Introduction With the continuous development of portable devices and power tools, the performance requirements for battery performance, especially rechargeable lithium batteries, are also constantly improving. At the same time, the design and optimization of lithium battery charging circuits have continued to improve. The lithium battery charging circuit designed in this article is a smart lithium battery charger designed based on three strings of 10.8V battery packs. The battery model used in the experiment is a INR18650-13Q battery from SAMSUNG. The battery has a rated capacity of 1300mAh and a rated voltage of 3.60V. The charging voltage reaches 4.20V when charging. The default is full state, and the battery requires CC-CV (constant current-constant voltage) charging method. Based on the control of the STM8S103F2 single-chip microcomputer and the adjustment of the current mode controller UC3842B, the constant current and constant voltage process and conversion in the charging process are realized through the combination of software and hardware. The article elaborates on the working principle of the software and hardware of the lithium battery charger, especially the concrete realization of constant current and constant voltage charging, and analyzes the experimental results of the charger. Comparison of charging methods At present, there are not many charging methods for lithium batteries at home and abroad, which can be roughly divided into the following two: 2.1 DC charging method This method is relatively simple, and the 220V voltage of the mains is converted to a certain voltage value through the transformer. Then use constant current to directly charge the lithium battery pack
until the battery voltage reaches a certain value. Although this method is simple and has a fast charging time, it does great damage to the battery, and it is almost impossible to charge the battery to a full state. Long-term use of this charger will lead to a decrease in battery life. u003cpu003eFigure 1: Conventional charging methodu003c/pu003e2.2 The conventional charging method is shown in Figure 1. This method is the most commonly used method. When the battery is inserted into the charger, the charger first enters the preconditioning state, which mainly checks the battery, uses a charging current of about C/15 to gently charge the battery, and tests whether its voltage can reach the threshold 2.5V. If not, it is considered that the battery is damaged; if it can, it will enter the constant current area and quickly charge the battery pack with a higher current (about 1C). When the battery constant current charging reaches the threshold voltage of 4.2V, it enters the constant voltage zone. In the constant voltage zone, the battery is charged at a constant voltage. As the battery voltage continues to rise, the charging current continues to decrease until the charging current drops to the magnitude of C/10 to C/15, the charging stops and the entire charging state end. In terms of charging effect and battery loss, the DC method is far inferior to the conventional method. Although the charging time is shorter, the frequency of use is still very low. In contrast, the charging effect of the conventional method and the loss of the battery can meet the requirements, and the realization of the conventional method is simpler than other methods. Therefore, the conventional method has become the most frequently used lithium battery charging method today. The design of this system is also based on conventional methods. u003cpu003eFigure 2: Simplified block diagram of UC3842B chipu003c/pu003e Chip introduction and circuit architecture The high-performance current mode control chip UC3842B is mainly used in this system. 3.1 UC3842B chip introduction UC3842B chip is a high-performance current-type PWM controller. The chip circuit switching frequency is adjustable, has the characteristics of current feedback and voltage feedback dual-loop control, and has a high voltage regulation rate and load regulation rate. And it has the advantages of simple peripheral circuit, convenient installation and debugging, excellent performance and low price. The UC3842B chip mainly includes: 5V reference voltage, input under-voltage lockout circuit, error amplifier, oscillator, pulse width modulator. Figure 2 is a simplified block diagram of the UC3842 chip in an 8-pin package. The functions of each pin are as follows: pin 1 is the error amplifier output; pin 2 is the error amplifier inverting input, usually connected to the switching power supply output through a resistor divider, for voltage feedback; pin 3 is the current sampling pin; 4 Used for PWMu003c/pu003e
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