Many devices that are powered by a Li-Ion battery can be recharged over 5V DC supplied by a USB connection.
Is the 5V voltage internally converted by the charging logic to something lower, and is that process energy efficient (e.g. by using a DC-DC converter), or is some of the power dissipated as heat (e.g. by simply using a resistor to reduce the voltage)?
If so, is there a difference in efficiency when the battery is empty vs. when it is almost fully charged?
My practical reason for asking is that I am using a portable USB battery pack to charge my devices and I want to use the energy stored in it as efficiently as possible. It would be useful to know whether charging from 50-100% is more or less efficient than charging from 0-50%.