I understand at a basic level that a USB DCP simply advertises that it is a DCP, and then basically puts 5V on Vbus, and then the battery can go to town.
With that in mind this is really a 2 part question:
How does a battery limit it’s current to the 1.5A specified by the USB BC 1.2 spec? Take the example of a charger capable of outputting 2A, advertising as a DCP. Assuming the device being charged is compliant with the spec, how does it limit the current draw to 1.5A given there is no communication between the two devices? Does it modulate some sort of load switch? Does it even limit in the first place?
Are USB portable devices responsible for implementing the typical lithium ion battery Constant-Current/Constant-Voltage charge cycle? If so, does it taper the current by some method described in (1)? Or perhaps it relies on the increasing impedance of the battery as it becomes more fully charged? Perhaps it doesn’t implement a CC/CV charge cycle at all?
I appreciate any feedback on this. It seems like the answer should be fairly straightforward, but it’s been bugging me for a couple days.