Charging a 3.6V pack begins by applying a constant current to a voltage peak of 4.20V/cell, at which point the voltage peaks and the current begins to taper off. The original USB port can only charge a small single-cell Li-ion battery.
To prevent overload, some hosts include current-limiting circuits that shut down the supply when overdrawn. Charging a device that draws 500mA connected together with other loads will exceed the port’s current limit, leading to a voltage drop and a possible system failure. There is, however, a danger of overloading a USB hub when attaching too many gadgets. With 5V and 500mA available on version USB 1.0 and 2.0, and 900mA on USB 3.0, the USB can charge a small single-cell Li-ion pack. The host cannot take power from an outside source. Data streams in both directions but the power is unidirectional and always flows from the host to the device. The USB port can also be used to charge personal devices, but with a current limit of 500mA on the original design, this might have been an afterthought.Ī typical USB network consists of a host that is often a PC and peripherals such as a printer, smartphone or camera. Compaq, DEC, IBM, Intel, NEC and Nortel contributed to the developments with the goal of simplifying the interconnection of peripheral devices to a PC, as well as to allow a greater data transfer rate than was feasible with earlier interfaces. The Universal Serial Bus (USB) was introduced in 1996 and has since become one of the most widespread and convenient interfaces for electronic devices.
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