Anything with a closed circuit has a correlative relationship to operating speed and temperature. Often, the higher one goes, so does the other. In the case of battery charging, either extreme of cold or heat can prove harmful to the cell and firmware would help best adjust charge rates to cope.
But when the Google Pixel 2 XL says it is “charging rapidly” while only charging at less than 4W, that’s just a mischaracterization of what’s going on.
Android Police heard some users’ claims of this behavior during cold weather. Under normal conditions, the second-gen Pixel device should charge above 10.5W, ideally at the billed 18W rate listed for its complementary USB Power Delivery adapter.
However, in comparison testing the Pixel 2 XL with the Pixel XL, OnePlus 5T, Essential Phone and Nextbit Robin, the publication found that while the other phones charged more or less the same cold as when warm, the difference between cold and warm peaks on the Pixel 2 XL was shown to be 12.5W. Full details on the test can be seen at the source link below this story.
It’s a replicable test, if not a perfect one given the nature of reportage from most power meters and component thermometers. But the major point seemed to have been affirmed by this trial: the Pixel 2 XL is slower to charge when it’s colder than room temperature.
In a statement, Google said that it would work on changing the interface — that would imply that the “charging rapidly” tag is due for replacement. But alas, it’s something for the rest of us to keep in mind.