Design of Lithium Battery Monitoring System Based on GPRS Short Message Communication

. The lithium battery in the new energy system works in the wilderness environment, and its data remote monitoring is often realized based on wireless communication, and this transmission method needs to set up exclusive base stations, which is costly. General Packet Radio Service (GPRS) short message communication realizes data transmission through satellite without setting up base stations. Based on the above situation, a lithium battery monitoring system based on GPRS short message communication is designed, and the system consists of monitoring terminal, GPRS receiver, and remote monitoring cloud platform. For the software and hardware requirements of data acquisition, data monitoring and GPRS short message communication, the corresponding design ideas and implementation scheme are given. Meanwhile, considering the impact of wilderness environment on satellite communication, a transmission error control method based on data backup is proposed to improve its transmission reliability. Finally, through practical testing, it is verified that the system can effectively transmit monitoring data and locate lithium batteries, and maintenance personnel can monitor data in real time through the cloud platform to reduce the construction and maintenance costs of remote monitoring.


Introduction
In recent years, with the rapid development of the energy Internet, new energy generation system has gradually become an important part of the energy Internet, and lithium battery is the key equipment for the stable operation of the new energy generation system, playing an important function of stabilizing the system bus voltage. Considering that lithium batteries are mostly in harsh environments, the factors affecting the normal operation of lithium batteries are variable and complex. In order to ensure the reliable operation and service life of lithium batteries, each lithium battery needs to be remotely monitored in real time so that maintenance personnel can find and replace abnormal lithium batteries in time [1][2][3]. At present, in order to achieve real-time monitoring of the status of lithium batteries, mainly through wireless communication, build a dedicated base station to achieve data upload. This approach is difficult to build, high infrastructure costs and post maintenance, especially in wilderness environments such as deserts and oceans. Also the change of terrain increases the difficulty of maintenance personnel to locate abnormal devices quickly [4][5][6].
In order to solve the above problems, the use of short message communication service can realize the remote transmission of lithium battery monitoring data, and the passive navigation service provided can realize the precise positioning of lithium batteries. In addition, the transmission distance of GPRS communication is not limited by geographical area and the transmission cost is low [7][8][9]. Therefore, using GPRS communication to achieve remote monitoring of lithium batteries in the wilderness environment is a cost-effective solution. Based on the above idea, a monitoring system containing a monitoring terminal, a communication master station, and a cloud platform was designed [10][11][12]. Real-time monitoring of lithium battery data and location information is achieved while taking into account the transmission error control problem.

Overall design solution
GPRS communication-based lithium battery monitoring system adopts the design idea of Remote Rerminal Unit (RTU) master-slave station, and the overall system architecture is shown in Fig 1. The lithium battery monitoring terminal and GPRS communication module are used as the slave station responsible for monitoring data acquisition, GPRS short message sending, GPRS communication error control, etc., and the GPRS receiver is used as the RTU master is used to receive data sent from the slave station and realize the conversion of GPRS protocol to TCP/IP protocol. Because the civilian three-stage GPRS communication card only supports a frequency of once-per-minute transmission frequency. So the timing of the error control process is taken into account. Each GPRS receiver is responsible for the data upload of 40 monitoring terminals. Finally, the cloud platform saves the monitoring and positioning data to the database and displays it on the front-end. The system hardware includes two major parts: monitoring terminal and GPRS receiver. Among them, the GPRS receiver mainly realizes the data transmission error system through software. The function of the monitoring terminal is closely related to the hardware circuit design. The main controller of the monitoring terminal is STM32F407, including lithium battery charging and discharging module, data acquisition module, GPRS communication module, power supply module, data backup module and display module, as shown in Fig 2. The power supply module is used to supply power to all modules of the monitoring terminal with the lithium battery, the display module is composed of a Flexible static memory controller (FSMC) combined with an LCD display to display the collected data, and the data backup module uses an SD card as the storage medium to temporarily store data when the transmission is interrupted. The main circuit for monitoring terminal charging and discharging uses a non-isolated Buck-Boost as the basic design idea. The power switching device is a field effect tube IRF3205, and the control chip drives a pair of field effect tubes by changing the duty cycle of the PWM signal, thus realizing the bidirectional flow of energy.
The monitoring terminal data collection includes four kinds of data: lithium battery charge/discharge current, lithium battery terminal voltage, internal resistance, and temperature, among which the lithium battery charge/discharge current is obtained through Hall element, terminal voltage is obtained through voltage divider circuit, and temperature is obtained using temperature sensor MF52-B3950. However, the change of the internal resistance of the lithium battery is the direct basis for the health judgment of the lithium battery, and the error needs to be kept within the milliohm level. The result is interfered by a variety of factors such as charge and discharge current. Therefore, this system uses the four-terminal AC injection method for measurement, which can achieve real-time acquisition of the internal resistance of the lithium battery while avoiding the above problems.

Lithium battery charging and discharging main circuit
Considering the experimental lithium battery parameters, the main circuit of charging and discharging with nonisolated Buck-Boost as the basic design idea. Power switching devices Q1, Q2 selected field effect tube IRF3205. microcontroller by changing the duty cycle of the PWM signal to drive the field effect tube to achieve the bidirectional flow of energy. The circuit design is shown in Fig 3.   Fig. 3. Li-ion battery charging and discharging main circuit circuit design diagram

Lithium battery voltage acquisition circuit
The Li-ion battery voltage measurement circuit specifically collects the terminal voltage of the Li-ion battery when it is in working condition, which is closely related to the healthy working condition of the battery. The voltage divider circuit with LM3581 voltage follower is used to realize the voltage data acquisition at both ends of the charge and discharge, as shown in Fig 4. Then the ADS1115 chip is used to realize the analog-todigital conversion of the signal, which has a built-in ultra-small 16bit high-precision analog-to-digital converter (ADC). The voltage signal of the lithium battery is input to the ADS1115 through two ports V1 and V2, and compared with the reference voltage AVDD (set to 6.048V in this paper) in the chip to get the corresponding voltage. The analog signal is converted into a digital signal through the analog-to-digital conversion circuit, and finally the digital signal is transmitted to the microcontroller through the I2C port, where the analog-to-digital conversion module is shown in Fig 5.   Fig. 4. Lithium battery voltage signal acquisition circuit.

Lithium battery current signal amplification circuit
Lithium battery current measurement circuit and voltage acquisition circuit design ideas are the same, by setting the sampling resistor in the main circuit combined with amplification circuit and voltage following circuit to measure the current, and then through the A/D conversion will get the current signal voltage to the microcontroller, according to the known fixed resistance calculation to get the current, here the main description for amplification, conversion current signal circuit. In this paper, we choose INA213 chip, which is an operational amplifier for current detection. INA213 can add the reference voltage 2.5V and the voltage after 50 times amplification of the resistor R0, and finally send the obtained voltage to the microcontroller to calculate the current, as shown in Fig 6.   Fig. 6. Lithium battery current measurement circuit.

GPRS short message communication module
The GPRS communication module consists of two major parts, one is the RD0538DGPRS satellite Radio-Determination Satellite Service (RDSS) single-mode module, RDSS realizes positioning by interacting with satellites, i.e., active positioning, and RDSS provides short message communication function, the design block diagram. The other part is the JS-U810 multimode positioning module which is used to realize the Radio Navigation Satellite System (RNSS), i.e. passive positioning. The chip supports GPRS/GPS/GLONASS positioning and navigation systems and is used to realize the positioning function of the system.RDSSGPRS short message communication module is the key to realize the data transmission of the monitoring system, which is realized by baseband signal modulation and demodulation with GPRS communication RF signal generation. The modulation and demodulation of the baseband signal is mainly done by using the TD1100A chip. The chip interacts with the controller for the baseband signal through asynchronous communication. When sending and receiving data, it acts as a middleware to convert the baseband signal to and from the IF signal.
The RF signal is realized by DT-A6 RF communication chip with peripheral RF signal processing circuit. When sending data, the chip further BPSK modulates the baseband IF digital signal to GPRS transmitting carrier band 1615.68 ± 4.08MHz, and finally through shaping, filtering, filtering and amplifying by the peripheral RF circuit to the satellite; when receiving data, the antenna listens to GPRS communication RF carrier band 2491.75 ± 4.08MHz, and then When receiving data, the antenna listens to the GPRS communication RF carrier band 2491.75 ± 4.08MHz, and then amplifies the signal into the chip through the low-noise amplifier, the chip internal signal mixing, IF filtering, analog-to-digital conversion and other processing, and finally outputs a 2bit or 4bit digital IF signal to the baseband signal processing chip.

Conclusion
In this paper, we focus on the research background of lithium battery pack status monitoring in wilderness environment, and design a monitoring terminal that combines data acquisition with GPRS communication, and a cloud platform for monitoring lithium battery status data and location. Meanwhile, considering the problem of wilderness environment, the transmission error control method of retransmission using storage backup device is proposed to improve the reliability and availability of data transmission. Finally, through testing, it is proved that this monitoring terminal and the data monitoring cloud platform when working properly. The above requirements are fully satisfied. The system can ensure the safe use of lithium batteries in the wilderness environment, while extending the maximum service life of lithium batteries and reducing the construction cost and manual maintenance cost of remote monitoring of lithium batteries