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USB usually provides 5V (or even more, for example 9V with Qualcomm Quickcharge) and the phones contain a single li-ion cell with 3.5-4.3V, so there must be some dc-dc converter in between.

I wonder if it is typically a switching regulator and what efficency they provide in the smartphone environment? After all, how much energy input is needed for a given nominal battery capacity?

I could not find concrete numbers on this, the more detailed power bank calculations only go to the 5V output and then assume the same current at the phone battery, like a simple linear regulator. Which seems to be pretty close in practice? Has nobody done any real mesurments so far?

Hints to battery managment IC datasheets from common smartphones are also appreciated.

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It just depends on the charge current. Newer, higher powered smart phones use DC-DC conversion. But older phones that make do with 500mA or less USB current typically used linear charge control. If you do the power dissipation calculations you will see why. Once the charge current is higher than a few hundred mW, the dissipation for a linear regulator becomes difficult to manage. This is the driving force behind the choice. Thermal management (not efficiency, per-se).

Look at TI single cell charger IC's. I am not trying to endorse a specific brand. They just have a large variety of single-cell charger IC's. Efficiency will be 85% or higher.

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The ICs that go to into smartphones are called ether "Battery Management" or "PMIC", plus other service chips as "battery gauges" etc. The list of silicon providers is vast, starting with

Texas Instruments,

Analog Devices/Linear Technology,

Maxim Integrated,

Intersil/Renesas,

Diodes/Pericom,

and maybe another dozen of less known Chinese manufacturers. It is really difficult to miss these offerings, together with many white papers about application efficiency etc.

Charger efficiency itself is usually above 90%, and coloumbic efficiency of the entire battery charge-discharge cycle itself is about 88-90%

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