Design and Implementation of Monitoring System for Lithium Ion Power Battery Pack

A battery is a power source that discharges electric energy by discharge and absorbs and restores electric energy by charging. A low-voltage power supply composed of a lithium-ion power battery is a key device in an underwater robot system. Improper maintenance and management of lithium-ion batteries will directly affect the use efficiency and longevity of lithium-ion batteries, and even directly damage the lithium battery, thereby affecting the overall performance of the underwater robot, and in serious cases, it will lead to safety accidents of the robot. By measuring the parameters of the lithium-ion battery pack online, it is possible to understand the working state and working characteristics of the lithium-ion battery and the maintenance of the lithium-ion battery. Therefore, the development of the online monitoring system for the lithium-ion battery is imperative.

In order to realize the monitoring of lithium ion power battery parameters, it is preferred to design a parameter acquisition module to collect the voltage, current, temperature and other parameters of the lithium ion power battery, and upload them to the MCU with the A/D conversion module. Record and display.

2 Overview of monitoring system for lithium-ion battery packs

The system uses distributed data acquisition and centralized data processing, respectively design voltage acquisition circuit, current acquisition circuit, temperature acquisition circuit, and then send the data to the single-chip microcomputer for centralized processing. The system structure diagram is shown in Figure 2-1.

Figure 2-1 System structure diagram.

The system is monitored by the lithium ion power battery pack of the National 863 Project underwater robot system, which uses the TS-LFP160AHA model lithium ion power battery produced by Shenzhen Leitian Technology. The battery pack consists of 8 single cells. It is necessary to monitor the terminal voltage of each single cell and make an overvoltage and undervoltage judgment; it is necessary to measure the temperature more, monitor the temperature of each battery and the temperature and humidity of the environment in which the battery pack is located; due to 8 single cells In series, it is only necessary to measure the series current and make an overcurrent judgment.

This article uses the TMS320LF2407A chip. The CPU that uses this chip as the battery monitoring system is also reflected in the following aspects:

1. Energy saving and energy saving have become a hot issue in the design of modern electronic equipment. When the device is powered by a secondary battery, the problem of energy saving becomes more prominent and important. The DSP used in this design is powered by a 3.3V supply, reducing controller losses. Chip power management includes a low-power mode that independently switches peripheral devices to low-power modes.

2.16 A/D converter for channel input. This makes sense for multiple acquisition subcircuits. The output of the acquisition circuit can be directly connected to the A/D conversion channel of the DSP. It is not necessary to set up an A/D conversion circuit outside the DSP.

3.40 I/O pins that can be individually programmed or multiplexed. Can be used for control of safety switches and other peripheral circuits.

4. Serial Communication Interface (SCI) and 16-bit Serial Peripheral Interface Module (SPI) can be connected to the display portion of the monitoring system.

3 system hardware design

The hardware design of the system mainly includes voltage acquisition circuit, current acquisition circuit and temperature acquisition circuit design. The acquisition circuit uses the TMS320LF2407A as the CPU. The TMS320LF2407A is a high-performance 16-bit fixed-point DSP device designed by TI for real-time control with a command cycle of 33ns. It integrates a front-end sampling A/D converter and back-end PWM output hardware to meet the real-time requirements of the system. Simplify hardware circuit design.

3.1 Voltage acquisition circuit design This design uses lithium ion power battery as the management object. The battery pack consists of eight 3.6V lithium batteries. Each battery cell is rated at 3.6V and has a full-end voltage of 4.25V. The voltage acquisition accuracy is required to be controlled within 1.5%. The minimum sampling frequency required by the battery management system is 20ms.

The system uses a linear optocoupler as the signal transfer sampling device for the isolation and data acquisition system, thus isolating the voltage of each cell in the front end. The large voltage of the battery is reduced by a certain ratio, so that the voltage value of the battery change is faithfully reflected to the DSP. It is then passed through a multi-way switch into the microprocessor for calculation. The advantage of optocoupler isolation is that the speed is fast (the speed of the optocoupler is microseconds, much smaller than the millisecond level of the relay), and the real-time performance is better. In addition, the signals at both ends of the optocoupler are completely isolated on the electrical connection, so there is no relationship, so even if a short circuit occurs at the output of the optocoupler, it will not affect the use of the battery. The optocoupler converts the voltage signal into a current signal for acquisition, which solves the common problem. Optocouplers are more cost effective than voltage sensors.

When selecting the device, we considered the economy and practicality. The photoelectric accident detector chose TLP521 produced by Toshiba Corporation of Japan and the dual operational amplifier TL082 selected by the operational amplifier.

The voltage measurement circuit of the battery cell is shown in Figure 3-1.

Figure 3-1 Single cell voltage acquisition circuit.

VIN is the cell voltage, which is looped through R1 and the LED in the optocoupler to convert the voltage signal (VIN) into a current signal (I11). I11 has a certain proportional relationship with I21 I11∝ I21. UU1 is used here as a comparator. When the voltage V at point A is greater than the voltage Vb at point B, UU1 outputs a higher voltage value. When the voltage Va at point A is lower than the voltage Vb at point B, UU1 outputs a lower voltage value. The comparator forms a feedback throughout the voltage sampling circuit. Keep the voltage values ​​of the two points A and B consistent. The purpose of this is that the voltage at point B is obviously 15 ∕ 2 = 7.5 volts, Va = Vb = 7.5 volts, indicating that the transistor in the upper and lower optocouplers is turned on. Thus, the conduction of the triode is controlled by the light emitting diode. It can be seen that when I21 = I22, I11 = I22. Thus, VIN ∕ = I11 = I22 = Vout ∕ R4. It can be seen that Vout is proportional to VIN.

3.2 Current Acquisition Circuit Design All the battery cells of the lithium-ion battery pack are connected in series to form the entire power supply system. Only one current collection point can be set.

This article uses Hall current sensor acquisition.

The schematic diagram of the Hall current sensor is shown in Figure 3-2. The measured current In flows through the magnetic field generated by the conductor, and is compensated by the magnetic field generated by the compensation current Im controlled by the output signal of the Hall element flowing through the secondary coil. When the magnetic field of the primary side and the secondary side are balanced, the compensation current Im can be Accurately reflect the primary current In value.

Figure 3-2 Schematic diagram of the Hall current sensor.

The system uses a closed-loop Hall current sensor of the Yusen CBH100SF model. The measuring frequency is 0-100KHz, the rated current is 100A, the measuring range is 0-±150A, the turns ratio is 1:1000, the precision is 0.2%-1%, and the corresponding time: “lus. The structure is shown in Figure 3-3:

Figure 3-3 CHB100 appearance and connection diagram.

The sampling resistor Rm is sampled by precision resistors. It is recommended to use high-precision metal film resistors with low temperature drift (not more than 2ppm). Because of the large parasitic inductance, precision wirewound resistors should be avoided in high frequency sampling. Sampling resistance × secondary output current rating should be less than the power supply voltage, the difference is greater than 4V. The power of the sampling resistor must be sufficient, Rm = 30 Ω.

3.3 Temperature acquisition circuit design In the calculation of the remaining battery power, the operating temperature of the battery is an important factor. In addition to this, it is also necessary to collect temperature parameters in real time in judging battery safety and heat treatment. In this design, the temperature signal acquisition of 8 single cells is designed, and the real-time acquisition of ambient temperature is also designed.

This system uses a thermistor to measure the temperature of the battery itself. Combined with the bridge circuit, the temperature signal is reflected as a voltage signal. The circuit is shown in Figure 3-4.

Figure 3-4 Single cell temperature sampling circuit

Among them, RMDZ1 is a thermistor, which is mainly considered to be cost-effective, and its small size is long and can be directly attached to the outer casing of the battery. The disadvantage is that the linearity is not good. The detection of the battery temperature is mainly to replace the upper and lower limit temperature, and calculate the temperature difference between the batteries to find the abnormal battery. It does not involve problems with functions and complex calculations, and the requirements for line type are not high, so the use of thermistors can meet the demand.

The ambient temperature is measured using a novel temperature sensor LM35, which is characterized by an output voltage proportional to the ambient Celsius temperature, which has been internally calibrated and requires no external calibration. The sensitivity is 10.0mV/°C, the accuracy is up to 0.5°C, the working voltage range is 4V-30V, the consumption of the electrode is small, and the output impedance is low. The LM35 full-scale [55°C, 150°C] connection method has been used since then. In order to prevent the subzero temperature from being output, the negative voltage is not convenient for sampling into the DSP, and a subtractor circuit is designed. Adjusted to ambient temperature in the range of [-45 ° C, 75 ° C], the output voltage is [0, 4.5V].

4 System software design The software design of this system adopts DSP (TMS320LF2407A) C language programming, and implements modular design, which increases the readability and portability of the program. This design is mainly designed for the lithium-ion power battery used by underwater robots. At the same time, it strives to have better compatibility. That is, it does not need to change the hardware for other batteries, just change the software and even change it as little as possible. The software is ready to use. For this system, the control software should meet the following requirements:

Collect current, voltage, temperature and other signals, determine the battery's fault signal, process and take appropriate protective measures to display fault information.

The acquisition of analog data includes battery cell voltage, current, battery cell temperature, and ambient temperature. The voltage acquisition needs to be completed by controlling the analog multi-way switch, and the voltage values ​​of the individual cells are time-divisionally entered into the DSP, and it is required to collect the voltage and current at the same time. Make full use of the TMS320F2407A/D module to collect four quantities at a time: voltage, current, battery temperature, ambient temperature, and use the cycle to complete analog sampling of multiple batteries in the battery pack.

5 Summary

In this paper, based on the characteristics and test requirements of lithium-ion battery packs, a monitoring system based on TMS320LF2407A is designed. The scheme of distributed data acquisition and centralized data processing is proposed. The software and hardware schemes for voltage, current and temperature acquisition of battery monitoring system are given. The bottom acquisition module frame of the battery monitoring system with up to 8 cells is built.

On this basis, the battery information can be conveniently collected into the DSP for recording and battery state estimation and judgment, and communicate with the central controller through the CAN network to form a complete battery monitoring system.

The main research content of this subject is the design of the overall scheme of the battery monitoring system and the design of the hardware circuit. At its core is a combination of decentralized data collection and centralized data processing. Collect the voltage, circuit and temperature of the single cell separately, and send these basic information to the DSP for centralized and comprehensive analysis and processing. The hardware design focuses on the design of several acquisition circuits and the application of DSP small systems in the monitoring system. The voltage acquisition circuit has flexibility and obvious price advantage on the basis of guaranteed performance. Inter-channel interference and acquisition speed are improved. It can meet the requirements of real-time and measurement accuracy of the system. By increasing the peripheral sample and hold, it is possible to collect the voltage and current at the same time. The current and temperature of the battery management system are measured by Hall high current sensor, thermistor and Hall temperature.