Electric vehicles, considered to be the cars of the future, are a combination of three major technologies: electric power source, motor and vehicle. The electric power source is the core component of electric vehicles. There has been a boom in the development of power lithium-ion batteries and their special materials. As a new type of power technology, lithium batteries must be connected in series to meet the required voltage. The uneven performance of individual cells is not entirely due to battery production technology problems. From the coating to the finished product, it has to go through There are multiple processes. Even if each process undergoes strict testing procedures to make the voltage, internal resistance, and capacity of each battery consistent, differences will occur after a period of use, making the technical issues of the use of lithium power batteries urgent and necessary. solve quickly. The service life of the power battery pack is affected by many factors. If the life of the battery pack is less than half of the average life of the battery pack, it can be inferred that it is due to improper use of technology. The primary reason is overcharging and over-discharging of the single battery. Early expiration. This article combines the characteristics of lithium power batteries, electronic power supplies, and computer control technology to study the use technology of power battery packs and explores the balance control and management of power battery packs. 1 Main performance parameters of power battery 1.1 Voltage Open circuit voltage = electromotive force + electrode overpotential, working voltage = open circuit voltage + voltage drop caused by current on the internal impedance of the battery. The electromotive force is determined by the material properties of the electrode and electrolyte. The overpotential of the electrode is related to the material activity, state of charge and working conditions. The standard electrode potential of metallic lithium is -3.05V, 3V lithium battery 3.3~2.3V, 4V lithium 4.2~3.7V, 5V lithium 4.9V~3.0V 1.2 internal resistance battery in a short time The steady-state model can be regarded as a voltage source, and its internal impedance is equivalent to the internal resistance of the voltage source. The size of the internal resistance determines the efficiency of the battery. The internal resistance of the battery includes two parts: ohmic resistance and polarization resistance. The ohmic resistance does not change with the frequency of the excitation signal. It is also called AC resistance. During the same charge and discharge cycle, the ohmic resistance changes very little except for the influence of temperature rise. Polarization resistance is caused by the resistance reaction of the battery's electrochemical characteristics to external charge and discharge, and is related to the battery charge, charge and discharge intensity, and material activity. Batteries of the same batch, if the internal resistance is too large or too small, it is abnormal. If the internal resistance is too small, it may mean the growth of dendrites and micro short circuits. If the internal resistance is too large, it may mean plate aging, loss of active materials, and capacity attenuation. Internal resistance The change can be used as one of the references for the adequacy of battery cracking. 1.3 Temperature rise Battery temperature rise is defined as the difference between the internal temperature of the battery and the ambient temperature. Most lithium batteries have an endothermic reaction when charging and an exothermic reaction when discharging, both of which include internal resistance heat consumption. In the early stage of charging, the polarization resistance is minimum, and the endothermic reaction is dominant. The battery temperature rise may be negative. In the later stage of charging, the impedance increases, the heat is released more than the heat is absorbed, and the temperature rise increases. When overcharging, irreversible reactions occur. , the gas escapes, the internal pressure and temperature rise, until it deforms and bursts. 1.4 Internal pressure The internal pressure of the battery. Due to the gas pressure escaping from the internal reaction of the battery, the air pressure increases. Excessive air pressure will burst the shell and cause explosion. Based on safety considerations, on the one hand, lithium batteries are designed with one-way explosion-proof valves. Plastic case manufacturing. The gas evolution reaction is often accompanied by an irreversible reaction, which means the loss of active materials and the decrease of battery capacity. No gas evolution and small temperature rise charge and discharge are the most ideal 1.5 electric power. In electricity, electric power is expressed in Wh, which is an energy unit. One kilowatt hour of electricity is equal to 1kWh. Batteries usually use Ah to calculate the power. For power batteries, the focus is on power and energy. It is more direct to use Wh, because the voltage of the battery changes, and the change in the whole process can reach about half of the maximum value. Ah is used to calculate the power. Calculating the power cannot accurately describe the power driving capability of the battery, but Ah as the battery power unit has its own history and reason. Both power units can be used where there is no ambiguity. 1.6 How much power is left in the charged battery, also known as the remaining power? The amount of electricity is often taken as the ratio to the rated capacity or actual capacity, and is called the degree of charge. It is the parameter data that people are most concerned about in use and is also the most difficult to obtain. People try to calculate the amount of charge by measuring internal resistance, changes in voltage and current, etc., and have done a lot of research work, but until now, no formulas and algorithms can be obtained Effectively supported by statistical data, the indicated degree of charge always changes non-linearly. The 1.7-capacity battery, after being fully charged, begins to discharge until it is discharged, and the maximum amount of electricity it can output. Capacity is related to the discharge current and the charge and discharge cut-off voltage, so the capacity is defined as the hourly rate capacity. Power batteries commonly use 1 hour rate (1C) or 2 hour rate (0.5C) capacity. Before the battery is formed, the activity of the material cannot be exerted normally and the capacity is very small. After the formation process begins, the battery enters its life cycle. During the entire life cycle, the activation and degradation processes of the battery are two aspects of the same problem. The initial activation effect is at In the dominant position, the battery capacity gradually increases. Later, the activation and degradation effects are not obvious or equivalent. In the later stage, the degradation effect is significant and the capacity attenuates. After the capacity attenuates to a certain proportion (60%), the battery life ends. 1.8 Power electrical definition The output power of a DC power supply is equal to the product of the output voltage and current. The lithium battery cell voltage is high. At the same output current, its power is 1.8 times and 3 times that of lead-acid and nickel-cadmium nickel-hydrogen respectively. . The load of the power battery pack for electric vehicles is the motor controller. The motor controller adjusts the output power according to changes in vehicle speed. In the short term, the battery pack drives a constant power load. The range of this power change is extremely large, and there are differences during braking. Similar reverse inverter power during acceleration. 1.9 Efficiency The efficiency of the battery refers to the charge and discharge efficiency or energy output efficiency of the battery. This article refers to the latter. For electric vehicles, driving range is one of the most important indicators. Under the premise of a certain battery pack capacity and output impedance, according to the law of conservation of energy, the energy output by the battery pack is converted into two parts, one part is lost to the resistor as heat dissipation , the other part is provided to the motor controller and converted into effective power. The ratio of the two parts of energy depends on the ratio of the output impedance of the battery pack and the equivalent input impedance of the motor controller. The smaller the impedance of the battery pack, the smaller the useless heat loss. , the output efficiency is greater. 1.10 Life The definition and test procedures of single battery life have been widely accepted and formed into many standards. When testing the life, it can be guaranteed that there will be no overcharge or over discharge, and it will not fail prematurely. Unlike the single battery, the life test of the battery pack is currently The approach is unscientific, which limits the practical application of power lithium batteries to a certain extent. The provider emphasized that the voltage of each battery should not exceed the specified limit. The life of the battery pack should be the smallest of the lifespans of each single cell. Its value should not be much different from the average life of the single cell. The tester simulated the use of the battery pack. In this case, use the same method for single cells to test the life. The voltage limit is the product of the single cell voltage limit and the quantity. The actual limit is the average cell voltage. The cell voltage in the group may be low or high. For dozens, For hundreds of battery packs, differences in voltage, capacity, and internal resistance always exist objectively. Overcharge and overdischarge cannot be avoided, and once the relevant batteries occur, they will be scrapped quickly. Therefore, there are electric vehicle power batteries tested by an expert group. The lifespan of the group has not exceeded a hundred times. 1.11 Safety The working conditions of power batteries are harsh. The main safety issues are the explosion, combustion and electric fire caused by the battery itself. During the development process of electric vehicles, many fire incidents have occurred, which have had a negative impact on the development of electric vehicles. Through We have learned from various sources that in these accidents, some batteries spontaneously ignited, some vehicles were burned, and fire brigade was even used to extinguish the fire. Many units were concerned about the impact and implemented confidentiality strategies. It was difficult to be present at the first scene of the incident. To summarize these incomplete reports According to the accident information, the following preliminary inferences are made: batteries that have been in stock for a long time have not experienced spontaneous combustion or explosion, nor have spontaneous combustion occurred during transportation; the battery explosion occurred in the late stage of charging or has ended, and the charging equipment and methods are difficult to isolate; the external circuit is short-circuited It can cause strong arcs or burn wires, or it can also cause spontaneous combustion. General voltage and current sources have this characteristic; overcharging and overdischarging of the battery cannot be avoided by using voltage or current limits; overcharging may cause the battery to deform, fail, and burn. , or even explode, and one over-discharge (reverse charge) is enough to cause the battery to be scrapped; some of the tested batteries passed harsh safety tests such as shooting with a submachine gun, extrusion, rupture, short circuit, water splash, and blisters. In short, the correct use of batteries is very important. 2 Charging and discharging characteristics of power battery packs Using single batteries as power sources, such as mobile phones, power management technology has been very perfect, but in battery packs, differences between cells always exist. Taking capacity as an example, the differences It never disappears, but gradually gets worse. The same current flows through the group. Relatively speaking, the one with a large capacity is always in a small current, shallow charge and shallow discharge, tending to slow capacity attenuation and extended life, while a small capacity one is always in a large current overcharge and overdischarge, tending to capacity attenuation. The battery life is accelerated and the lifespan is shortened. The difference in performance parameters between the two is getting larger and larger, forming a positive feedback characteristic. The small capacity will fail prematurely and the battery life will be shortened. The overcharge and overdischarge processes must be included in the analysis of the charge and discharge characteristics below. 2.1 Charging At present, lithium battery charging is mainly based on the voltage limiting and current limiting method. In the initial stage of constant current (CC) charging, the battery has the strongest acceptance capacity and is mainly an endothermic reaction. However, when the temperature is too low, the material activity decreases and may enter the constant current stage in advance. , so when the temperature is low in northern winter, preheating the battery before charging can improve the charging effect. As the charging process continues, polarization intensifies, the temperature rise intensifies, and along with gas evolution, the electrode overpotential increases and the voltage rises. When the charge reaches about 70~80%, the voltage reaches the maximum charging limit voltage and switches to constant voltage. (CV) stage. Theoretically, there is no objective overcharge voltage threshold. If it is understood that gas evolution and temperature rise mean overcharging, overcharging to varying degrees will always occur at the end of the constant current stage, and the temperature rise reaches 40~50 degrees Celsius. It becomes easier to sense, some of the escaped gases can be recombined, and some of them lose capacity as a result of irreversible reactions. This can be regarded as the current intensity exceeding the battery's ability to accept. In the constant voltage stage, known as trickle charging, it takes about 30% of the time to charge 10% of the electricity, the current intensity decreases, gas evolution and temperature rise no longer increase, and change in the opposite direction. 2.2 Overcharging The above process considers the total voltage or average voltage control of the battery pack. In fact, there are always cells with higher voltages than other batteries in the pack that have entered the overcharge stage. During overcharging, if it occurs during the constant current stage, due to the large current intensity, the voltage, temperature rise, and internal pressure will continue to rise. Taking 4V lithium as an example, when the voltage reaches 4.5V, the temperature rises by 40 degrees and the plastic case becomes hard. , the temperature rise can reach 60 degrees at 4.6V, and the shell deforms obviously and cannot be recovered. If it continues to be overcharged, the air valve will open, the temperature rise will continue to rise, and the irreversible reaction will intensify. In the constant voltage stage, the current intensity is smaller, and the overcharge symptoms are not as obvious as in the constant current stage. As long as the temperature rises and the internal pressure is too high, there will be side reactions, and the battery capacity will decrease. Side reactions have inertia and develop to a certain extent, which may cause the internal materials of the battery to burn during charging or in a short period of time after charging. , causing the battery to be scrapped. Overcharging accelerates the decline of battery capacity and causes battery failure, which is harmful to all. 2.3 Discharge During constant current discharge, the voltage drops suddenly, which is mainly caused by ohmic resistance. This resistance includes the wire resistance and contact resistance connecting the single electrode. The voltage continues to drop, and after a period of time, it reaches a new electrochemical equilibrium. , entering the discharge plateau period, the voltage change is not obvious, and the exothermic reaction plus resistance heat release causes the battery temperature to rise higher. The discharge voltage curve is similar to the single cell discharge curve. When discharge continues, the voltage curve enters the horsetail descending stage, the polarization impedance increases, the output efficiency decreases, the heat consumption increases, and the discharge stops when it is close to the termination voltage. The above process uses constant current characteristics to simulate the load motor. When the actual car is driving, the changes in the motor output power are very complex, and the current changes in bipolarity. Even when driving at a constant speed, road bumps and small turns will cause the output power to change in real time. In a short period of time, Here, constant current discharge simulation analysis can be used. In short, the general direction is discharge, and occasionally irregular zero pulses (without inverter function) or negative pulses (with inverter function, the battery is being charged) appear. 2.4 Over-discharge Considering the single cells in the group, there must be relative over-discharge. In the later stage of discharge, the voltage is close to the horsetail curve, the cell capacity in the group is normally distributed, and the voltage distribution is very complex. The voltage of the cell with the smallest capacity drops earliest and fastest. If the voltage drop of other cells is not obvious at this time, the small cell will The voltage drop of the capacity cell is covered up and it has been over-discharged. Observe the over-discharge of the cell. After entering the horsetail curve, if the current continues to be large, the voltage decreases rapidly and reverses quickly. At this time, the battery is charged in the opposite direction, or It is called passive discharge. The structure of the active material is destroyed and another side reaction occurs quickly. After a period of time, the active material of the battery is almost completely lost, which is equivalent to a passive resistor. The voltage is negative, which is numerically equal to the reverse charging current. The voltage drop generated on the equivalent resistance will cause the electromotive force of the original battery to disappear after the discharge is stopped, and the voltage cannot be restored. Therefore, one reverse charge is enough to cause the battery to be scrapped. Over-discharge of the cells in the group is easy to occur and difficult to control. The voltage and current limiting methods of the motor controller are ineffective. The ohmic and polarization voltage fluctuations caused by changes in the battery output power are enough to drown out the cell voltage drop signal, and the group voltage monitoring Lost the purpose. 2.5 Economic Speed and Driving Range Traditional cars consume the most fuel when driving at economic speed, which is evaluated by fuel consumption per 100 kilometers. Economic speed is determined by engine efficiency, power transmission efficiency and friction. Electric vehicles also have economic speed, which is determined by battery usage efficiency. , the efficiency of the motor and controller, and frictional resistance are determined. The economic speed is directly related to the internal resistance of the battery pack and changes within a certain range. Driving at economical speeds, electric vehicles can achieve their maximum driving range. Fixing the vehicle and electric motor, the driving range can examine the energy supply capability of the power battery pack. The economic speed reflects the power supply capability of the battery pack. Electric vehicles hope that the power battery pack can provide large capacity and high power.
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