USB additional information.

The Universal Serial Bus (USB) was introduced in 1996 and has since become one of the most widespread and convenient interfaces for electronic devices. The USB port is a bi-directional data port that provides a supply voltage to power memory sticks, keyboards, mice, wireless interfaces, cameras, MP3 players and chargers.


With 5V and 500mA of available current, the USB bus can charge a small single-cell Li-ion pack, but there is a danger of overloading the USB hub when attaching too many gadgets. Plugging in a charger that draws 500mA along with other devices will exceed the port’s current limit, leading to a voltage drop and a possible system failure. To prevent overload, some hosts include current-limiting circuits that shut down the supply when overdrawn. Another method is limiting the current of all attachments to 400mA to reserve 100mA for housekeeping.


The most common USB chargers are designed for single-cell Li-ion. The charge begins with a constant current charge to 4.20V/cell, at which point the voltage caps and the current begins to decrease. Due to a voltage drop in the cable, which is about 350mV, and losses in the charger circuit, it is possible that the 5V supply cannot supply the battery’s 4.2V charge threshold. This is no problem; the battery does not suffer but will deliver shorter than expected runtimes.

The rectangular Type A USB plug has four connector pins and a shield. The rightmost contact is number 1 and carries 5V; the leftmost contact is number 4 and forms the ground. The two shorter pins in the middle are reserved for data transfer and have no function in the USB charger. Figure 1 illustrates the rectangular Type A USB plug.




With the USB supply current limited to 500mAh, doing active work on a laptop or watching a video on a tablet with a bright screen can result in a net discharge. A larger internal load than what the charger can provide will gradually drain the battery; however, the pack will replenish itself when the activity ends. 

Some USB chargers plugging into the AC main or the cigarette lighter of a car deliver higher peak currents than 500mAh. This allows connecting several devices via a USB bar without causing overload.


Note that the USB port is unidirectional and cannot take power from an outside source. In other words, power only flows out.