SOC and SOH are common contents of intelligent lithium-ion battery packs, and it can also be said that they are the key points in the software design of BMS. There are many articles about BMS on SES Power's website, which have descriptions for SOC and SOH. The reason why we know so much about the parameters of these intelligent lithium batteries is because our products, such as fully intelligent lead-acid replacement products (lithium iron phosphate 12V100Ah, 12V200Ah), UPS high voltage lithium battery system (up to 860V), 3Kw ~20Kw off-grid, grid-connected lithium battery energy storage system, base station communication backup battery system (standard 19 inches), intelligent electric forklift lithium battery, etc., all use highly intelligent BMS, the software inside is for SOC, SOH The requirements are very high.

　　Some customers don’t know much about smart lithium batteries, and often come to consult us about SOH content. The following is a senior engineer from SES Power to explain the SOH content for you:

　　A: What does SOH mean? Why SOH Monitoring for Lithium Batteries?

　　The whole process of SOH is Status of Healthy.

　　The aging of lithium batteries is a long-term gradual process, and the health of the battery is affected by various factors such as temperature, current rate, and cut-off voltage. The research of SOH includes battery degradation mechanism and aging factor analysis, battery health management, battery state monitoring and estimation, battery life prediction, etc.

　　The battery SOH characterizes the ability of the current battery to store electrical energy relative to the new battery, and represents the state of the battery from the beginning of its life to the end of its life in the form of a percentage. There are many definitions of SOH, and the concept lacks unity. At present, the definition of SOH is mainly reflected in several aspects such as capacity, electricity, internal resistance, number of cycles and peak power.

　　1 Capacity Definition SOH

　　There are most literatures on the definition of SOH by battery capacity decay, and the definition of SOH is given as follows:

　　In the formula: Caged is the current capacity of the battery; Crated is the rated capacity of the battery.

　　2 Electricity definition SOH

　　The definition of SOH for electricity consumption is similar to the definition of capacity. Because the rated capacity of the battery has the actual effective capacity and the maximum capacity, the actual capacity of the battery is somewhat different from the nominal rated capacity, so some literatures define SOH from the perspective of battery discharge capacity.

　　In the formula: Qaged-max is the maximum discharge capacity of the current battery; Qnew-max is the maximum discharge capacity of the new battery.

　　3 Internal resistance defines SOH

　　The increase of the internal resistance of the battery is an important manifestation of the aging of the battery, and it is also the reason for the further aging of the battery. Many literatures use the internal resistance to define the SOH.

　　In the formula: REOL is the internal resistance at the end of the battery life; Rc is the internal resistance of the current battery; Rnew is the internal resistance of the new battery.

　　4 The number of remaining cycles defines SOH

　　In addition to using battery performance indicators such as capacity and internal resistance to define the SOH, there are also literatures that define the SOH of the battery by the number of remaining cycles of the battery.

　　In the formula: Cntremain is the number of remaining cycles of the battery; Cnttota is the total number of cycles of the battery.

　　The SOH definitions of the above four types of batteries are relatively common in the literature. The definition of capacity and charge is highly operable, but the capacity is the external performance of the battery, while the definition of internal resistance and remaining times is not very operable. The internal resistance is related to SOC and temperature, and it is not easy to measure. The number of remaining cycles and the total number of cycles are not easy to measure. cannot be predicted accurately.

　　B. What are the factors that affect the SOH of lithium batteries?

　　It is generally believed in the industry that lithium ion deposition, SEI film thickening and active material loss are the main reasons for battery aging and capacity fading. Abuse of lithium batteries will accelerate battery aging, and normal charging and discharging of batteries will also affect battery health and accelerate battery aging.

　　1 Influence of temperature on battery SOH

　　Temperature is usually considered to be the main factor affecting the state of health of a battery. Temperature has a dual impact on the performance of the battery. On the one hand, high temperature will speed up the chemical reaction inside the battery, improving the efficiency and performance of the battery. At the same time, high temperature will also accelerate some irreversible chemical reactions. The reaction occurs, resulting in a decrease in the active material of the battery, causing aging and capacity decay of the battery. Experimental data show that high temperature will accelerate the growth of the SEI film of the battery electrode, and the difficulty of lithium ions penetrating the SEI film will increase, which is equivalent to an increase in the internal resistance of the battery.

　　2 Influence of current rate of charge and discharge process on battery SOH

　　The charging and discharging rate will affect the life of the battery. The Sony 18650 battery was tested for 300 cycles at three different discharge rates. and 27.7%, while high-rate discharge will generate more heat inside the battery, accelerating battery aging, and it is observed under electron microscope that the electrode surface SEI film of high-rate discharge is thicker than that of low-rate discharge.

　　3 Influence of depth of discharge on battery SOH

　　The depth of charge and discharge of the battery has an impact on the health and aging of the battery. Some people believe that the battery has accumulated total transfer energy, and the capacity attenuation and aging analysis of the battery are carried out based on the total transfer energy.

　　4 Influence of cycle interval on battery SOH

　　The battery charge-discharge cycle interval will also affect the battery aging process. The internal resistance of the charge-discharge battery is different in different cycle intervals. Therefore, the battery heat and reaction during the cycle are slightly different, which will affect the battery's health and aging in the long run. Therefore, some experts suggest that the battery SOC range is 20%~80%, which is beneficial to battery health and cycle life.

　　5 Influence of cut-off voltage of charge and discharge on battery SOH

　　Overcharge and overdischarge of the battery will affect the health of the battery, and inappropriate upper and lower voltage limits will affect the battery. The lower the discharge cut-off voltage, the greater the internal resistance of the battery, resulting in internal heating of the battery, increased side reactions, reduction of battery active materials and collapse of the negative graphite sheet, accelerated aging and capacity decay of the battery. Excessive charging cut-off voltage causes the internal resistance of the battery to increase, the internal heat of the battery increases, and the overcharge causes the “lithium precipitation” phenomenon of the negative electrode and the corresponding increase in side reactions, which affects the capacity and aging of the battery.

　　C. SOH model of lithium battery

　　The health status of lithium batteries cannot be obtained by direct measurement, and the battery health status can be obtained through model evaluation. However, the aging and health of batteries will be affected by various factors. At present, there are three main models for evaluating the state of health of lithium batteries: electrochemical model, equivalent circuit model and empirical model.

　　1 Electrochemical model

　　The electrochemical model analyzes the changes of the state of health of the battery during the operation process from the electrochemical reaction mechanism of the battery, and considers the influence of the aging factors of the battery on the internal and external state variables (temperature, current rate, cut-off voltage, etc.) of the battery. The research on electrochemical models of lithium batteries includes complex electrochemical models based on SEI mechanism models, electrochemical first-principles models, and single-factor and multi-factor comprehensive electrochemical models.

　　2 Equivalent circuit model

　　Equivalent circuit model from the perspective of battery electrical engineering, combined with a large number of state data analysis, the lithium battery is equivalent to a basic circuit model, and the circuit model is used to evaluate the state of health of the battery. There are three basic equivalent circuit models of lithium battery: Rint model, RC model and Thevenin model. PNGV model and GNL model are improved models based on Thevenin equivalent circuit model.

　　3 Empirical Models

　　The empirical model is a model recommended by SES Power. It obtains the change of battery performance state through a large number of experimental data analysis, fitting, trial and error, empirical formula and statistical processing, and finally summarizes the change rule of battery health state. It is very reasonable to use it in mass customized products, such as electric vehicles.

　　D. Difficulties in SOH research on lithium batteries

　　More and more attention is paid to the research on the health status and lifespan of lithium batteries. However, the research on the SOH of batteries is still in its infancy, mainly for the following three reasons.

　　1 The research period is long and the experimental conditions are strictly controlled

　　The cycle life of lithium batteries is long, and the aging test cycle of batteries is very long. During the test, the temperature, charge-discharge current, and charge-discharge cut-off voltage need to be strictly controlled, and the aging of the battery needs to be evaluated at regular intervals.

　　2 It is difficult to monitor and analyze the internal state of the battery

　　The SOH research of lithium batteries involves the internal state variables of the battery, such as the internal temperature, electrolyte concentration and internal resistance of the battery in the electrochemical model. It is very difficult to accurately monitor the internal state of the battery. It is also necessary to quantitatively analyze these state variables. This makes the SOH research of batteries difficult to solve.

　　3 Coupling of various influencing factors

　　The operating temperature, charge-discharge rate, and discharge depth of the battery are all factors that affect the aging and life of the battery, and these factors work together. The study of battery SOH requires the decoupling of various influencing factors. However, these factors are interrelated, the decoupling conditions are difficult to control, and it is difficult to perform decoupling analysis at present.

　　E: The significance of lithium battery SOH

　　Battery SOH research is a long-term project with high difficulty and slow progress, but SOH research is of great value to the use, maintenance and evaluation of batteries.

　　1 Significance of battery management

　　The battery management system mastering the aging law and health status of the battery will help it manage the battery in the full life cycle of the battery.

　　2 Significance of battery use and maintenance

　　It is beneficial to master the factors affecting battery aging and provide theoretical guidance for the use and maintenance of batteries. For the use and maintenance of batteries, understanding the factors affecting battery aging can reduce high and low temperature, overcharge and overdischarge, etc., which are harmful to battery use; knowing the current health status of the battery can help determine the inherent hidden dangers and life of the battery. Provides reference for battery maintenance and replacement.

　　3 Significance of battery economic evaluation

　　It is of great significance to the economic evaluation of batteries. Different application scenarios, usage methods and maintenance methods of lithium batteries cause differences in battery life, resulting in differences in economic evaluations such as battery use costs and economic benefits. The battery aging model is established through SOH research, which provides data support for analyzing the economics of batteries, and will provide effective assistance for corporate investment decisions, government policy formulation and industrial development planning.

　　SES Power has more than ten years of rich experience in the intelligentization of lithium battery systems. Our BMS has evolved from simple detection, calculation to cloud big data analysis, online system access, etc. The increase of these functions is inseparable from our engineers and the efforts of partners.