Research on the principle characteristics and remaining power of lithium batteries

　　1.1 Working principle of 18650 battery pack 12v

　　Early lithium batteries used metallic lithium directly in the negative electrode, which easily caused lithium deposition and corrosion during the charging process, which greatly shortened the cycle life of the battery. In serious cases, it could cause the battery to short circuit or even explode. To solve this problem, lithium-ion batteries were developed. The so-called lithium-ion battery uses a crystalline structure active material that can accommodate lithium ions in the positive and negative electrodes, so that lithium ions can be transferred from the positive electrode to the negative electrode or from the negative electrode to the positive electrode with charging and discharging. The charging and discharging process of lithium-ion batteries is achieved through the intercalation and deintercalation of the positive and negative electrodes of the lithium-ion battery. When the battery is charged, the positive electrode releases lithium ions into the electrolyte. This process is deintercalation, and the negative electrode is removed from the electrolyte. Inhaling lithium ions, this process is intercalation. When the battery is discharged, the opposite process to the above occurs. This intercalation and deintercalation process of lithium ions during charging and discharging is like a rocking chair rocking back and forth, so some people call lithium-ion batteries " Rocking Chair Battery”. Generally, the negative electrode of lithium-ion batteries is made of carbon (C) material, and the positive electrode is made of lithium metal oxide (LiMO2). The main chemical reactions are: Lithium-ion batteries have many superior characteristics, such as high energy, higher safety, and workability. It has a wide temperature range, stable operating voltage and long storage life (compared to other batteries). In terms of safety, lithium-ion batteries are much safer than other batteries. Especially after taking control measures, the safety of lithium-ion batteries is greatly guaranteed. The batteries have undergone abuse tests (ABUSETEST) such as overcharge, short circuit, puncture, impact (pressure), etc., and there is no danger. Lithium-ion batteries, like nickel-cadmium batteries (Cd-Ni) and nickel-metal hydride batteries (MH-Ni) batteries, can be charged quickly and have no memory effect, which is far superior to Cd-Ni batteries; its self-discharge rate is far higher than MH- Ni battery is low. From the perspective of environmental protection, the World Environmental Protection Organization has already listed three elements: cadmium (Cd), mercury (Hg), and lead (pb) as hazardous substances. Therefore, the use of batteries containing these three elements has been restricted, especially in Europe, where some governments have significantly increased environmental taxes on certain batteries. In contrast, lithium-ion batteries do not have these problems. Of course, lithium-ion batteries also have some shortcomings, such as low low-temperature discharge rates and relatively high battery prices. 1.2 Charge and discharge characteristics of lithium batteries

　　The charging process of the battery is a complex electrochemical change process, and its complexity is shown as:

　　(1) There are many multi-variable factors that affect charging, such as the concentration of the plate and dielectric, the state of the plate active materials, the charging environment temperature, etc., all of which have a direct impact on the maximum charging current that the battery can withstand. (2) Nonlinearity Generally speaking, the charging current decreases exponentially with the charging time during the charging process, and it is impossible to control the entire charging process with just a simple constant current or constant voltage. (3) Complex electrochemistry Even batteries of the same type and capacity have different discharge history and remaining power, and their charge acceptance capabilities are also very different when used. As a battery pack that provides power for underwater robots, due to the complexity of the use environment, its charge and discharge process is also more complicated. In particular, overcharge and overdischarge will cause irreparable damage to the battery structure and greatly affect its health. extent and performance. 18650 battery pack 12v technology has great performance advantages compared with traditional battery technology, but it also has higher requirements for monitoring systems. Improper control will not only cause damage to the battery structure, but also cause danger. The nominal voltage of lithium-ion battery is 3.6V, and the full voltage is 4.2V. It is sensitive to overcharge and overdischarge. In order to minimize the damage of overcharging, deep discharge and short circuit to which lithium batteries are susceptible, the charging voltage of single lithium-ion batteries must be strictly limited. Different batteries may have different capacities. The same current value with the same capacity value is called 1C. The charge and discharge characteristics of lithium batteries will be different under different currents. Therefore, study its charge and discharge characteristics and use the correct charge and discharge characteristics. Discharge control and protection methods have great theoretical and application significance for its safe use and improved service life.

　　Lithium batteries have the following characteristics during charging:

　　(1) In the first half of charging, the voltage gradually rises. (2) After the voltage reaches 4.2V, the internal resistance changes and the voltage remains unchanged. (3) During the whole process, the power continues to increase. (4) When nearly full, the charging current will reach a very small value. After years of research, a better charging control method has been found:

　　(1) First use constant current to charge, so that the voltage basically reaches 4.2V. The safe current is less than 0.8C.

　　(2) The constant current stage can basically reach 80% of the power.

　　(3) Switch to constant voltage charging, and the current gradually decreases. (4) When the current reaches a smaller value (such as 0.05C), the battery reaches a full state. This constant current-constant voltage charging method can effectively reach the full state of the battery without damaging the battery, and has become the main charging method for lithium batteries. However, when the battery voltage is already very low, the lithium ion activity inside the battery weakens. If a relatively large current is used to charge at this time, it may also cause damage to the battery. Just like people must carry out necessary warm-up activities before strenuous exercise, the activity of lithium ions must also be gradually activated. The pulsation method can be used in the low voltage section of the battery to effectively activate the battery voltage to above 3.0V, and then the constant current-constant voltage charging method can be used to effectively protect the battery. Lithium-ion batteries have very good load capacity. Its discharge characteristics at different currents are as shown in the figure: (1) The maximum discharge current that can be safely provided is 3C. (2) Different discharge currents have different effects on voltage and power, and the effective power that can be output also varies greatly. (3) Discharge to about 3V, the battery power has basically been output. According to the discharge characteristics, this article will study the use of appropriate discharge management and protection technology to achieve dynamic management of parameters. 2 Research on remaining capacity of 18650 battery pack 12v

　　2.1 Factors affecting the remaining capacity of the battery

　　During battery use, battery capacity is affected by many uncertain factors. Therefore, how to accurately estimate the existing battery capacity status based on measurable battery parameters is a difficult problem. At present, the battery's state of charge SOC (State of Charge) is commonly used at home and abroad as a battery capacity state description parameter to reflect the remaining capacity of the battery. Its value is defined as the ratio of the remaining capacity of the battery to the battery capacity: where: CQ is the battery's remaining capacity. The remaining energy, 0C, is the nominal capacity of the battery, that is, the capacity that can be released when it is in an ideal state under specified current and temperature. Usually, the state where the battery is charged to the point where it can no longer absorb energy at a certain temperature is defined as 100% state of charge, and the state where the battery can no longer release energy is defined as 0% state of charge. The discharge process of the battery is a complex electrochemical change process. The remaining capacity of the battery is affected by many factors such as battery temperature, discharge rate, self-discharge, number of charge and discharge cycles, etc., making it very difficult to estimate the remaining capacity. When the battery is discharged, the following factors will have a major impact on the actual remaining capacity of the battery. (1) Discharge rate. The discharge current directly affects the discharge termination voltage. Under the specified discharge termination voltage, the greater the discharge current, the smaller the amount of electricity that the battery can discharge. (2) The effect of battery temperature on capacity. Battery temperature has a greater impact on its capacity. This is because as the battery temperature increases, the chemical reaction of the plate active material gradually improves, so a higher battery temperature during discharge will cause the battery to release more power. However, excessively high temperature during charging will cause more oxygen to precipitate, and the electrode voltage will more easily reach the maximum value, which will reduce the charging effect. (3) Self-discharge rate. During the storage period of the battery, due to the effect of impurities in the battery, such as the positive electrode active metal ions and the negative electrode active material forming a micro-battery, the negative electrode metal is dissolved and hydrogen gas is precipitated. Another example is the impurities dissolved from the positive electrode plate in the solution. If the standard electrode potential is between the standard electrode potentials of the positive electrode and the negative electrode, they will be oxidized by the positive electrode and reduced by the negative electrode. Therefore, the presence of harmful impurities causes the active materials of the positive and negative electrodes to be gradually consumed, resulting in a loss of battery capacity. This phenomenon is called self-discharge. (4) Life span. A battery's charge and discharge is called a cycle or cycle. The number of cycles that the battery can withstand before the battery reaches a certain specified capacity value under certain discharge conditions is called cycle life. For lithium-ion batteries, as the battery life changes, the battery capacity also changes.

　　2.2 Common methods for estimating remaining capacity

　　Introduction Commonly used methods in remaining capacity estimation generally include:

　　1. Open circuit voltage measurement method.

　　Utilize the corresponding relationship between the battery's open circuit voltage and the battery's discharge depth, and estimate the SOC by measuring the battery's open circuit voltage. For lithium-ion batteries, the open circuit voltage has a certain proportional relationship with its SOC. This method can be used to obtain the battery more directly. The SOC and open circuit voltage method is relatively simple, but the open circuit voltage cannot be detected during charging and discharging, so this method cannot be used to dynamically estimate the SOC of the battery. 2. Electricity accumulation method (ampere-hour integration method).

　　The intuitive expression of the power accumulation method is: the remaining power of the battery = (total power) - (discharged power). Here, we do not need to study the relatively complex electrochemical reactions and the relationship between the internal parameters of the battery. Instead, we regard the battery as a closed system and only focus on the external characteristics of the system. In this way, in the power monitoring, the power entering and exiting the closed system of the battery is accumulated, and the SOC of the battery is estimated by accumulating the power of the battery during charging or discharging. At the same time, the SOC is compensated according to the temperature and discharge rate of the battery. Discharge capacity and discharge current are closely related. When the battery is discharged to the cut-off voltage with a certain current, it does not mean that the remaining capacity has been zero. At this time, if a smaller current is used to continue discharging, a part of the battery can still be discharged. of electricity, so the size of the discharge current must be given while providing a prediction of the remaining capacity. The power accumulation method (ampere-hour integration method) uses the integration method to calculate the power charged into the battery and discharged from the battery in real time, and records and monitors the battery power for a long time, so that it can be compared with the full power at any time, so as to The remaining power corresponding to that moment can be obtained. This method is relatively simple to implement, is less restricted by the battery itself, and is suitable for taking advantage of microcomputer monitoring. However, the SOC estimate obtained using the ampere-hour method will have an error that becomes larger and larger as time goes by. 3.Measuring internal resistance method.

　　This method was proposed by Japan's CHUGOKU Electricpower Co. Inc. for the detection of hybrid electric vehicle battery state of charge SOC. This method uses alternating current of different frequencies to excite the battery and measure the AC resistance inside the battery. Then, the SOC estimate is obtained through the established calculation model. It should be noted that the battery state of charge obtained by this method reflects the SOC value of the battery under a specific constant current discharge condition. This method is difficult to implement because the working conditions of the battery have a great impact on the internal resistance of the battery. The calculation of the internal resistance needs to consider the magnitude of the electromotive force, terminal voltage, and discharge current value, which is difficult to model using traditional mathematical methods. Therefore, this method is rarely used in battery management systems to determine the state of charge of the battery. 4. Establish a mathematical model of the battery.

　　The main method is to obtain battery data (voltage and current of the entire battery pack) through experiments, establish a multi-input-single-output linear model of the battery, and obtain the dynamic model parameters of the battery through system identification methods, and use this experimental modeling The research results are used to discuss the correction method for battery SOC estimation. 5. Fuzzy inference and neural network methods. Fuzzy logic reasoning and neural networks are two branches in the field of artificial intelligence. Their common feature is that they both adopt parallel processing structures and are model-free predictors. They can obtain the input-output relationship of the system from the input and output samples of the system. In the prediction of the remaining battery capacity, considering that there are many factors that affect the battery state and the system model is difficult to establish, using fuzzy logic reasoning and neural network methods to determine the battery's state of charge has always been a hot research topic. These complex algorithms are difficult to implement on single-chip microcomputer systems, so they are still rare in practical applications, but this is the direction of future development. 2.3 The solution adopted by this battery management system

　　From the above analysis, it can be seen that there are many methods for estimating battery capacity, and different SOC estimation methods have their own characteristics. Through experimental analysis, this paper obtains an empirical model suitable for lithium-ion batteries, using a combination of ampere-hour integration method and open circuit voltage measurement method. plan. The ampere-hour integral method is used to calculate the remaining capacity of the 18650 battery pack 12v in the working state. The basic idea is to equate the discharge capacity under different currents to the discharge capacity under a specific current, and then determine the SOC based on the remaining capacity. The equivalent discharge capacity formula is as follows: where: t: charge and discharge time;

　　λ: coefficient of different charge and discharge;

　　i: charge and discharge current. In the formula: 0C is the capacity of the battery when it is discharged at a calibrated constant current. In the ampere-hour integration method, due to problems with battery self-discharge and charge-discharge efficiency, errors continue to accumulate and the SOC estimate may eventually deviate seriously from the actual value. A fundamental solution has not yet been found. In addition to recording battery capacity changes using the ampere-hour integration method. Through experiments, each time the system is powered on, the current SOC is corrected to a certain extent based on the battery power-off time and the open circuit voltage when powered on, that is, compensation. Compensation for batteries requires some prior knowledge of the batteries they manage. Although this approach is more complicated. But it is considered to be a more effective approach in actual battery energy management systems. The terminal voltage of lithium-ion batteries changes greatly during the charge and discharge process. Therefore, we cannot use the terminal voltage to estimate the remaining capacity of the battery during operation. However, when the battery is powered off (that is, after it is stationary), its terminal voltage will gradually become stable over time. At this time, the relationship between the terminal voltage and its capacity is relatively clear. The longer the battery is parked, the better the terminal voltage is indicative of its internal capacity. Based on this, the parking time t of the battery can be considered as a parameter. The capacity 0SOC before the battery is parked and the capacity SOC1 represented by its terminal voltage after the battery is stabilized are weighted to a certain extent, as follows: where: T: battery terminal voltage is stable the time required;

　　t: battery parking time (i.e. the time between two uses);

　　SOC0: remaining capacity of the battery before parking;

　　SOC1: The remaining capacity of the battery's terminal voltage mark when the battery is stable. Theoretically, T is an infinite time. However, in practical applications, it is generally believed that the terminal voltage is stable when the change rate of the terminal voltage is less than a certain value. After the battery is stabilized, the capacity 1SOC represented by its terminal voltage can be determined by the battery parameters provided by the battery supplier. The 18650 battery pack 12v managed by this battery management system is the 90AH power 18650 battery pack 12v of Leitian Company. The parameters are as shown in the figure below:

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