Design of battery online monitoring system based on Labview

Since AC mains power may experience power outages, voltage sags and surges, continuous undervoltage and overvoltage, and frequency fluctuations during supply, these factors will affect the continuous operation of the network and even damage network equipment and servers in operation. When building a network system, each enterprise will take necessary measures to provide high-quality UPS power supply in terms of computer network power supply. Among them, the battery pack is undoubtedly the last insurance in the power supply as the last guarantee of power supply. The working state of the battery will directly affect the stability of the UPS system, so the working state of the battery pack must be monitored in real time. It can be seen that accurate monitoring of the battery in the power supply has become very important. In order to achieve accurate detection of various parameters of the battery, a design scheme of an online battery monitoring system (hereinafter referred to as "monitoring system") based on Labview is proposed and designed on the basis of demand analysis. The system can complete accurate detection.

1 Monitoring demand analysis
In order to meet the demand for testing the battery parameters of a certain type of UPS power supply, the analysis of the battery intelligent comprehensive monitoring and management system shows that the system acquisition signals are divided into the following 4 parameters: the main parameters such as battery voltage, current, temperature, and power are sampled. To complete the test of the above signals, the following aspects need to be done. First, the monitoring system should be able to detect input signals in various working conditions; second, it should be able to communicate the detected data with the PC; third, it should also be able to display and process the data.

2 Overall Design
The structure diagram of the monitoring system is shown in Figure 1. In the process of detecting battery parameters, the acquisition module monitors the operation of the battery, monitors whether the current is within the normal range, monitors whether the single battery voltage is normal, and uses the MCU controller (AT89S52) and DS2438 devices to collect various battery parameters; the collected data is sent to the computer through the RS232 serial interface circuit; at the same time, based on the collected and uploaded data, the capacity is estimated and calculated, and the battery monitoring system is constructed with the help of battery data (voltage, current, temperature, and power).

3 System hardware design
The monitoring system hardware is mainly composed of RS232 serial communication interface circuit, AT89S52 controller, DS2438 battery parameter acquisition circuit, etc. The system hardware structure diagram is shown in Figure 2. The system is based on Labview serial communication for data acquisition, with PC as the host computer and single-chip microcomputer (AT89S52) as the slave computer. The host computer sends acquisition instructions to trigger the slave computer to read the battery parameter value collected by DS2438 through the P2 port, and use the serial input and output terminals of P3.0 and P3.1 to transmit to the serial port of the host computer through the serial port chip MAX232, collect and convert to decimal using Labview, and then process the data through Labview.

3.1 Measurement of battery temperature parameters
The battery temperature measurement is stored in the DS2438 temperature register (bytes 1 and 2 on page 0) through the internal temperature sensor, and the serial data transmission is completed through the single bus input and output port (DQ) and the microcontroller P2.0 port. The battery parameter acquisition circuit is shown in Figure 3.

3.2 Measurement of battery voltage parameters
DS2438 has a built-in 10-bit voltage A/D converter. When a resistor R1=1 MΩ is selected, R2=390 kΩ is obtained by the formula 14×1 MΩ/(1 MΩ+R)=10 V. Uactual is the actual voltage of a single battery, and Umeasured is the voltage value measured by DS2438. According to the following formula, Uactual=Umeasured(1 MΩ+0.39 MΩ)/1 MΩ, the measured value can be converted to the actual value in the microcontroller.
3.3 Measurement of battery current parameters
DS2438 has a built-in current A/D converter. When the microcontroller sends an A/D converter enable signal, DS2438 automatically measures the current flowing through the sampling resistor, and the measurement result is stored in the current register (bytes 5 and 6 on page 0). The selection of the current collection resistor should not affect the use of the battery, so a small resistance resistor is selected, and the resistance accuracy requirement is high. The design uses a resistor with Rsers=0.025 Ω.
In order to resist battery interference, an RC low-pass filter is designed. R: 100 kΩ, C: 0.1 μF, and the cut-off frequency are selected by calculation:
F = 1/(2πRC) = 15.9 Hz (1)
For the AD conversion frequency of DS2438 is 36.41 Hz, the low-pass filter effectively filters out the Jianfeng pulse to ensure that the current accumulator accurately obtains the sampling signal.
3.4 Measurement of the remaining battery capacity
The remaining battery capacity is obtained by the value of the integrated current accumulator (ICA). ICA is a register that accumulates all the current flowing into and out of the battery after the battery pack is put into use. Its value is changed by the DS2438 after automatically measuring the voltage of the external resistor Rsers. There is no need to control it. The microcontroller only needs to read the value of the ICA register, and then the remaining battery capacity is calculated by the following formula:
Remaining capacity = ICA/(2048xRsers) (2)
The unit of Rsers is Ω.

4 Test system software design
The software of this monitoring system is programmed with Labview, which is a graphical programming environment launched by National Instruments (NI) for data acquisition, instrument control, data analysis and data expression. It is an open development environment with all functional functions of various instrument communication bus standards such as PCI, PXI, RS-232/485, USB, etc. Developers can use these functions to interact with data acquisition hardware with different bus standard interfaces. This system uses NI_VISA serial port Serial function to access and control the serial port, thereby realizing the serial port communication function. First, use VISA Con2figure SeriM Port.vi to initialize the serial port, then use VISA write.vi to send data read instructions to the write buffer, and finally use VISA read.vi to read the 8-bit binary number in the data buffer in the form of a string, and use the HexadeeimalString To Number node to convert the 8-bit string data into digital data and provide it to the control circuit. Modular programming is used in the design process to facilitate updating, maintenance and expansion. The entire system consists of temperature data acquisition module, voltage data acquisition module, current data acquisition module, power data acquisition module, communication module and system help module. The monitoring system uses Labview program to send hexadecimal data to the microcontroller, start each acquisition module to collect data, record parameters in real time, and use the host computer to process and display data. It not only realizes the function of acquisition monitoring, but also can further process and analyze data. The structure diagram of the system software design is shown in Figure 4.

The lower computer software is written in C language, including two main parts: reading and writing of DS2438 and serial communication. The upper computer software is written in Labview. The relevant program segments are shown below.
System lower computer main function:
5 Experimental application
This test system is used to test the temperature of a certain type of battery. When testing, first run the monitoring application software, initialize it, complete the relevant settings such as detection settings and communication configuration, and then click the corresponding module detection button on the main program interface to perform the corresponding test, in which the upper computer sends the control command word and then receives the data sent back by the lower computer; and displays the results. Part of the interface of the program panel is shown in Figure 5. Through actual application, it is found that the test results of the test system are accurate, stable and reliable.

6 Conclusions
The battery online monitoring system designed in this paper can not only collect and display battery parameters in real time, but also realize remote data control, which can meet the measurement requirements of the system. The test system has been used to test a certain type of battery system. The actual application shows that the test system has the characteristics of accurate detection, stable and reliable, and friendly human-computer interface, which meets the design requirements. Moreover, after the system is expanded, it can be used for remote data collection and measurement and control of UPS power battery packs.