What are the factors that cause the inconsistency of the battery pack?

Factors that cause inconsistent battery packs:

　　1 Differences in parameters between single cells

　　The state difference between the single cells mainly includes the initial difference of the single cell and the parameter difference produced during the use process. There are many uncontrollable factors in the design, manufacture, storage and use of the battery, which will affect the consistency of the battery. Improving the consistency of single cells is a prerequisite for improving the performance of the battery pack. The mutual influence of single battery parameters, the current parameter state is affected by the initial state and the cumulative effect of time.

　　 Battery capacity, voltage and self-discharge rate

　　 Inconsistent battery capacity will make the depth of discharge of each single cell in the battery pack inconsistent. A battery with a small capacity and poor performance will reach the full charge state in advance, causing a battery with a large capacity and good performance to fail to reach the full charge state. The inconsistency of battery voltage will cause the single cells in the parallel battery pack to charge each other, and the battery with higher voltage will charge the battery with lower voltage, which will accelerate the degradation of battery performance and consume the energy of the entire battery pack. A battery with a high self-discharge rate has a large loss of capacity, and the inconsistency of the battery's self-discharge rate will cause differences in the state of charge and voltage of the battery, which will affect the performance of the battery pack.

　　Battery internal resistance

　　In a series system, the internal resistance difference of the single battery will cause the charging voltage of each battery to be inconsistent. The battery with large internal resistance reaches the upper voltage limit early, and other batteries may not be fully charged at this time. A battery with a large internal resistance consumes a lot of energy and generates high heat. The temperature difference further increases the difference in internal resistance, leading to a vicious circle.

　　In a parallel system, the internal resistance difference will cause the inconsistency of the current of each battery. The voltage of the battery with a large current changes quickly, which makes the charging and discharging depth of each single battery inconsistent, making the actual capacity value of the system difficult to reach the design value. The operating current of the battery is different, and its performance will be different during use, which will eventually affect the life of the entire battery pack.

　　2 Charge and discharge

　　The charging method affects the charging efficiency and state of charge of the lithium battery pack. Overcharge and overdischarge will damage the battery, and the battery pack will show inconsistency after repeated charging and discharging. Currently, there are several charging methods for lithium-ion batteries, but the common ones are segmented constant current charging and constant current and constant voltage charging. Constant current charging is an ideal method, which can carry out safe and effective full charging; constant current and constant voltage charging effectively combines the advantages of constant current charging and constant voltage charging, and solves the problem of general constant current charging methods that are difficult to accurately fully charge. The constant voltage charging method avoids the influence of excessive current on the battery at the initial stage of charging, and the operation is simple and convenient.

　　3 Temperature

　　 The performance of lithium batteries will be significantly attenuated at high temperatures and high discharge rates. This is because lithium-ion batteries will cause the decomposition of the positive electrode active material and electrolyte when used under high temperature conditions and high currents. This is an exothermic process. A short time of such heat can cause the battery's own temperature to rise further. It also accelerates the decomposition phenomenon, forming a vicious circle, and accelerated decomposition will further reduce the battery performance. Therefore, if the thermal management of the battery pack is improper, it will cause irreversible performance loss.

　　4 External circuit of battery

　　 Connection method

　　In a large-scale energy storage system, batteries will be combined in series and parallel, so there will be many connection circuits and control components between the battery and the module. Because the performance and aging speed of each structural member or component are different, and the energy consumed by each connection point is inconsistent, different devices have different effects on the battery, resulting in inconsistencies in the battery pack system. The inconsistency of the battery decay speed in the parallel circuit will accelerate the deterioration of the system.

The impedance of the connecting piece will also affect the inconsistency of the battery pack. The resistance of the connecting piece is different, and the resistance value of the pole to each single battery branch is different. The battery far away from the pole has a longer resistance value due to the longer connecting piece. If the current is large, the current is small. The connecting piece will make the single battery connected to the pole reach the cut-off voltage first, which will reduce the energy utilization rate and affect the battery performance. Moreover, the early aging of the single battery will cause the battery connected to it to overcharge. , Causing safety hazards.

　　 As the number of battery cycles increases, the ohmic internal resistance will increase, the capacity will decrease, and the ratio of the ohmic internal resistance to the resistance of the connecting piece will change. In order to ensure the safety of the system, the influence of the resistance of the connecting piece must be considered.

　　BMS input circuit

　　 Battery Management System (BMS) is the guarantee for the normal operation of the battery pack, but the BMS input circuit will adversely affect the consistency of the battery. Battery voltage monitoring methods include precision resistor divider, integrated chip sampling, etc. These methods cannot avoid the leakage current outside the sampling line due to the existence of resistors and circuit board paths. The voltage sampling input impedance of the battery management system will increase the battery state of charge ( The inconsistency of SOC) affects the performance of the battery pack.

　　5 SOC estimation error

　　The inconsistency of SOC is caused by the inconsistent initial nominal capacity of the single battery and the inconsistent decay rate of the nominal capacity of the single battery during operation. For parallel circuits, the difference in internal resistance of the single cells will cause uneven current distribution, which in turn leads to inconsistent SOC. SOC algorithms include ampere-hour integration method, open circuit voltage method, Kalman filter method, neural network method, fuzzy logic method, discharge test method, etc.

The ampere-hour integration method has better accuracy when the initial state of charge SOC0 is relatively accurate, but the Coulomb efficiency is greatly affected by the state of charge, temperature and current of the battery, and it is difficult to accurately measure it, so the ampere-hour integration method is difficult Meet the accuracy requirements of state-of-charge estimation. Open circuit voltage method After a long period of standing, the open circuit voltage of the battery has a definite functional relationship with SOC, and the estimated value of SOC is obtained by measuring the terminal voltage. The open circuit voltage method has the advantage of high estimation accuracy, but the disadvantage of long standing time also limits its scope of use.