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Mobile phones (simplified)Edit
When applying a plain 5V DC to a mobile phone's USB charging port, the phone usually draws 0.5A.
If the data lanes of the USB cable are shortened, it signals the mobile phone to draw an indefinite amount of current.
The charging current could be limited by an insufficient voltage, because Lithium-Ion batteries require 4.3 volts to reach their maximum charging speed.
Some mobile phones (and also powerbanks) do not accept any voltage below a minimum such as 4.5V or 4.4V, despite their battery might be entirely depleted (3.0V), therefore making charging at a lower voltage such as 4.0V, although slow, technically possible.
Mobile phones usually maintain the initial charging current, even after increasing the voltage.
When decreasing the voltage while the mobile phone is charging, towards 4.5V or lower, the mobile phone will reduce the charging current accordingly, and keep hold of that current even after increasing the voltage again, until reconnecting the power.
Power banks (simplified)Edit
Power banks do not care whether the initial charging voltage was too low. If the low voltage is increased belatedly (e.g. 4.7V to 5.6V), the power bank does not hold on to the lower current, but acts as if the voltage had been there initially, and draws more current accordingly.
“Spare current” is the method of battery charging, where the device, during usage, does not decrease the power that arrives at the battery.
Instead, it utilizes the spare output power of the power supply. The total charging throughput (the power pulled from the power supply) gets increased to cover the electricity needed to power the device itself.
Most laptops ever released, and more recent mobile phones use this method for charging.