A design scheme for efficient battery performance monitoring system

　　As a convenient, safe and reliable DC power source, batteries have been widely used in the fields of electricity, communication, military, etc. Temperature is an important parameter of batteries. It can indirectly reflect the performance of batteries, and the batteries can be managed intelligently based on this temperature parameter to extend the battery life. During the charging and discharging maintenance and working process of battery packs, the heat generated inside the battery will cause the battery temperature to change. In particular, overcharging of the battery and abnormal changes in the electrolyte inside the battery may cause the battery temperature to be too high and cause battery damage.

　　The traditional method of manual timing measurement is labor-intensive, has poor measurement accuracy, and a harsh working environment. In particular, it is not possible to detect abnormal single cells in time, which can easily lead to damage to the single cells, or even cause failure or damage to the entire battery pack. The wired multi-point temperature monitoring system based on the bus structure can realize intelligent temperature measurement, but it has the disadvantages of complex wiring and difficult maintenance and expansion. In view of this, a battery temperature wireless monitoring system based on a single bus temperature sensor and a wireless transceiver module is designed, which can effectively overcome the shortcomings of thermistor temperature measurement and bus structure control system, and is conducive to improving the intelligent level of battery performance monitoring.

　　1 Single bus temperature sensor DS18B20

　　1.1 DS18B20 chip features

　　The DS18B20 digital temperature sensor is a new generation of intelligent temperature sensor adapted to microprocessors produced by DALLAS Semiconductor Corporation in the United States. It integrates temperature sensors, A/D converters, registers and interface circuits in one chip, adopts 1-wire bus protocol, and can directly output and test digitally. Compared with other temperature sensors, it has the following main features:

　　It adopts unique single-wire interface technology and only needs one port line to connect to the microprocessor to achieve two-way communication. It occupies fewer ports of the microprocessor and can receive a large number of leads and logic circuits. It does not require any peripheral circuits during use, and all sensor elements and conversion circuits are integrated in an integrated circuit shaped like a transistor. The temperature measurement range is -55~+125℃, the accuracy can reach ±0.5℃, the programmable 9~12-bit A/D conversion accuracy, the temperature measurement resolution can reach 0.0625℃, and high-precision temperature measurement can be achieved. The measurement result directly outputs the digital temperature signal, and the CRC check code can be transmitted at the same time, with strong anti-interference and error correction capabilities; it supports multi-point networking function, and multiple DS18B20 can be hung on the bus to achieve networking multi-point temperature measurement. It has a wide adaptability voltage range: 3.0~5.5V. It can be powered by the data line in the power supply mode. The connection between DS18B20 and the microcontroller is shown in Figure 1. The single-bus device has only one data line. Data exchange and control in the system are all completed on this line. A 4.7Ω pull-up resistor is connected to the single bus to ensure that the state is high when the bus is idle.

　　Figure 1 DS18B20 and MCU hardware connection diagram

　　1.2 DS18B20 control timing

　　Serial data transmission is used between DS18B20 and microprocessor. When reading and writing programming, the read and write timing must be strictly guaranteed, otherwise the temperature measurement results will not be read. The DS18B20 control timing mainly includes initialization timing, read operation timing and write operation timing, as shown in Figure 2.

　　Figure 2 DS18B20 control timing

　　(1) Initialization timing. The timing is shown in Figure 2 (a). The host bus sends a reset pulse (a low-level signal with a minimum duration of 480 seconds) at time t0, then releases the bus at time t1 and enters the receiving state. After detecting the rising edge of the bus, the DS18B20 waits for 15 to 60 μs, and then sends a presence pulse (a low-level signal lasting 60 to 240 seconds) at time t2, as shown by the dotted line in the figure.

　　(2) Write operation timing. When the host bus is pulled from high to low level at time t0, a write time gap is generated. The bits to be written should be sent to the bus within 15μs from time t0. DS18B20 samples the bus between 15 and 60μs after t0. If the bit written by the low level is 0, if the bit written by the high level is 1, the gap between writing two consecutive bits should be greater than 1μs, as shown in Figure 2 (b).

　　(3) Read operation timing. When the host bus is pulled from high to low level at t0, the bus only needs to remain at a low level for 6 to 10 μs. At t1, the bus is pulled high to generate a read time gap. The read time gap is valid from t1 to t2. The time from t2 to t0 is 15 μs. That is, the host must complete the read bit before t2 and release the bus within 60 to 120 μs after t0, as shown in Figure 2 (c).

　　2 System Hardware Design

　　The monitoring system mainly consists of three parts: temperature monitoring nodes, main control unit and host computer. The system structure is shown in Figure 3. The temperature monitoring nodes are distributed on each single cell of the battery pack, collect the temperature information of each single cell, and transmit it to the main control unit through the wireless network; the main control unit communicates with all monitoring nodes, receives commands from the host computer and temperature information from the monitoring nodes, and reports the temperature information to the host computer; the host computer displays the temperature information of the battery in real time, analyzes and processes the data, and starts the alarm program according to the set alarm threshold to detect abnormal batteries in time.

　　Figure 3 System overall structure

　　2.1 Temperature monitoring node design

　　The function of the temperature monitoring node is to complete the temperature information collection, processing and wireless data transmission of the single battery. The single-chip microcomputer controls the wireless transceiver chip nRF2401 and the single-bus digital temperature sensor DS18B20 to realize the intelligent measurement of temperature. It mainly includes the single-chip microcomputer system, temperature collection circuit, wireless transceiver circuit, display circuit, alarm circuit and power supply. Its hardware structure is shown in Figure 4.

　　Figure 4 Temperature monitoring node hardware structure

　　The DS18B20 temperature measurement circuit is shown in Figure 1. The DS18B20 is adhered to the surface of the battery with a heat-conducting adhesive. The difference between the core temperature and the surface temperature is about 0.2°C. Wireless transmission is achieved using the nRF2401 wireless transceiver chip. The nRF2401 is a single-chip integrated receiver and transmitter chip with an operating frequency range of the globally open 2.4GHz frequency band. It has built-in first-in first-out stack area, address decoder, demodulation processor, GFSK filter, clock processor, frequency synthesizer, low noise amplifier, power amplifier and other functional modules. It requires very few peripheral components and is very convenient to use. In this system, nRf2401 communicates with the microcontroller through the P2 port, and the P2.0 and P2.1 ports of the AT89S51 are connected to the CLK1 and DATA of the nRF2401 respectively. The CS of nRF2401 is the chip select terminal, CE is the send or receive control terminal, and PWR_UP is the power control terminal, which are controlled by the P2.3, P2.4 and P2.5 pins of the microcontroller respectively. When the DR1 of nRF2401 is high, it indicates that there is data in the receive buffer, which is connected to the P2.2 of the microcontroller.[page]

　　Since the supply voltage range of nRF2401 is 1.9~3.6V, and the supply voltage of AT89S51 microcontroller is 5V, in order for the chip to work properly, level conversion and voltage division processing are required. The design uses MAXIM's MAX884 chip to perform 5V to 3.3V level conversion, as shown in Figure 5.

　　Figure 5 5V to 3.3V conversion circuit

　　2.2 Main control unit design

　　The main control unit and monitoring nodes form a wireless network, and the data communication between the host computer and the monitoring unit is realized through the main control unit. The basic structure of the main control unit is similar to that of the monitoring unit, mainly consisting of a single-chip microcomputer system, a wireless transceiver module, a display circuit, a serial communication circuit and a power supply.

　　The serial port is a very common device communication protocol on the computer. Most computers contain two serial ports based on RS232. The serial port of the PC is RS232C level, while the serial port of the microcontroller is TTL level. When the two communicate through the serial port, level conversion must be performed. The design uses the MAX232A chip to complete the data transmission between the microcontroller and the PC. The hardware connection circuit is shown in Figure 6.

　　Figure 6 MCU and MAX232A hardware connection circuit

　　3 Control Program Design

　　The system control program is mainly composed of a single bus temperature measurement control program, a wireless transceiver control program, and a host computer monitoring program. The single bus temperature measurement program is responsible for the initialization of the single bus device, collecting the battery temperature and transmitting it to the nRF2401 module; the main function of the wireless transceiver control program is to be responsible for the establishment of the wireless network and the wireless transmission of data information; the main function of the host computer monitoring program is to communicate data with the main control unit through the serial port, and to display and store data information in real time. Taking the monitoring node as an example, Figure 7 is a program flow chart of the monitoring unit. The monitoring unit is first initialized, mainly including the communication, interruption and timing initialization of the single chip system, and then collects the temperature information of the single battery, saves it and displays it with a digital tube, and monitors the data transmission command of the main control unit in real time. If there is any, the battery temperature data will be sent out through the wireless module.

　　Figure 7 Monitoring node program flow

　　4 Test results

　　A test prototype was designed, and the actual monitoring node test circuit is shown in Figure 8.

　　Figure 8 Monitoring node test circuit

　　Temperature tests were conducted indoors using four monitoring nodes at distances of 4m, 8m, and 12m from the main control unit. The test data are shown in Table 1.

　　Table 1 Temperature measurement test data

　　As can be seen from Table 1, the temperature measurement accuracy can reach ±0.3°C, and the accuracy of wireless transmission is high, which can meet the needs of wireless temperature monitoring.

　　5 Conclusion

　　Aiming at the temperature monitoring problem of single cells in battery packs, this paper proposes a design scheme for a battery performance monitoring system based on DS18B20 digital temperature sensor and wireless transceiver chip. The system in the scheme consists of a host computer, a main control unit and multiple monitoring nodes. The main control unit communicates with the host computer through a serial port. Compared with the traditional wired multi-point temperature measurement system, it has the characteristics of high efficiency, easy layout, expansion, maintenance and update, and has certain practical engineering application value.