CR1225 battery

　　CR1225 battery charging circuit design based on USB interface

　　Lithium-ion (Li-ion) battery, referred to as lithium battery, is a new type of battery that has become increasingly 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 it also has some fatal flaws.

　　1 Overview of CR1225 battery and USB interface

　　Lithium-ion (Li-ion) battery, referred to as lithium battery, is a new type of battery that has become increasingly 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, and it cannot be overcharged or overdischarged, otherwise it will easily cause irreversible damage. In extreme cases such as short circuit and overcharge, it may explode and cause danger. .

　　Generally, 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, constant current charging, that is, the current is constant, and the charging current is 0.2C according to the low rate charging specified by the national standard (arbitration charging standard), the maximum does not exceed 1C; and the battery voltage gradually increases with the charging process. When the battery terminal voltage reaches the termination voltage of 412±0105V, constant current charging should be changed to constant voltage and small current (about 011C) charging. This state is normal It is called the trickle charge state. The charging current gradually decreases as the charging process continues based on the saturation degree of the battery core. When it decreases to 0101C, the charging is considered terminated.

　　USB interface is the abbreviation of English UniversalSerialBus, and its 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, while 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 a USB interface provided by a USB host or a powered hub, the minimum available voltage of the connected peripheral is 4.5V, while a hub driven by the USB bus can provide The minimum should be 4.35V. It is worth noting that although the USB specification defines that the upper limit of the current provided shall not exceed 0.15A, in fact the current output by the USB port often exceeds several amps.

　　It can be seen from the above analysis that when using the USB interface to charge lithium batteries, the current should be sufficient. The key is how to control the size of 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 voltage, which makes the voltage drop of the charging circuit extremely important and difficult to design.

　　2 Design scheme of charging circuit based on PC USB interface

　　2.1 Option 1

　　The internal circuitry of some USB charging cables on the market. The main principle is to use a resistor to limit the maximum current, and use the voltage drop of the diode (about 0.7V) to reduce the output voltage of the USB interface of about 5V to about 4.2V and then charge the lithium battery. This circuit has the advantages of simplicity and low cost, but the disadvantages are also obvious: the charging current and charging voltage vary depending on the USB interface and the battery status, and are basically impossible to grasp. In many cases, the battery may not be fully charged. (The charging voltage cannot reach 4.2V), and sometimes it may be overcharged and damage the battery. This circuit is acceptable for some lithium batteries with complete internal protection circuits, but it is dangerous for ordinary lithium batteries.

　　2.2 Option 2

　　A practical USB lithium battery charging circuit composed of MAX1551/1555. MAX1551/1555 is a USB single-cell lithium battery charging chip designed and produced by MAXIM Company. It is a 5-pin thin SOT23 package and allows a voltage input in the range of 3.7V~7V from the USB interface or wall AC adapter. There is also a temperature limit circuit inside the chip.

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