3.7V 18650 lifepo4 battery

　　3.7V 18650 lifepo4 battery management system hardware design technology

　　Electric vehicles are vehicles that are powered in whole or in part by electric motors. At present, there are three main types of pure electric vehicles, hybrid electric vehicles and fuel cell vehicles. The commonly used power sources for electric vehicles currently come from lead-acid batteries, lithium batteries, nickel-metal hydride batteries, etc.

　　Lithium batteries have high cell voltage, high specific energy and high energy density, and are currently the batteries with the highest specific energy. However, precisely because of the relatively high energy density of lithium batteries, safety accidents will occur when misuse or abuse occurs. The battery management system can solve this problem. When the battery is under charging overvoltage or discharging undervoltage, the management system can automatically cut off the charging and discharging circuit, and its power balancing function can ensure that the voltage difference of a single battery is maintained within a small range. In addition, it also has functions such as over-temperature, over-current, and remaining power estimation. What this article designs is a battery management system based on a single-chip microcomputer [1].

　　1Battery management system hardware composition

　　The hardware circuit of the system can be divided into MCU module, detection module and equalization module.

　　1.1MCU module

　　MCU is the core of system control. The MCU used in this article is the GZ16 model of the M68HC08 series. All MCUs in this series use the enhanced M68HC08 central processing unit (Cp08). This microcontroller has the following features:

　　(1) 8MHz internal bus frequency; (2) 16KB built-in FLASH memory; (3) 2 16-bit timer interface modules; (4) Clock generator supporting 1MHz ~ 8MHz crystal oscillator; (5) Enhanced serial communication Interface (ESCI) module.

　　1.2 Detection module

　　In the detection module, the voltage detection, current detection and temperature detection modules will be introduced respectively.

　　1.2.1 Voltage detection module

　　In this system, the microcontroller will detect the overall voltage and single cell voltage of the battery pack. There are two methods for detecting the overall voltage of the battery pack: (1) using a dedicated voltage detection module, such as a Hall voltage sensor; (2) using precision resistors to build a resistor voltage dividing circuit. Using a dedicated voltage detection module is more expensive, requires a specific power supply, and the process is complicated. Therefore, a voltage dividing circuit is used for detection. The voltage variation range of a 10-cell lithium manganate battery pack is 28V ~ 42V. Use 3.9M? and 300k? resistors to divide the voltage. The variation range of the collected voltage signal is 2V ~ 3V, and the corresponding AD conversion results are 409 and *.

　　For the detection of single cells, flying capacitor technology is mainly used. The schematic diagram of flying capacitor technology is shown in Figure 1 [2], which is a protection circuit diagram for the last four cells of the battery pack. The voltage of any one of the last four batteries can be collected into the microcontroller through a four-channel switch array. Outputs the drive signal to control the on and off of the MOS tube, thereby protecting the charging and discharging of the battery pack.

　　As shown in Figure 1, it is the protection circuit diagram of the last 4 cells of the battery pack. Through the four-channel switch array, the voltage of any one of the last 4 batteries can be collected into the microcontroller. The microcontroller outputs the drive signal to control the conduction of the MOS tube. On and off, thus protecting the charging and discharging of the battery pack.

　　The above 6 batteries can be realized using 2 three-channel switch arrays. MAX309 is a 4-select-1, dual-channel multi-channel switch that realizes channel selection through address selection. Switches S5, S6, and S7 are responsible for connecting the positive terminal of the battery to the positive terminal of the flying capacitor. Switches S2, S3, and S4 are responsible for connecting the negative terminal of the battery to the negative terminal of the flying capacitor. The structure of the three-channel switching array is similar to that of the four-channel switching array, except that the number of channels is one less. When working, the microcontroller sends a channel address signal, connects the positive and negative poles of one of the batteries to the capacitor, charges the capacitor, then turns off the channel switch, turns on the switch that follows the amplifier, and the microcontroller quickly detects the voltage of the capacitor. This completes the voltage detection of 1 battery. If the detection voltage is found to be less than 2.8V, it can be inferred that the battery may be short-circuited, over-discharged, or the detection line from the protection system to the battery is disconnected, and the microcontroller will immediately send a signal to cut off the main circuit MOS tube. Repeat the above process, and the microcontroller will complete the detection of the battery managed by this module.

　　1.2.2 Current sampling circuit

　　During current sampling, the parameters in the battery management system are an important basis for battery overcurrent protection. The current sampling circuit in this system is shown in Figure 2. When the battery is discharged, a constantan wire is used to detect the current signal, and the detected voltage signal is amplified by a differential mode amplifier and converted into a voltage signal of 0~5V and sent to the microcontroller. If the discharge current is too large and the voltage signal detected by the microcontroller is relatively large, it will drive the transistor to change the gate voltage of the MOS tube and turn off the discharge circuit. For example, for a 36V lithium manganate battery, the protection current is set to 60A. The resistance of constantan wire is about 5mΩ. When the current reaches 60A, the voltage of the constantan wire reaches about 300mV. In order to improve the accuracy, the voltage is amplified 10 times through the amplifier and sent to the microcontroller for detection.

　　1.2.3 Temperature detection

　　During the charging and discharging process of the battery pack, part of the energy is released in the form of heat. If this part of the heat is not removed in time, it will cause the battery pack to overheat. If the temperature of a single nickel-metal hydride battery exceeds 55°C, the battery characteristics will deteriorate, and the charge and discharge balance of the battery pack will be disrupted, resulting in permanent damage or explosion of the battery pack. In order to prevent the above situation from happening, the battery pack temperature needs to be monitored in real time and heat dissipated.

　　A thermistor is used as a temperature sensor for temperature sampling. Thermistor is a heat-sensitive semiconductor resistor whose resistance value decreases as the temperature increases. The resistance temperature characteristics can be approximately expressed by the following formula:

　　1.3 Balance module

　　Commonly used equalization methods for battery packs include shunt method, flying capacitor equalization charging method, inductive energy transfer method, etc. In this system, more I/O ports are needed to drive the switch tubes, and the I/O ports of the microcontroller are limited, so the charging balancing method of converting full charge to single charge is adopted. The schematic is shown in Figure 3. Q4 is the switch that controls the overall charging of the battery pack, and Q2, Q3, and Q5 are the switches that control the charging of a single battery. Taking a 10-cell lithium manganate battery pack as an example, the voltage at both ends of the main coil of the transformer is 42V, and the voltage of the secondary coil is the battery's rated voltage of 4.2V. At the beginning, Q4 is turned on, Q2, Q3, and Q5 are turned off. The voltage of a single battery continues to rise. When it is detected that the voltage of a certain battery reaches the rated voltage 4.2V, the voltage detection chip sends a driving signal, turns off Q4, and turns on Q2. , Q3, Q5, the entire system enters the single charging stage, and the battery that is not fully charged continues to be charged, so that the battery that reaches the rated voltage keeps the rated voltage unchanged. After testing, the voltage difference will not exceed 50mV.

　　2SOC power detection

　　In lithium-ion battery management systems, commonly used SOC calculation methods include open circuit voltage method, Coulomb calculation method, impedance measurement method, and comprehensive table lookup method [3].

　　(1) The open circuit voltage method is the simplest measurement method, which mainly determines the size of the SOC based on the size of the battery's open circuit voltage. It can be seen from the operating characteristics of the battery that there is a certain corresponding relationship between the open circuit voltage of the battery and the remaining capacity of the battery.

　　(2) The Coulomb calculation method measures the charging and discharging current of the battery, integrates the product of the current value and the time value, and then calculates the amount of electricity charged and discharged by the battery, and uses this to estimate the SOC value.

　　(3) The impedance measurement method uses a certain linear relationship between the internal resistance of the battery and the state of charge SOC, and calculates the internal resistance of the battery by measuring the voltage and current parameters of the battery, thereby obtaining an estimate of the SOC.

　　(4) In the comprehensive look-up table method, the remaining capacity SOC of the battery is closely related to the voltage, current, temperature and other parameters of the battery. By setting up a related table and inputting parameters such as voltage, current, temperature, etc., the remaining capacity value of the battery can be queried.

　　In this design, the software programming method is adopted from the aspects of circuit integration, cost, and performance of the selected MCU. Combining several methods, it is more appropriate to use the Coulomb calculation method.

　　(1) Use C to represent the total power released when the lithium battery pack drops from 42V to 32V.

　　(2) Use eta to represent the ratio of the amount of electricity released to C after the current i has passed time t.

　　Among them, CRM is the remaining power. Let ΔCi = i × Δt, which represents the discharge amount of the battery pack discharged by i during the stationary time t; or the charging amount of the battery pack charged by i. The remaining capacity is actually the calculation and accumulation of ΔCi. Set an appropriate sampling time Δt, measure the current current value, and then calculate the product to obtain the change in the remaining capacity CRM within Δt time, so as to continuously update the value of CRM to achieve SOC power detection.

　　3 test results

　　Conduct charge and discharge tests on lithium manganate battery packs through the battery management system. Figure 4(a) shows the discharge test chart of the lithium battery pack. The discharge current is 8A. When the battery pack voltage drops to 32V, the discharge MOS tube is turned off.

　　The battery management system of this article uses M68HC08GZ16 as the core to realize the collection of voltage, current, and temperature signals of the battery pack cells. After the charging capacity is balanced, the voltage difference of the single battery does not exceed 50mV. The overall system has good operating performance and can meet the application needs of electric vehicle 3.7V 18650 lifepo4 battery packs.

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