Lithium ion (Li-ion) battery, referred to as lithium battery, is a new type of battery that has gradually become popular in recent years. It has the advantages of small size, light weight, large capacity (high energy density), low self-discharge rate and no memory effect. , But at the same time, it also has some fatal flaws: the requirements for charging and discharging are relatively strict. Overcharge and overdischarge are not allowed, otherwise it is easy to cause irreversible damage. In extreme cases such as short circuit and overcharge, explosion may occur. Danger.

　　The technical experts of Sunhe Energy Technology Co., Ltd. pointed out that the nominal voltage of a single lithium battery is 316～317V. When charging, it is generally required to use the voltage limiting and current limiting method. First, the constant current charging, that is, the current is constant, and the charging current is in accordance with the national standard. The low-rate charging of the battery is 0.2C (arbitration charging mode), and the maximum is not more than 1C; and the battery voltage gradually increases with the charging process. When the battery terminal voltage reaches the termination voltage of 412±0105V, the constant current charging should be changed to constant voltage. Current (about 011C) charging, this state is generally called trickle charge state. According to the saturation of the battery cell, the charging current gradually decreases as the charging process continues. When it decreases to 0101C, the charging is considered to be terminated.　　USB interface is the abbreviation of English UniversalSerialBus, and the Chinese meaning is "Universal Serial Bus". It is a new interface technology applied in the PC field. USB uses a 4-pin plug as a standard plug to connect to external devices. In this 4-pin plug, two pins are data communication lines, and the outer two pins provide power for external devices. According to the USB specification, each USB interface should be able to provide a current output of 500mA; for the USB interface provided by a USB host or a hub with a power supply, the minimum available voltage of the connected peripheral is 4.5V, and the hub driven by the USB bus can provide The minimum should be 4.35V. It is worth noting that although the USB specification defines the upper limit of the current provided shall not exceed 015A, but in fact the current output from the USB port often exceeds a few amperes.　　 It can be seen from the above analysis that the current should be sufficient for charging the lithium battery using the USB interface. The key is how to control the current. In addition, when using a USB interface with a minimum voltage of 4.35V to charge a lithium battery with a typical requirement of 4.2V, there is only a small margin in the voltage, which makes the voltage drop of the charging circuit extremely important and a design difficulty.