Lithium-ion battery consists of positive and negative electrodes, electrolyte, and separator, and is the basic structural unit that makes up the battery module and the battery pack. As an electrochemical power source, the battery has natural parameters such as voltage, internal resistance, capacity, energy and power. In the power lithium battery system, each parameter can characterize the different performance of the system. This article lists the various parameters of the lithium battery. Lithium battery cell Lithium-ion battery consists of positive and negative electrodes, electrolyte, and separator, and is the basic structural unit that makes up the battery module and the battery pack. As an electrochemical power source, the battery has natural parameters such as voltage, internal resistance, capacity, energy and power. People mainly want to measure and evaluate the battery parameters for two purposes. One is to achieve the purpose of active control, for example, the battery cell voltage is inconsistent, so that the system energy storage capacity is reduced, if you can actively adjust the voltage of the two poles, you can play a role in amplifying the system capacity. The other is that for safety reasons, the parameters of the battery have a fixed range, the battery parameters are detected, and the monitoring of its boundaries can be used to characterize the battery safety status. Voltage The monomer voltage mainly depends on the type of the positive and negative electrode materials. Generally, the lithium cobalt oxide and the ternary positive electrode with the graphite negative electrode can obtain a full charge voltage of about 4.2V, and the lithium iron phosphate can only reach 3.6V. The voltage here, precisely, should be that the potential depends on the properties of the material, and the potential is numerically equal to the open-circuit voltage of the battery after standing for a sufficiently long time. The single-ended voltage in the closed loop is the voltage value that we detected with an external instrument. Its value is equal to the battery potential minus the internal resistance of the battery. The internal resistance of the battery is not constant and will be affected by a variety of factors, which are described in the following section. Continuing with the voltage, the voltage of the cell, other than the material, will change with the change of the charge, and it is a one-to-one relationship. Therefore, in many cases, the battery charge (SOC) that cannot be directly measured simply is often Use the open circuit voltage of the battery to estimate. The cell voltage is related to the activity of the active material inside the battery, so the temperature that affects the activity can also affect the cell voltage in a small range. The higher the cell voltage, the more energy the cell with the same capacity contains. Therefore, under the premise of ensuring safety, increasing the cell voltage upper limit is a technical route to increase the system energy density. Internal resistance Inside the lithium battery, lithium ions move from one pole to another, and the factors that hinder ion movement in the process together constitute the internal resistance of the lithium battery. Its main parts include the physical resistance of the conductive parts; the inherent resistance of electrochemical materials such as motor materials, separators and electrolytes; and the impediments to the movement of lithium ions that are temporarily increased inside the battery by the presence of current; these three parts together constitute the The main body of the resistance. The internal resistance is the most sensitive to temperature, and the internal resistance can vary greatly at different temperatures. One of the important reasons for the drop in battery performance at low temperatures is due to excessive battery internal resistance at low temperatures. As a power source for lithium batteries, from the outside, the internal resistance must be as small as possible. Especially in the case of power applications, small internal resistance is a necessary condition. capacity Lithium battery capacity, measurable capacity, is the maximum amount of power that can be charged and discharged within the battery's reasonable maximum and minimum voltage range. The capacity of the cell can be measured by charging and discharging before being mounted on the vehicle. Once on the train, battery capacity can only rely on algorithmic estimates. In the battery management system BMS, accurately estimating the battery state of charge SOC is an important indicator of its design level. It is well known that the current practice is to integrate the loop current at the time of dynamic operation, and to check the battery power with the battery open circuit voltage in the non-operating state. Although there are many other methods, but it is not stable or the amount of calculation is too large, it is rare to really be applied to batches. The capacity of the monomer is obviously affected by the degree of aging. As we all know, the capacity decay to a limit value is the time when the battery is eliminated. It can be seen that the two have absolute relevance. Secondly, the capacity also receives the influence of temperature. At low temperatures, the activity of the active material decreases, the amount of available ions decreases, and the capacity necessarily decreases. power Here, the power, precisely, should be the specific power, which is the charge/discharge power capability of a single cell or the charge/discharge power capacity of a unit cell or a unit cell. Lithium batteries are capable of high-power charge and discharge, which was decided when it was designed. The same is lithium iron phosphate material or ternary material, the use of process means, change the electrode thickness or add additives, adjust the structure of the active material, electrolyte properties, electrode SEI film properties, can play a role in adjusting the battery power performance. In general, power performance and energy density often cannot co-exist, and the same material, the pursuit of high power, will partially sacrifice energy density. Once the cell is produced, its reasonable maximum charge and discharge current has been determined. In addition to adjusting the heat dissipation condition of the battery, it can change its maximum charge and discharge power in a small range, and there is almost no room for further adjustment. In addition to specific power, there are several parameters that are equivalent to the unit mass or volume, and the battery level can be more clearly seen. Specific capacity, specific energy The volumetric capacity is the capacity divided by the volume of the battery, and the mass ratio capacity is the capacity divided by the mass. Extend from here, the battery cost is converted to unit capacity, which is to talk about the price from the battery charge and discharge capacity point of view: The battery capacity is calculated by the ratio of price to capacity, that is, the unit of the battery can discharge the amount of electricity. However, this method is less commonly used. Similarly, the mass-to-cell mass ratio energy is the energy divided by the mass, ie, the energy that can be emitted by a unit mass of the battery; the volume-specific energy, that is, the energy that the unit volume of the battery or the active material can emit; and the battery price is defined from the unit energy price. , is a more common way in the industry, talking about battery prices, is how much 1kWh. Lithium-ion battery module The battery module is formed by a series of parallel connection of a plurality of battery cells and is an element constituting the battery pack. The battery module is seldom evaluated as a subject in actual operation, and occasionally some systems will detect the voltage value. In fact, people often see the module as a large battery. The difference is that the module has a monomer consistency problem, and the internal cell voltage difference is the focus of the balanced function. The performance of a battery module is often restricted by the lowest-performance battery element that makes up the battery module, and is mainly embodied in the capacity index. When charging, the high-voltage cells are filled first; when discharging, the low-voltage cells are discharged first. And it is likely that these two batteries are not the same. Therefore, the consistency of the module parameters inside the module has a decisive influence on the module performance. Consistency is a parameter that needs to be taken into consideration when the module level is greater than the unit. At the beginning of module grouping, this parameter will be screened by various means to guarantee the guarantee of the cell; after the module is produced, consistency is an important indicator of its acceptance; in the course of operation, it can only rely on the balanced function of BMS. Guarantee. Lithium-ion battery system The battery pack is generally composed of modules in series. In addition to inheriting all the parameters of the module, the total voltage of the battery pack determines the voltage platform of the electric vehicle power system and is a very important parameter. There are also several indicators related to safety on the battery pack that are continuously monitored. The battery pack outputs positive and negative ground resistance, system leakage current, high voltage interlock signal, system maximum and minimum temperature, system maximum temperature difference, system maximum temperature rise rate, system maximum and minimum unit voltage and so on. The above are all based on the electrical performance. As a whole, the battery pack has many parameters that need continuous attention, as well as environmental tolerance performance and abuse tolerance performance, which are not listed here.
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