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I know that fast charge technology allow chargers to communicate with the battery management chip inside the mobile phone in order to use different voltages to charge the mobile phone instead of just using 5V and have a high current like 2A or 3A.

But I don't know how that charge is transfer inside the mobile phone to the battery. I know normally batteries are 3.7V.

How a fast charge using the same power (2A x 5V vs 1.11A x 9V) will affect the heat in the battery of the mobile phone ? I know that 2A will produce more heat in the wire/components, but not sure how it will affect the battery.

EDIT: I think the right answer is in another topic here.

Li-ion cell charging voltage ranges from ~3.7V to 4.2V. 18W/4.2V = 4.29A. The charger has a DC/DC voltage converter circuit which pulses current through an inductor to 'transform' the voltage down and current up. This process is typically 85~90% efficient, so it should be able to deliver 4A for most of the charge cycle.

Help me to understand VOOC fast charging for lithium ion batteries?

So the heat will be the same, since its convert 9V voltage to 4.2V or 4.4V and increase current, no matter the method you use for fast charge.

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So-called "fast chargers" convert the input voltage (5V or 9 V) into charging voltage for the Li-Ion battery, which is typically 4.2 or 4.35 V most of time, depending on battery's particular type. The idea of "fast charging" at elevated input voltage is to diminish input losses across charging cable and connectors by having "more voltage at less current", the same idea as is used in high-voltage AC transmission lines in common electrical grid.

Having less losses on input side, the "fast charger" can deliver higher current to the battery, in order to charge it "faster". And higher current produces more heat inside the battery (heat gets generated from parasitic internal resistance of the battery), and from DC-DC converters (higher voltage difference is less effective, and higher currents on output side also generate more heat. So the "fast-charging" smartphone usually feels quite warmer than when charged in ordinary mode.

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The battery cell itself probably charges at about 2.7V at the start - 4.2V at termination. The phone needs some sort of power management circuit to accept power at a different voltage and current and properly charge the cell. During the charge, a properly functioning controller is likely to push the same amount of power into the cell, so you're interested in the efficiency and the ripple of the output and how it relates to the input voltage. The higher voltages are just a way to get around the current limits that make transmitting the required power through small cords and connectors challenging.

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