AA Carbon battery.Technology and solutions for power lithium-ion battery pack monitoring system

　　Lithium-ion batteries with the advantages of high voltage, high capacity, long cycle life, and good safety performance have broad application prospects in portable electronic equipment, electric vehicles, space technology, special industries, etc. Power lithium-ion battery packs composed of several lithium-ion batteries connected in series are currently the most widely used. Since the voltage of each single cell is inconsistent, the battery is not allowed to be overcharged or overdischarged during use, and the performance and life of the battery are greatly affected by temperature. The series lithium-ion battery pack must be monitored to ensure that the lithium-ion battery pack is in use. If the battery is in good condition, or if there is a problem with the battery during use, the alarm will be reported immediately. The power management system will immediately take safeguard measures and remind relevant personnel for maintenance. Cell voltage and battery pack temperature are the main technical indicators to identify whether the series-connected lithium-ion battery pack is working properly. Literature [1] uses the direct sampling method to store the single cell voltage to be measured on a non-capacitor for measurement. This method has slow response time, large error, and complex control; the literature [2] uses operational amplifiers and photocoupler relays to measure the cell voltage of the series battery pack. This method requires high linearity of the optocoupler, resulting in higher hardware costs. Currently, series-connected lithium-ion battery pack monitoring systems that directly use integrated chips are favored. However, this method has a fixed number of series-connected batteries, resulting in inflexible applications and high hardware costs. In this article, a power lithium-ion battery pack monitoring system is developed to conduct online monitoring of the cell voltage of the series-connected lithium-ion battery pack and the temperature of the battery pack. When the cell voltage deviates from the specified range, the monitoring system starts an alarm program to sound and Light alarm; when the temperature of the battery pack deviates from the specified range, the monitoring system starts the fan or heating control circuit and stores relevant data to ensure the normal operation of the battery pack. The entire monitoring system has the characteristics of continuous component measurement, simplicity and economy, high precision and high reliability.

　　1Technology and solutions

　　1.1 System structure

　　The series lithium-ion battery pack monitoring system includes the core control module using the 51 series microcontroller, the lithium-ion battery pack status acquisition module, the signal conditioning module, the alarm and processing system module, and the monitoring system can form a distributed monitoring system with the PC machine through the RS485 interface. To realize the monitoring of multiple series-connected battery packs by one PC, the system structure diagram is shown in Figure 1.

　　The status acquisition module includes the collection of parameters such as the voltage of a single battery and the temperature of the battery pack, and then processes the measured signal, samples it through the A/D converter and transmits it to the microcontroller for data processing, and transmits the valid data to the local through the serial port. PC machine, monitoring personnel can understand the working status of the battery pack by analyzing the status data, handle unsafe status in a timely manner, and ensure the reliability of its work.

　　1.2 Common ground problem of series-connected lithium-ion battery packs

　　There are many methods for measuring the voltage of series lithium-ion battery packs. The simplest is the resistor voltage division measurement method. The disadvantage of this method is that the drift error of large resistance resistors and resistor leakage current lead to low measurement accuracy and affect the consistency of the battery pack. . Another common method is to use an isolated operational amplifier for each single cell, but it is large in size and expensive, and is suitable for situations where high measurement accuracy is required and leakage current and cost are not considered. The design uses Texas Instruments' INA117 to solve the common ground problem of series-connected lithium-ion battery packs [3]. The distortion of INA117 is 0.001%; the minimum common-to-analog ratio is 86dB, and the common-mode input voltage range is ±200V, which is suitable for high-precision Measurement.

　　INA117 has three built-in resistors of 380kΩ, 20kΩ and 21.1kΩ, so the external circuit eliminates the need for precision resistors, reducing errors and system complexity caused by precision resistors. Figure 2 is the connection method for INA117 to output the voltage of one battery. The voltage between pin 6 and pin 1 is the voltage difference between the two ends of the battery.

　　The detection system uses 16 INA117 to select the cell voltage of 16 lithium-ion batteries respectively. If their pin 1 is connected to the same ground, 16 INA117s can all have the same signal ground, and the A/D converter can perform sampling. The common point is chosen at the connection between the negative electrode of the 8th battery and the positive electrode of the 9th battery.

　　The maximum voltage of each lithium-ion battery is 5V. From Figure 3, it can be seen that the input potential of pin 3 of the first INA117 is the highest 40V. Similarly, the input potential of pin 2 of the 16th INA117 is the lowest -40V. The input potential of pin 1 to INA117 is the lowest -40V. The output voltage of the 8 INA117s is positive, and the output voltage of the 9th to 16th INA117 is negative, so the multiple-select analog switch and A/D converter are required to input positive and negative voltages. The multi-select one analog switch uses MUX16, which can select 1 from 16 positive and negative voltage input analog switches, so 16 batteries only require one MUX16. However, due to the limited IO port of the microcontroller, a 74LS154 is used in this article to expand the IO port, and only the microcontroller's IO port is used. The 4 IO ports can control MUX16 to separately select a single lithium-ion battery for voltage sampling.

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