AA rechargeable battery

　　AA rechargeable battery technology development process and performance parameter analysis

　　Decompose according to the customer's needs and carry out step-by-step decomposition design. Each process will eventually be converted into file input and output. The product design process is explained in detail below.

　　GB/T34013-2017 "AA rechargeable battery product specifications and dimensions for electric vehicles"

　　Because the aluminum-plastic film is light in weight and has high internal space utilization, soft-pack batteries are suitable for the development of batteries with larger energy density. However, metal casings are limited by internal space and generally have slightly lower energy density than soft-pack batteries.

　　Safety: Because aluminum-shell batteries are protected by metal casings, their safety is higher, while soft-package batteries can only rely on the performance of the material itself to pass the safety test, which currently seems to be more difficult.

　　In terms of process difficulty, because soft-pack batteries have many small pole pieces, they require high die-cutting equipment and are prone to large self-discharge and local micro-short circuits. At the same time, due to internal space limitations, there is less free electrolyte, and the cycle performance may be slightly lower. Difference.

　　Rolled batteries are relatively better, with some margin, and are easy to realize automated production. In terms of cost, because rolled batteries require high welding of the shell, the cost is slightly higher, while soft-pack batteries do not involve lasers. Welding focuses on packaging and equipment investment is low.

　　Calculate the number of positive and negative electrodes and separator layers of the battery core based on the internal space of the battery. According to the development status of the industry, the relevant parameters of the material are all experimentally verified based on past testing experience. It is necessary to verify the compaction density and material performance. The performance of auxiliary materials (including verification of SBR, CMC, PVDF, conductive agents, etc.) is based on the development of platform models. The development of the final process also needs to be matched with the materials to derive the final control plan and process flow chart.

　　In order to shorten the time, manufacturers now combine experimental verification and process development, but the risks are often relatively high. After all, the material system itself develops with the development of technology.

　　Each performance of each material has relevant testing standards. Certain performance indicators of the positive and negative electrodes are directly related to the performance indicators of the battery. However, there is currently no suitable model for forward electrochemical performance simulation. It is only based on existing Empirical data for patching.

　　Voltage(V)

　　Open circuit voltage, as the name suggests, means that the battery is not connected to any external load or power supply, and the potential difference between the positive and negative electrodes of the battery is measured, which is the open circuit voltage of the battery. The operating voltage corresponds to the open circuit voltage, that is, when the battery is connected to an external load or power supply, current flows through the battery, and the potential difference between the positive and negative electrodes is measured.

　　Due to the existence of the internal resistance of the battery, the operating voltage is lower than the open circuit voltage when discharging (external load), and the operating voltage is higher than the open circuit voltage when charging (external power supply).

　　Battery capacity (Ah)

　　The charge Q that can be accommodated or released, Q=It, that is, battery capacity (Ah) = current (A) x discharge time (h), the unit is generally Ah (ampere hour) or mAh (milliamp hour).

　　For example, if the battery in the car is marked 16Ah, then when the working current is 1A, it can theoretically be used for 16 hours.

　　Battery energy (Wh)

　　The energy stored in the battery is measured in Wh (watt hours). Energy (Wh) = voltage (V) × battery capacity (Ah).

　　After reviewing high school knowledge, let’s have some useful information...

　　Energy density (Wh/L&Wh/kg)

　　The amount of energy released by a battery per unit volume or unit mass.

　　If it is unit volume, it is volume energy density (Wh/L), which is directly referred to as energy density in many places;

　　If it is unit mass, it is the mass energy density (Wh/kg), which is also called specific energy in many places.

　　For example, if a lithium battery weighs 300g, has a rated voltage of 3.7V, and a capacity of 10Ah, its specific energy is 123Wh/kg.

　　According to the "Energy Saving and New Energy Vehicle Technology Roadmap" released in 2016, we can have a rough idea of the development trend of power batteries. As shown in the figure below, by 2020, the specific energy of a pure electric vehicle battery cell will reach 350Wh/kg

　　Power density (W/L&W/kg)

　　Divide energy by time and you get power, measured in W or kW. In the same way, power density refers to the power output of the battery per unit mass (also called specific power directly in some places) or unit volume, and the unit is W/kg or W/L.

　　Specific power is an important indicator to evaluate whether the battery meets the acceleration performance of electric vehicles.

　　What is the difference between specific energy and specific power?

　　Give a vivid example:

　　Power batteries with high specific energy are like the tortoise in the tortoise and the hare race. They have good endurance and can work for a long time, ensuring a long driving range of the car;

　　Power batteries with high specific power are like the hare in the tortoise and the hare race. They are fast and can provide high instantaneous current to ensure good acceleration performance of the car;

　　The following parameters are a bit convoluted...

　　Battery discharge rate (C)

　　The discharge rate refers to the current value required to discharge its rated capacity (Q) within a specified time, which is numerically equal to a multiple of the battery's rated capacity. That is: charge and discharge current (A) / rated capacity (Ah), the unit is generally C (abbreviation for C-rate), such as 0.5C, 1C, 5C, etc.

　　For example, for a battery with a capacity of 24Ah:

　　When discharging at 48A, the discharge rate is 2C. Conversely, when discharging at 2C, the discharge current is 48A and the discharge is completed in 0.5 hours;

　　When charging at 12A, the charging rate is 0.5C. Conversely, when charging at 0.5C, the charging current is 12A and the charging is completed in 2 hours;

　　The charge and discharge rate of a battery determines how quickly we can store a certain amount of energy into the battery, or how quickly we can release the energy in the battery.

　　State of charge (%)

　　SOC, the full name is State of Charge, state of charge, also called remaining capacity, represents the ratio of the remaining capacity of the battery after discharge to its capacity in the fully charged state.

　　Its value range is 0~1. When SOC=0, it means the battery is completely discharged. When SOC=1, it means the battery is fully charged. The battery management system (BMS) mainly ensures the efficient operation of the battery by managing SOC and making estimates, so it is the core of battery management.

　　At present, SOC estimation mainly includes open circuit voltage method, ampere-hour measurement method, artificial neural network method, Kalman filter method, etc. We will explain it in detail later.

　　internal resistance

　　Internal resistance refers to the resistance to current flowing through the interior of the battery when the battery is working. Including ohmic internal resistance and polarization internal resistance, among which: ohmic internal resistance includes the resistance of electrode materials, electrolytes, diaphragm resistors and various parts; polarization internal resistance includes electrochemical polarization resistance and concentration polarization resistance.

　　Let the data speak. The figure below shows a battery discharge curve. The X-axis represents the discharge amount and the Y-axis represents the battery open circuit voltage. The ideal discharge state of the battery is the black curve, and the red curve is the true state when the internal resistance of the battery is taken into account.

　　The unit of internal resistance is generally milliohms (mΩ). Batteries with large internal resistance consume large internal power and generate serious heat during charging and discharging, which will cause accelerated aging and lifespan of the battery, and will also limit high-rate charging. discharge applications. Therefore, the smaller the internal resistance is, the better the battery life and rate performance will be. Usually, the measurement methods of battery internal resistance include AC and DC testing methods.

　　self discharge

　　Battery self-discharge refers to the phenomenon of voltage drop during open circuit resting, also known as the battery's charge retention capability. Generally speaking, battery self-discharge is mainly affected by manufacturing processes, materials, and storage conditions. Self-discharge is divided into two types according to whether the capacity loss is reversible: reversible capacity loss, which means that the capacity can be restored after recharging; irreversible capacity loss, which means that the capacity cannot be restored.

　　At present, there are many theories on the causes of battery self-discharge, which can be summarized as physical reasons (storage environment, manufacturing process, materials, etc.) and chemical reasons (instability of electrodes in the electrolyte, internal chemical reactions, and consumption of active materials). etc.), battery self-discharge will directly reduce battery capacity and storage performance.

　　life

　　The life of the battery is divided into two parameters: cycle life and calendar life. Cycle life refers to the number of times a battery can be charged and discharged. That is, under ideal temperature and humidity, charge and discharge at the rated charge and discharge current, and calculate the number of cycles experienced when the battery capacity decays to 80%. Calendar life refers to the time span in which a battery reaches end-of-life conditions (capacity decays to 80%) under specific usage conditions under environmental conditions. Calendar life is closely combined with specific usage requirements, and it is usually necessary to stipulate specific usage conditions, environmental conditions, storage intervals, etc.

　　Cycle life is a theoretical parameter, while calendar life has more practical significance. However, the calculation of calendar life is complicated and time-consuming, so generally battery manufacturers only provide cycle life data.

　　The picture above shows the charge and discharge characteristics of a ternary lithium battery. It can be seen that different charging and discharging methods have different effects on the life of the battery. As shown in the data above, the life of charging and discharging at 25%-75% can reach 2500 times. That is what we call shallow charging and shallow discharge of the battery. We will discuss the topic of battery life in depth later.

　　Battery Pack Consistency

　　This parameter is quite interesting. Even if the battery cells of the same specifications and models are grouped together, the performance of the battery packs such as voltage, capacity, internal resistance, and lifespan are very different. When used in electric vehicles, the performance indicators often fall short of the standard. to the original level of a single battery.

　　The most reasonable explanation at present:

　　After the single cells are manufactured, due to process problems, the internal structure and materials are not completely consistent, and there are certain performance differences. The initial inconsistency accumulates with the continuous charge and discharge cycles of the battery during use. In addition, the usage environment in the battery pack is also different for each single cell, resulting in greater differences in the status of each single cell. It is gradually amplified during use, which in some cases accelerates the degradation of the performance of some single cells and ultimately causes premature failure of the battery pack.

　　It should be pointed out that the performance of the AA rechargeable battery pack is determined by the performance of the battery cells, but it is by no means a simple accumulation of the performance of the single cells. Due to the inconsistent performance of single cells, various problems will occur when the AA rechargeable battery pack is used repeatedly in electric vehicles, resulting in shortened lifespan.

　　In addition to the requirement to strictly control the process and maintain the consistency of single cells during the production and assembly process, the industry currently generally uses battery management systems with balancing functions to control the consistency of cells in the battery pack to extend the product life. service life.

　　form

　　Let’s talk about the last parameter, which is mainly related to the battery manufacturing process.

　　After the battery is made, it is necessary to charge the battery core with a small current to activate the positive and negative electrode materials inside it and form a passivation layer - SEI (solid electrolyte interface) film on the surface of the negative electrode to make the battery performance more stable. After the battery is formed, In order to reflect its true performance, this process is called formation.

　　The sorting process during the formation process can improve the consistency of the battery pack and improve the performance of the final battery pack. The formation capacity is an important indicator for screening qualified batteries.

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