Lithium Battery Grouping Inconsistency and Optimization Solution (Ⅰ)

The inconsistency of lithium-ion battery will affect the service life of the battery pack and reduce the performance of the battery pack. The inconsistency of lithium battery group refers to the difference of capacity, voltage, internal resistance, self-discharge rate and other parameters of single battery cell, which is caused by the different combination structure, operating conditions, operating environment and battery management of battery pack.

The cycle life of single cell will be reduced after being grouped. In order to improve the overall performance of the battery pack and obtain a longer service life, we should pay attention to the matching consistency of the single cell, provide appropriate working conditions and adopt appropriate thermal management measures for timely repair and maintenance. Based on the analysis of the causes of inconsistency of lithium-ion batteries, the improvement measures and optimization methods of battery inconsistency are proposed.

Ⅰ. Inconsistency Mechanism
1. Parameter Difference Between Single Cells

The state difference between single cells mainly includes the initial difference and the parameter difference in the process of using. There are many uncontrollable factors in the process of battery design, manufacture, storage and use, which will affect the consistency of battery cell. Improving the consistency of single battery cell is the prerequisite to improve the performance of battery pack. The current parameter state is affected by initial state and time accumulation.

Battery Capacity, Voltage and Self discharge rate

Different battery capacity will lead to different discharge depth of each cell. The battery cell with small capacity and poor performance will reach the full charge state ahead of time, resulting in the battery cell with large capacity and good performance cannot reach the full charge state. The inconsistency of battery cell voltage will lead to the mutual charging of single battery cell in parallel battery pack. The battery cell with higher voltage will charge the battery cell with lower voltage, which will accelerate the decline of battery performance and consume the energy of the whole battery pack. The capacity loss of batteries with high self-discharge rate is large. The inconsistency of self discharge rate will lead to the difference of battery state of charge and voltage, and affect the performance of batteries.

Battery Internal Resistance

In series system, the difference of internal resistance of battery cell will lead to different charging voltage of each cell. The cell with large internal resistance will reach the upper limit of voltage in advance, and other cell may not be fully charged at this time. The cell with large internal resistance has large energy loss and high heat. The temperature difference further increases the internal resistance difference, leading to a vicious circle.

In the parallel system, the difference of internal resistance will lead to the inconsistency of the current of each battery. The voltage of the cell with large current changes quickly, which makes the charge and discharge depth of each single battery cell inconsistent, and makes the actual capacity of the system difficult to reach the design value. The performance of batteries will be different with different working current, which will eventually affect the life of the whole battery pack.

Ⅱ.Charging and Discharging Conditions

Charging methods affect the charging efficiency and charging state of lithium battery pack. Overcharge and over discharge will damage the battery, and the battery pack will show inconsistency after multiple charging and discharging. At present, there are several ways to charge lithium-ion batteries, but the common ways are constant current charging and constant current and constant voltage charging. Constant current charging is an ideal way for safe and effective full charging; Constant current and constant voltage charging effectively combines the advantages of constant current charging and constant voltage charging, solves the problem that the general constant current charging method is difficult to accurately charge, avoids the impact of excessive current on the battery in the initial charging stage of constant voltage charging method, and has simple and convenient operation.

.Temperature

The performance of Li ion battery will decrease obviously at high temperature and high discharge rate. This is because the decomposition of positive active material and electrolyte will be caused when lithium-ion battery is used at high temperature and high current, which is an exothermic process. The short-term release of heat can lead to the further increase of battery temperature, which accelerates the decomposition phenomenon, forming a vicious circle, accelerating the decomposition and further reducing the battery performance. Therefore, if the thermal management of the battery pack is improper, it will bring irreversible performance degradation.

The difference of battery pack design and use environment will cause the temperature environment of battery cell is inconsistent. According to Arrhenius law, the electrochemical reaction rate constant of the battery is exponential with the degree of oxidation, and the electrochemical characteristics of the battery are different at different temperatures. Temperature will affect the operation, coulomb efficiency, charge and discharge capacity, output power, capacity, reliability and cycle life of electrochemical system. At present, the main work is to carry out the quantitative research on the influence of temperature on the inconsistency of battery pack.

.External circuit of battery

Battery External Connection Mode

The impedance of the connecting piece will also affect the inconsistency of the battery pack. The resistance of the connecting piece is not the same. The resistance of the branch from the pole to each cell is different. The battery far away from the pole has a larger resistance and a smaller current because of the longer connecting piece. The connecting piece will make the cell connected with the pole reach the cut-off voltage first, which will reduce the energy utilization and affect the battery performance, Moreover, the premature aging of the single cell will lead to overcharge of the battery connected with it, causing potential safety.

With the increase of battery cycles, the ohmic internal resistance will increase, the capacity will decay, and the ratio of ohmic internal resistance to connector resistance will change. In order to ensure the safety of the system, the influence of the resistance value of the connector must be considered.

BMS Input Circuit

Battery management system (BMS) is the guarantee of the normal operation of the battery pack, but the BMS input circuit will have a negative impact on the consistency of the battery. Battery voltage monitoring methods include precision resistor voltage divider, integrated chip sampling and so on. Due to the existence of resistance and circuit board path, these methods cannot avoid the leakage current of sampling line. The input impedance of voltage sampling in battery management system will increase the inconsistency of battery state of charge (SOC) and affect the performance of battery pack.

Ⅴ.SOC Calculation Deviation

The reason of SOC inconsistency is that the initial nominal capacity of single cell is inconsistent and the attenuation rate of nominal capacity of single cell is inconsistent. For parallel circuit, the difference of internal resistance of single cell will lead to uneven current distribution, and then lead to SOC inconsistency. SOC algorithm includes ampere hour integration method, open circuit voltage method, Kalman filter method, neural network method, fuzzy logic method, discharge test method and so on.

The ampere hour integration method has better accuracy when the initial state of charge SOC 0 is more accurate, but the coulomb efficiency is greatly affected by the state of charge, temperature and current of the battery, so it is difficult to accurately measure, so the ampere hour integration method is difficult to achieve the accuracy requirements of state of charge estimation. Open circuit voltage method has a definite functional relationship between the open circuit voltage and SOC after a long period of standing. The SOC estimation is obtained by measuring the terminal voltage. Open circuit voltage method has the advantage of high estimation accuracy, but the disadvantage of long-standing time also limits its application range.

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