18650 li-ion battery

　　Effective management solution for 18650 li-ion battery based on MCU

　　18650 li-ion battery have the advantages of small size, light weight, high capacity, long service life, no pollution, and no memory effect, and have been widely used in consumer electronics and other occasions. Using a battery manager to effectively manage the charge and discharge of 18650 li-ion battery can extend the service life of the battery. At present, lithium-ion battery charger solutions mainly include two solutions: the use of dedicated chip control and the use of MCU (single chip microcomputer) to control step-down (Buck) converters. The dedicated chip control scheme has a simple structure but a single function, and can usually only charge 18650 li-ion battery with specific parameters. However, different models of portable products often use 18650 li-ion battery of different specifications. If special chips are used, it will cause repeated development and waste of resources. It is controlled by microcontroller. The Buck circuit has high precision, low cost, and flexible control method, which can be easily improved and upgraded, making it suitable for different types of 18650 li-ion battery.

　　This article designs a safe and efficient battery manager based on the introduction of the charging and discharging characteristics of 18650 li-ion battery. A microcontroller is used to control the Buck converter to control battery charging. At the same time, an external circuit is added to protect the battery during the battery charging and discharging process to achieve effective management of 18650 li-ion battery.

　　1Lithium-ion battery charging and discharging characteristics

　　The positive electrode material of 18650 li-ion battery is LiCoO2, and the negative electrode material is graphite crystal. Both materials have a layered structure that allows lithium ions to enter and exit. The following main chemical reactions occur in 18650 li-ion battery when charging: Positive electrode:

　　negative electrode:

　　The above reactions are all reversible reactions, and the reverse reaction occurs when the battery is discharged. Under certain conditions, some side reactions will occur inside the battery. In extreme cases, these side reactions can cause the battery electrolyte to burn or explode. Therefore, the safety performance of 18650 li-ion battery has always attracted people's attention. However, the current understanding of the process of electrolyte combustion or explosion in 18650 li-ion battery is not uniform. The main reactions that may cause the battery to catch fire or explode are:

　　a) Li1-xCoO2 formed after Li+ is embedded in the positive and negative electrodes will release oxygen when heated, and Lixc6 will burn when exposed to oxygen, generating a large amount of heat.

　　b) After multiple charges and discharges, a layer of SEI film often forms on the surface of the graphite negative electrode, preventing the interaction between the electrolyte and the graphite negative electrode. But when the temperature rises, the sEI film will undergo a decomposition reaction, causing an irreversible reaction between the electrolyte and the surface of the negative electrode, resulting in the formation of irreversible capacity and the generation of heat, causing the temperature to further rise.

　　c) When the temperature rises, the solvent and electrolyte will also react and release heat.

　　It can be seen that the safety performance and battery capacity of 18650 li-ion battery are closely related to temperature. When the battery temperature rises, a series of chemical reactions will occur inside the battery, resulting in the formation of irreversible capacity and the generation of a large amount of heat. If the heat generated by the internal reaction of the battery is much greater than the heat dissipated by the battery, the battery temperature will reach the ignition point, causing the battery to burn or explode. It is precisely because of these internal characteristics of 18650 li-ion battery that their charge and discharge rates are limited. They cannot charge rapidly in a short time like nickel-cadmium batteries, nor can they discharge at large currents. Otherwise, the capacity of 18650 li-ion battery will be reduced. , the life span will be reduced, or even cause the battery to explode or burn. Taking into account the safety and speed of the charging process and the efficiency of battery use, 18650 li-ion battery usually adopt a constant current to constant voltage charging method. In the early stage of charging, charge at a constant rate of 1c, and the battery voltage gradually rises. When the single cell voltage rises to 4.1V (or 4.2V), the charger switches to constant voltage charging mode. The single cell voltage fluctuation is controlled within 50mV. At this time, the charging current gradually decreases. When the current drops to a certain Once the value is set, the battery is considered fully charged. Figure 1 is a schematic diagram of the charging characteristic curve of a lithium-ion battery. In order to ensure the discharge capacity of 18650 li-ion battery, its maximum discharge rate is usually required to be 1c.

　　When using 18650 li-ion battery, overcharging and over-discharging of the battery is also a problem worth noting. When a lithium-ion battery is overcharged, there is no negative electrode material for the excess Li+ to be embedded, and that part of Li+ will be reduced to metallic lithium on the surface of the negative electrode and precipitated, thereby posing the risk of short circuit and causing irreversible changes in the structure of the positive electrode active material and electrolysis. The liquid decomposes, produces a large amount of gas, and releases a large amount of heat, which increases the temperature and internal pressure of the battery, posing risks such as explosion and combustion. When a lithium-ion battery is over-discharged, all the Li+ in the negative electrode and the SEI film on its surface may come out, and the SEI film will be destroyed. When the battery is charged and discharged again, the stability and density of the re-formed SEI film may become worse, requiring a larger amount of Li+, resulting in a reduction in discharge capacity and charge and discharge efficiency. Therefore, when charging and discharging 18650 li-ion battery, it is usually required that the voltage of a single cell should not be higher than 4.5V or lower than 2.2V. 2Lithium-ion battery manager solution design

　　To simplify battery charging requirements, the manager is placed inside the battery pack housing along with the battery. When charging, the AC adapter can be used to charge the battery through the input port of the manager. When discharging, the battery is discharged through the output port of the manager.

　　The following uses two 2000mA·h 18650 li-ion battery as an example to design a Buck-type battery manager. The main interface parameters are as follows: input voltage is 9V, constant current charging current is (2±0.1)A, charging cut-off voltage is (8.35±0.05)V, and single battery discharge cut-off voltage is 2.3V.

　　2.1 Main circuit design

　　The manager is mainly composed of three parts: power circuit, control circuit and protection circuit. The main circuit and control block diagram of the battery manager are shown in Figure 2. L1, C1, D2, Q1, etc. constitute the Buck circuit. R1 and R2 are connected in series and connected to both ends of the battery to provide sampling voltage. R3 is connected in series in the charging circuit to provide sampling current. Q2 forms the battery discharge circuit. The control circuit consists of 5V power supply, Mcu control, and Q1 drive circuit. MCU is used to monitor the charging process of the battery so that the battery can be charged safely and efficiently. Based on the achievable range of the microcontroller and the comprehensive consideration of pwM accuracy, the switching frequency is selected to be 20kHz.

　　2.1.1 Working principle of circuit

　　When the AC adapter is connected to the power supply R, Q2 is turned off, the battery does not participate in power supply, and the input power supplies power to the load through Dl. At the same time, the 5V power supply is working, Mcu generates a pwM signal to enable the Q1 drive circuit, and the input power charges the battery through the Buck circuit. When the AC adapter is disconnected from the power supply, the 5V power supply is cut off. At this time, Q1 is turned off, Q2 is turned on, and the battery supplies power to the load through Q2, achieving low voltage drop discharge.

　　2.1.2 Design principles and selection of circuit parameters

　　When the charging current drops to c/lO, the battery is considered fully charged. In order to ensure that the battery charging current is continuous during the entire charging process, the critical continuous current of the inductor u is required to be no higher than c/10, that is, 0.2A. In addition, when the battery is charging, the battery voltage fluctuation range must be limited to -0.05V ~ +0.05V, that is, the peak-to-peak ripple voltage of capacitor c1 is required to be lower than 0.05V. 1V. From this the required L1 and cl values can be calculated.

　　In order to ensure sampling accuracy and reduce circuit loss, select R1=R2=150kΩ. Since R3 is in series with the charging circuit, R3 must be as small as possible. Otherwise, the charging circuit voltage drop will increase, the loss will increase, the charging efficiency will decrease, and the manager will generate a lot of heat. Choose R3=0.02Ω here.

　　Since R3 is very small, the current signal sampled through R3 is also very small. In order to reduce the relative error of the sampling data, the current sampling signal must be amplified. This article uses a proportional amplification circuit to amplify the current signal. The amplification factor is selected according to the maximum forward output voltage of the operational amplifier and the charging current of the battery. If the amplification factor is too large, the operational amplifier will operate in the nonlinear region, resulting in sampling errors. If the amplification factor is too small, the relative error of the sampling data will increase.

　　2.2 Software design

　　2.2.1 Voltage and current control algorithm

　　In order to realize the constant current to constant voltage charging mode, this article adopts voltage and current double closed-loop control, and its control block diagram is shown in Figure 3.

　　First, the voltage given value is subtracted from the voltage sampling value, and the resulting error is subjected to pI calculation. After the voltage is processed by pI, the current reference value is obtained through limiting processing, and is output to the current digital pl regulator. Then, the current reference value is subtracted from the current sampling value, and the resulting error is subjected to pI calculation. After the current is processed by pI, the required duty cycle is obtained through limiting. MCu achieves a constant current to constant voltage charging mode by adjusting the duty cycle of the pwM signal.

　　2.2.2 Charging process control

　　The battery charging process is roughly divided into four stages: precharging, fast charging, top-up charging and trickle charging.

　　When charging starts, if the battery voltage is not within the range allowed by fast charging, a pre-charge stage is inserted in the early stages of battery charging. In the precharge stage, the battery is charged at a constant current of C/10 until the battery voltage rises to an undetermined threshold and then enters the fast charge stage. When the battery voltage meets the fast charging conditions, the charging process enters the fast charging stage. The fast charging stage adopts constant current charging method, charging with 1C constant current until the single battery voltage rises to 4.1V (or 4.2V). At this point, the battery should enter the top-up charging stage.

　　The supplementary charging phase uses constant voltage charging. In this stage, the battery voltage remains unchanged and the current gradually decreases. When the current is less than C/10, the battery is fully charged and enters the trickle charging stage.

　　Charging control and status switching are implemented in the main program, and charging timing and status display are implemented in the timer interrupt program. Figure 4 is the main program flow diagram.

　　2.3 Battery protection

　　There is a certain difference in the capacity of each single battery in series. During the charging process, if one battery is fully charged and the other is not, if the series battery pack continues to be charged, the fully charged battery will be overcharged. During the discharge process, if one battery is fully discharged and the other battery still has a certain amount of remaining power, if the discharge continues, the battery that is discharged first will be over-discharged. It can be seen that series-connected battery packs are prone to overcharge and over-discharge of single cells.

　　The single-chip microcomputer can be used to monitor the voltage and discharge current of the single battery to prevent overcharge and over-discharge of the single battery. However, this requires the single-chip microcomputer to be in working state at all times, and the static power consumption is large. When the battery is not in use, the battery still needs Supply energy to the microcontroller, which has a greater impact on the output capacity of the battery pack. This article uses the lithium-ion battery special protection chip S-8232. This circuit can realize overcharge, over-discharge, and over-current protection, and has a small operating current. It has a variety of parameter models and can meet the different protection parameter needs of the battery pack.

　　3Experimental results

　　Based on the above design scheme, a lithium-ion battery manager is designed. Experimental results show that the manager can effectively prevent overcharging and overdischarging of single cells. Figure 5 is the battery voltage and current change curve obtained based on the battery charging process. It takes about 4.5 hours for the battery to be fully charged from the time it is discharged. During pre-charging, the manager charges the battery with a constant current of C/10, and the battery voltage gradually rises to 6V. Then the charging current quickly rises to 2A and stabilizes at around 2A. At this time, the battery voltage continues to rise. When the battery voltage rises to 8.35V, the charging current begins to decrease, but the battery voltage is always stable at 8.34V~8.37V. Question. When the charging current drops to about 0.2A, the indicator light shows that the charge is full, and the manager performs a trickle charge on the battery. After a period of time, the charging ends. At this time, the battery voltage drops slightly.

　　4 Conclusion

　　This article designs a Buck-type battery manager based on the introduction of the charging and discharging characteristics of 18650 li-ion battery. The manager uses a microcontroller to control the charging voltage and current of the battery. In addition, it also uses a dedicated chip to monitor the voltage and discharge current of the single battery, preventing overcharge and overdischarge of the single battery and limiting the battery discharge rate. It ensures the safety and efficiency of the lithium-ion battery charging and discharging process, and helps extend the service life of the lithium-ion battery.

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