as well? The spectre 13 single usb-c gen1 port requst charging at 15V 3A (confirmed by gtrusted) and I think that the two thunderbolt 3 ports (like tb3 ports on elite x2 and elitebook folio) only accept 12V 3A. The macbook charger can only support 5V and 15V. So I would guess that it won't work on the two tb3 ports.