What are the factors influencing lithium battery SOH?

The aging of lithium battery is a long-term gradient process, the health status of the battery is affected by temperature, current magnus, cut-off voltage and other factors. At present, there have been some achievements in the research and modeling analysis of battery health status, and the relevant research includes the analysis of battery degeneration mechanism and aging factors, the health management of batteries, the monitoring and estimation of battery conditions, and the prediction of battery life.

However, there is still a lack of perfect induction and summary in the evaluation of the health status of lithium batteries. This paper systematically introduces the current situation and progress of battery health status from the definition, influencing factors, evaluation model, research difficulties and research significance.

Ⅰ. Battery health status definition

The battery SOH characterizes the current battery's ability to store power relative to the new battery, represents the state in percentage seamount situated by the battery from the beginning of life to the end of its life, and is used to quantify the performance state of the current battery. Battery performance indicators are more, domestic and foreign SOH has a variety of definitions, the concept of the lack of unity, the current definition of SOH is mainly reflected in capacity, power, internal resistance, cycle times and peak power and other aspects.

1, Capacity definition SOH

The most documented definition of SOH with battery capacity SOH attenuation is given as follows:

Caged is the current capacity of the battery and Crated is the rated capacity of the battery.

2, Power definition SOH

The definition of power consumption SOH is similar to the capacity definition, because the rated capacity of the battery has the actual effective capacity and the maximum capacity, the actual capacity of the battery and the nominal rated capacity is somewhat different, so there is literature to define SOH from the point of view of battery discharge.

Qaged-max is the maximum discharge power for the current battery, and Qnew-max is the maximum discharge for the new battery.

3, Internal resistance definition SOH

The internal resistance increase of battery is an important manifestation of battery aging, and also the cause of further battery aging, many literatures use internal resistance definition SOH.

REOL is the internal resistance at the end of battery life, Rc is the internal resistance of the current battery, and Rnew is the internal resistance of the new battery.

4, Number of remaining loops defines SOH

In addition to defining SOH with battery performance metrics such as capacity and internal resistance, there is also documented that the battery's SOH is defined with the number of cycles remaining on the battery.

Cntremain is the number of cycles remaining for the battery and Cnttota is the total number of cycles for the battery.

SOH definitions of these four types of batteries are common in the literature. Capacity and power definition is high operability, but capacity is the external performance of the battery, and the internal resistance and the remaining number of times defined operability is not strong, the internal resistance is related to SOC, temperature, not easy to measure, the number of remaining cycles and the total number of cycles can not be accurately predicted.

Ⅱ, Lithium battery health status factors

In recent years, there have been many literatures at home and abroad to study the mechanism and law of lithium battery aging, it is generally believed that lithium-ion deposition, SEI membrane thickening and loss of active substances are the main causes of battery aging and capacity decay. The abuse of lithium batteries will accelerate battery aging, the normal charging of batteries will also affect the health of the battery, accelerate battery aging.

1, The effect of temperature on battery SOH

Temperature is often considered to be a major factor affecting the health of the battery, temperature has a double effect on the performance of the battery, on the one hand, high temperature will speed up the chemical reaction inside the battery, improve the efficiency and performance of the battery, while the high temperature will also accelerate some irreversible chemical reactions, resulting in the battery's active substance reduction, causing the battery aging and capacity decay. Experimental data show that high temperature will accelerate the growth of the SEI membrane of the battery electrode, and the difficulty of lithium-ion penetration of the SEI membrane will increase, equivalent to the increase of intra-battery resistance.

2, Charge and discharge current magnifying effect on battery SOH

Charge-discharge multiples affect battery life, with three different discharge multiples for Sony 18650 battery 300 cycle experiments, its battery capacity decayed by 9.5%, 13.2% and 16.9%, respectively, the battery resistance increased by 12.4%, 18.3% and 27.7%, respectively. At the same time, high-multiplier discharge will generate more heat inside the battery, accelerate battery aging, electron microscope observed high-rate battery discharge electrode surface SEI membrane than low-rate discharge.

3, The impact of discharge depth on battery SOH

Battery charge and discharge depth has an impact on battery health and aging, there is a view that the battery has a cumulative total transfer energy, based on the total transfer energy for battery capacity decay and aging analysis. High-flying and so on through the lithium battery different discharge depth cycle test, analysis of the relationship between the cumulative transfer energy of the battery and the decay of battery capacity, to obtain the battery capacity decay to 85% before, the cumulative transfer of energy in the deep and shallow medium in the two modes are basically the same, when the battery capacity decay to 85% to 75%, The deep-filled mode of cumulative transfer energy and energy efficiency of the battery is better than the shallow-fill mode.

4, The effect of the cycle interval on the battery SOH

Battery charge and discharge cycle interval will also affect the battery aging process, the cycle interval is different from the corresponding charge and discharge battery resistance is not the same, so the cycle process battery heating and reaction slightly different, long-term will affect the health and aging of the battery. Therefore, some experts suggest that the battery SOC range of 20% to 80%, which is conducive to battery health and cycle life.

5, Charge and discharge cut-off voltage on the battery SOH effect

Overcharging and over-release of batteries have an impact on battery health, and inappropriate voltage limits and low voltage limits have an impact on the battery. The lower the discharge cut-off voltage, the greater the internal resistance of the battery, causing the battery internal heat, while causing an increase in side reactions, battery active substances reduced and negative graphite sheet layer collapse, battery aging and capacity decay. Excessive charging cut-off voltage causes the battery to increase, the internal heat of the battery increases, overcharging causes the negative pole to produce "lithium" phenomenon and the corresponding by-effects increase, affecting the capacity and aging of the battery.

In summary, the temperature, charge and discharge multiple, discharge depth, cycle interval and charge-discharge cut-off voltage of the battery will have an impact on the health status and life of the battery. At present, the research on the influence factors of battery health status is in the qualitative research stage, and the quantitative analysis of battery aging and the coupling of various factors are the difficulties of the research, and also the hot spot of future battery health and life span research.

Ⅲ. Lithium battery health assessment model

The health status of lithium battery can not be obtained by direct measurement, the health status of the battery can be obtained through model evaluation, the aging and health of the battery is affected by a variety of factors, the current health status evaluation model of lithium battery mainly has electrochemical model, equivalent circuit model and experience model three.

1, Electrochemical model

The electrochemical model analyzes the change of health state in the battery operation from the mechanism of the electrochemical reaction of the battery, and considers the effect of the aging factor son of the battery on the internal and external state variables (temperature, current magnus, cut-off voltage, etc.). The research on lithium battery electrochemical model sq.e. includes complex electrochemical models based on SEI mechanism model, electrochemical first principle model, single-factor and multifactor integrated electrochemical model.

2, Equivalent circuit model

The equivalent circuit model from the electrotechnical angle of the battery, combined with a large amount of state data analysis, the lithium battery is equivalent to a basic circuit model, the circuit model is used to evaluate the health status of the battery. The basic equivalent circuit model of lithium battery is Rint model, RC model and Thevenin model 3 kinds, PNGV model and GNL model are the model smodels that are improved on the basis of Thevenin equivalent circuit model.

3, Experience model

The empirical model obtains the change of battery performance state through a large number of experimental data analysis, fitting, trial, empirical formula and statistical processing, and summarizes the change law of the healthy state of the battery, mainly the battery impedance experience model and the battery capacity estimation experience model.

Ⅳ. Difficulties in the study of the health status of lithium batteries

The research on the health and life of lithium battery is more and more important, however, the SOH research of battery is still in the primary state, mainly for the following three reasons.

1, Long research cycle, strict control of experimental conditions

Lithium battery cycle is long, battery aging test cycle is very long, in the test process, temperature, charge and discharge current and charge and discharge cut-off voltage need to be strictly controlled, and every certain time need to evaluate the aging of the battery.

2, Battery internal state monitoring and analysis difficulties

The SOH study of lithium battery involves the internal state variables of the battery, such as the internal temperature of the battery, the concentration of electrolyte and the internal resistance in the electrochemical model, and the internal state of the battery is very difficult to monitor accurately, and the quantitative analysis of these state variables is also required, which makes the SOH study of the battery difficult to solve the problem.

3, All kinds of influence factors coupling

The temperature, charge and discharge multiple and discharge depth of battery operation are all factors that affect the aging and life of the battery, and these factors are co-effects, and the research 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 decoupling analysis at present.

V. The significance of the study on the health status of lithium batteries

Battery SOH research is difficult and slow, but SOH research is of great value to battery use, maintenance and evaluation, and can provide a basis and reference for planning, policy and industrial development.

1, The significance of battery management

Battery management system estimates the battery load state and the remaining power capacity is related to the battery capacity, if the battery management system can master the aging law and health status of the battery, will help it to do a good job in battery life cycle battery management.

2. Meaning for battery use and maintenance

SOH research is beneficial to grasp the factors of battery aging and provide theoretical guidance for battery use and maintenance. For battery use and maintenance, understanding the factors that affect battery aging can reduce high and low temperatures and overcharge, and overcharging can be used to damage battery use, and knowing the current health of the battery can help determine the inherent pitfalls and life of the battery and provide a reference for battery maintenance and replacement.

3, The significance of battery economy assessment

The accurate evaluation of SOH is of great significance to the economic evaluation of batteries, and the application scenarios, usage methods and maintenance methods of lithium batteries cause differences in battery life, so that the cost of battery use, economic benefits and other economic evaluation differences. Through SOH research to establish the aging model of batteries, to analyze the economy of batteries to provide data support, will provide effective support for enterprise investment decision-making, government policy-making and industrial development planning.