Lithium-ion batteries with high voltage, high capacity, long cycle life and good safety performance have broad application prospects in portable electronic equipment, electric vehicles, space technology, and defense industry. A power lithium-ion battery pack consisting of several lithium-ion batteries connected in series is currently the most widely used. Since the voltage of each unit cell is inconsistent, the battery in use does not allow overcharging and overdischarging, and the performance and life of the battery are greatly affected by temperature. The series lithium ion battery pack must be monitored to ensure lithium ion in use. The battery has a good state, or the battery immediately has an alarm in use, and the power management system immediately takes safeguard measures and reminds the relevant personnel to overhaul. The cell voltage and the temperature of the battery pack are the main technical indicators for distinguishing whether the series lithium-ion battery pack is working properly. Reference  uses the direct sampling method to store the cell voltage to be measured on a non-capacitor for measurement. The method has slow reaction time, large error and complicated control. The literature  uses op amp and diaphragm relay to measure the cell voltage of the series battery. This method requires high linearity of the optocoupler, resulting in high hardware cost. At present, the series lithium ion battery monitoring system directly adopting the integrated chip is favored, but the method has a fixed number of series batteries, which leads to disadvantages such as inflexible application and high hardware cost. In this paper, a power lithium-ion battery monitoring system is developed to monitor the cell voltage of the series lithium-ion battery pack and the temperature of the battery pack. When the cell voltage deviates from the specified interval, the monitoring system starts the alarm program to sound, Light alarm; When the battery pack temperature deviates from the specified interval, the monitoring system activates the fan or heating control circuit and stores the relevant data to ensure that the battery pack is working properly. The entire monitoring system features continuous measurement components, simple economy, high precision and high reliability.
1 Technology and solutions
1. 1 system structure
The series lithium ion battery monitoring system includes a core control module of the 51 series single chip microcomputer, a lithium ion battery state acquisition module, a signal conditioning module, an alarm and processing system module, and the monitoring system can form a distributed monitoring system through the RS485 interface and the PC. Realize one PC to monitor multiple series battery packs. The system block diagram is shown in Figure 1.
The state acquisition module includes collecting parameters such as the voltage of the single battery and the temperature of the battery pack, and then processing the signal to be measured, sampling by the A/D converter, and transmitting the data to the single chip for data processing, and transmitting the valid data to the local through the serial port. The PC can monitor the status of the battery pack by analyzing the status data, and timely process the unsafe state to ensure the reliability of its work.
1. 2 Common problems of series lithium-ion battery packs
There are many methods for voltage measurement of series lithium-ion battery packs. The simplest method is the measurement method of resistance voltage division. The disadvantage of this method is that the drift error of large resistance resistors and the resistance 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 cell, but it is bulky and expensive, suitable for applications where measurement accuracy is high and leakage current and cost are not considered. The design uses Texas Instruments' INA117 to solve the common problem of series lithium-ion battery packs . The distortion of INA117 is 0.001%; the common analog ratio is 86 dB minimum, and the common-mode input voltage range is ± 200 V, suitable for High precision measurement.
The INA117 has three resistors of 380 kΩ, 20 kΩ and 21. 1 kΩ, so the external circuit eliminates precision resistors and reduces the error and system complexity caused by precision resistors. Figure 2 shows the connection of the INA117 output to the 1st battery voltage. The voltage between pin 6 and pin 1 is the voltage difference across the two cells.
The detection system uses 16 INA117s to select the cell voltages of the 16-cell lithium-ion batteries. If their 1 pin is connected to the same ground, the 16 INA117s can have the same signal ground and the A/D converter samples. The total location is selected at the junction of the 8th battery negative and the 9th battery positive.
The maximum voltage of each lithium-ion battery is 5 V, which is available from Figure 3. The input potential of pin 3 of the first INA117 is up to 40 V. Similarly, the input voltage of pin 2 of the 16th INA117 is at least - 40 V. The output voltages of the 1st to 8th INA117 are positive, and the output voltage of the 9th to 16th INA117 is negative, so the selection of both the analog switch and the A/D converter requires input of positive and negative voltages. Multi-select an analog switch selects MUX16, which is 16-select 1 positive and negative voltage input analog switch, so 16-cell battery only needs 1 MUX16. However, due to the limited IO port of the microcontroller, the article uses a 74LS154 to expand the IO port, using only the single-chip microcomputer. Four IO ports can control the MUX16 to individually strobe a single-cell Li-Ion battery for voltage sampling.
1. 3 A/D converter
Monitoring the battery pack does not require sampling the voltage of each battery at a very high sampling speed. The sampling of 16 battery voltages shares one A/D converter . The measured voltage of each battery input is connected to the A/D converter by a multi-selection analog switch MUX16. According to the update cycle and voltage requirements of the battery voltage, the voltage conversion value error transmitted by the A/D converter to the MCU is up to 10 mV. Select Maxim MAX1272.
The MAX1272 is a fail-safe, software-selectable input range 12-bit serial-to-analog converter that uses SPI three-wire communication protocol, + 5 V supply, analog input voltage range 0 ~ 10 V, 0 ~ 5 V, ± 10 V , ± 5 V. Internal with + 4. 096 V reference voltage. When using the internal + 4. 096 V reference voltage, the digital output corresponding to the analog voltage input is ideally shown in Table 1.