Analysis and Research on lithium ion battery 18650 price Monitoring System

1 Overview of Lithium-ion Battery Monitoring System

Lithium-ion battery is a new energy source, which has many advantages, such as stable discharge voltage, wide operating temperature range, low self-discharge rate, long storage life, no memory effect, small size, light weight and pollution-free.

Therefore, since the 1990s, people have conducted a lot of research and processing on it, and now it has gradually replaced lead-acid batteries and nickel-cadmium batteries. The scope of use of lithium-ion batteries is also very wide. For example, laptops, communication radios, portable electronic devices, space satellites, electric bicycles and electric vehicles all use it as their power source.

However, when using, lithium-ion batteries also have their own shortcomings:

⑴ When charging lithium-ion batteries, too many lithium ions in the positive electrode dissolve and cannot return to their original state, causing the lithium ions to be unable to fill the positive electrode channel when the battery is discharged. It manifests as the battery cannot be charged and causes permanent damage. In this case, it is necessary to take measures to control the amount of lithium ion dissolution by limiting the charging voltage;

⑵ When the lithium-ion battery is discharged to the end, the internal material will undergo a qualitative change, that is, all the lithium ions in the graphite layer on the negative electrode will fall off. When charging next time, the negative electrode graphite layer without lithium ions cannot ensure smooth circuit, so measures must be taken to control the size of the discharge voltage;

⑶ If the battery is accidentally short-circuited or the charging and discharging current is too large, the internal temperature of the battery will be too high and energy will be consumed, which will shorten the discharge time.

Therefore, the monitoring and management of lithium-ion batteries has become one of the key technologies to promote the development of lithium-ion batteries.

This system protects the battery pack from overcharging, over-discharging, short-circuiting and balancing during the charging and discharging process by testing and judging the voltage, current and temperature signals of the lithium-ion battery pack, and uses fuzzy control methods to diagnose battery faults.

(1) Signal test: including the test of voltage signal, current signal and temperature signal. The voltage signal includes the total voltage test of the lithium-ion battery pack and the single cell voltage test of each battery; for the current signal test, since the battery pack is connected in series, it is only necessary to test the total current; in addition, the temperature signal of the lithium-ion battery pack should be tested, and the safe temperature parameter range of the battery model should be set according to the battery type. When the temperature is found to be out of the safe range, corresponding solutions should be taken and an alarm message should be issued.

(2) Protection circuit: The voltage, current and temperature parameters of the lithium-ion battery pack are collected in real time through the signal test circuit. According to the parameter value, abnormal conditions are discovered in time, thereby realizing the protection of the lithium-ion battery pack from overcharge, overdischarge, short circuit and balance.

(3) Battery fault diagnosis: that is, fuzzy control technology is used to make fault diagnosis and solution based on the measured parameters such as temperature and voltage of a single battery.

(4) Power estimation: that is, the measurement of the remaining power of the battery, and the state of charge (SOC) of the lithium-ion battery pack is estimated based on the measured voltage value.

From the current situation, the key issues of this system are:

(1) After collecting voltage, current and temperature parameters through the signal test circuit, how to establish fuzzy control rules for fault diagnosis based on these parameters and some historical data during the charging and discharging process, that is, fuzzy control fault diagnosis technology.

(2) Lithium-ion battery packs are prone to imbalance between single cells during the charging and discharging process, which will greatly affect the service life of lithium-ion batteries. Therefore, balanced charging technology is another key technology that this system is committed to studying.

1.2 The current status of the system at home and abroad and several common methods

1.2.1 The current status of the system abroad In the past 20 years, foreign countries have conducted extensive and in-depth research on the monitoring and management system of lithium-ion battery packs. These studies are all to ensure that lithium-ion batteries can work normally and stably. Especially in the past decade, with the emergence of electric vehicles, some large foreign automobile processors and battery processors have done a lot of research and experiments on lithium-ion batteries and successfully developed a variety of lithium-ion battery pack management systems. For example:

⑴bADICHEQ system and bADICOaCH system

After a lot of experiments, Mentzer Electronic GmbH and Werner Retzlaff of Germany designed the bADICHEQ system in 1991. The system has the following functions:

⑴ Multiple batteries can measure voltage at the same time (up to 20), and can also measure current and temperature;

⑵ The battery unit can control the charging current of the charger;

⑶ Can store historical data;

⑷ Can achieve balanced charging of single cells;

⑸ Has a dashboard display function, which can display the remaining power of the battery unit and various abnormal alarms;

⑹ Can communicate data with the PC machine.

Based on the bADICHEQ system, some improvements were made to design the bADICOaCH system, which has the following functions:

⑴ Each battery cell is added with a nonlinear circuit, through which the voltage of the single battery is measured, and the voltage values of all batteries in the battery pack are sent to the system for decoding through a signal line;

⑵ Use RS232 standard to communicate with the PC machine;

⑶ When the worst battery cell is found, the remaining power of the battery

unit is displayed first, and then the battery is over-discharged and stopped; ⑷ Store the specific data of the last 24 charge and discharge cycles; ⑸ When judging whether the battery is good or bad, allow quick search of basic battery information and incorrect use; ⑹ Can control the charging current and voltage value, using two pWM signal output lines to achieve.

⑵ bATTMAN system.

This system was designed by b.Hauck of Germany. This system can realize the management of battery packs of different models, make different types of battery packs into a system, and then select them through hardware and software. In terms of hardware, by changing the hardware jumper; in terms of software, by adding a method to select parameters.

⑶ Battery management system on electric car EV1.

This system is processed by General Motors of the United States. The biggest advantage of this system is that the reliability of the battery pack is very high.

The system consists of four parts:

Battery module (used for vehicle drive and other power systems);

Software bpM (batterypackModule);

Battery pack thermal system;

Battery pack high voltage power-off protection device (HighVoltageDisconnect).

Software bpM mainly completes the following tasks:

Single cell voltage test, high voltage protection function, six-way temperature sampling, current sampling, battery pack charging control, over-discharge protection and power estimation, etc.

⑷SmartGuard system

This system was developed by Aerovironment of the United States. The system measures the voltage and temperature of the battery through a distributed dedicated IC management device, and can also start the current bypass circuit when there is a signal from the main control component.

The main functions of the system are:

● Overcharge protection;

● When the discharge is reversed, an alarm signal is issued;

● Can record historical information;

● Provide the remaining power information of the worst battery cell.

⑸ batOpt system.

This system was developed by ACpropulsion, an American company. In this system, each battery is equipped with a monitoring module, which communicates with the central control unit through the two-wire bus and reports the battery voltage, temperature and other information to the main control unit. After receiving the above information, the main control unit issues a corresponding control command, thus forming a distributed system.

1.2.2 Domestic status of the system

Compared with foreign countries, the domestic lithium-ion battery pack monitoring and management system is mainly jointly developed by universities and some automobile manufacturers and battery suppliers. Universities have their own technological advantages. After years of hard work, they have developed many reliable battery management systems and have put them into use.

⑴ Battery management system for EV-6580 light electric buses.

This system was developed by Tsinghua University. The system can measure and monitor the battery's charging and discharging current, voltage and other parameters in real time, and provide overcharge and over-discharge protection, which greatly improves the battery life. At the same time, a charging system matching the system has been developed.

⑵ The lithium-ion battery management system developed by Tongji University was jointly developed by Tongji University and Beijing Xingheng Battery Co., Ltd. The main functions of the system are: accurate acquisition of voltage, current and temperature, battery power estimation, balanced protection, accident handling and recording, etc.

⑶ The battery management system developed by Beijing Institute of Technology is specially developed for batteries on electric vehicles.

The system uses a single-chip microcomputer as a microprocessor and has the following functions:

It can test various operating parameters such as battery power, total voltage of the battery pack, total current, voltage of each single cell, and temperature of the battery pack in real time;

It has fault diagnosis and alarm functions;

It adopts a distributed network control system structure, and the system parameters are sent to the PC machine. After calibration in the PC machine, it communicates with other systems through the CAN bus to achieve information sharing.

The system has been installed and used. The results show that the system has greatly extended the life of the battery pack and improved the operating efficiency of electric vehicles. When a fault occurs, it can be discovered and solved in time. At the same time, it can accurately estimate the remaining power of the battery pack, which improves the reliability of electric vehicles.

1.2.3 Common methods for protecting lithium-ion batteries

From the previous analysis, we can see that during the use of lithium-ion battery packs, lithium-ion batteries are susceptible to overcharging and over-discharging, which greatly reduces the service life of lithium-ion batteries. Therefore, the protection circuit of the lithium-ion battery pack is very important.

The lithium-ion battery pack protection circuit should first have the functions of overcharge protection, over-discharge protection, and equalization protection for the lithium-ion battery pack. In addition, in addition to being able to complete the above functions, its protection circuit must also have the following characteristics:

⑴ Low power consumption current.

The protection circuit works when the battery condition is abnormal, so the current consumed by the protection circuit should be as small as possible to achieve the purpose of reducing losses.

⑵ The test accuracy should be high. The overcharge test requires high circuit accuracy. If the test accuracy is not high, the battery capacity cannot be fully used.

⑶ Low working voltage.

When the protection circuit protects the lithium-ion battery, since the voltage of a single lithium-ion battery is low, when the battery is discharged, the voltage will further decrease. Therefore, the protection circuit should be able to work at a low voltage.

In view of the functions and characteristics that should be completed as mentioned above, there are currently two common lithium-ion battery management methods:

⑴ Lithium-ion battery management system using dedicated chips. In common portable devices, lithium-ion batteries with smaller capacity are usually used. First of all, considering the needs of portable devices, the system's protection circuit is usually required to be as small as possible. Secondly, considering that the battery capacity is relatively small, do not over-consider the system's balance and protection issues. Usually, a dedicated management chip and peripheral circuits can be used to achieve simple battery charge and discharge management and protection functions.

⑵ Lithium-ion battery management system based on monitoring. In large-capacity lithium-ion battery use systems, dedicated lithium-ion battery management chips can no longer meet the requirements, and microcontrollers must be used to achieve system management. We collect various data of the battery pack through the signal acquisition circuit, and then send these data to the microcontroller for decomposition. According to the decomposition results, we judge the working status of each battery in the battery pack. According to the hardware and software design of the system, the lithium-ion battery pack is intelligently managed and protected, so as to achieve the effective utilization of lithium-ion battery power and extend the battery life.

This paper is designed for balanced protection of 16 lithium-ion battery packs, and the dedicated chip can only protect 13 batteries at most. Therefore, if a dedicated chip is used for protection, at least 2 dedicated chips are required to achieve it, which increases the cost of the system. In addition, if a dedicated chip is used, the number of batteries in the lithium-ion battery pack will be fixed, so the flexibility of the system is low. Considering various aspects, this paper adopts a method based on a microcontroller lithium-ion battery monitoring system, and the microcontroller uses a low-power Msp430 single-chip microcomputer.

1.3 Significance of the topic and important research content

1.3.1 Significance of this paper

Lithium-ion batteries are green and high-efficiency rechargeable batteries that only appeared at the end of the 20th century. At present, with the promotion and large-scale use of lithium-ion batteries, lithium-ion batteries are deeply welcomed by society and users. In daily life, people's mobile phones, laptops, digital cameras and many portable devices have used lithium-ion batteries as power sources.

At present, the development of the world's battery industry technology has gradually shown the following three development trends: First, highlighting the green concept, including lithium-ion batteries, mercury-free alkaline manganese batteries, etc., to meet people's needs for batteries. Second, the transformation of primary batteries to secondary batteries. Just as reusable lithium-ion batteries were developed on the basis of lithium-ion battery technology, rechargeable mercury-free manganese batteries were developed on the basis of mercury-free manganese batteries. Third, portable batteries are the first choice for users. With the emergence of various batteries, when users choose batteries, they pay more attention to the portability of batteries while considering the environmental protection and cost-effectiveness of batteries. It is precisely because lithium-ion batteries have high volume energy and environmental protection performance that they are in line with the current development trend of world battery technology. According to authoritative departments, the total output value of lithium-ion batteries in my country will challenge 10 billion US dollars. According to IIT and other research institutions, the annual growth rate of my country's lithium-ion battery industry will exceed 20%, and the overall demand for batteries in 2016 will reach about 5 billion pieces. It can be seen that lithium-ion batteries will be the leader of my country's battery industry at present and for a considerable period of time in the future. It can be seen that the study of lithium-ion battery management system is particularly important and is one of the key factors for the further promotion of lithium-ion batteries.

1.3.2 The main research content of this article

This article is designed for the monitoring and management of lithium-ion battery packs on civilian electric vehicles. The main functions of this system include charging and discharging protection of lithium-ion batteries, balancing protection of single cells, battery fault diagnosis, and real-time monitoring of battery packs by the host computer.

The main content of this article is divided into the following parts:

Chapter 1, Research purpose and significance. Briefly analyze the current status and current protection methods of lithium-ion battery monitoring system and its protection circuit.

Chapter 2, Basic research on lithium-ion battery monitoring system. Analyze battery characteristics, power management, and the reasons and common methods for taking balancing measures for lithium-ion batteries.

Chapter 3, System hardware design. Specifically analyze the overall hardware method of the lithium-ion battery monitoring system designed in this article, and specifically analyze the hardware circuit design of each module.

Chapter 4, MCU software design. The system's lower computer software design process and the software design methods of each module of the system are analyzed in detail.

Chapter 5, upper computer program design. The system's upper computer software design process and the software design methods of each module of the system are analyzed in detail.

Chapter 6, fuzzy fault diagnosis. Fuzzy control technology is used to realize the diagnosis of battery faults.

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