The consistency of lithium-ion power batteries means that after the cells are used in groups, the voltage, internal resistance, capacity and other parameters of each cell are not exactly the same due to the influence of factors such as production and use environment. The performance parameters such as charge-discharge capacity and cycle life of a lithium-ion power battery are generally determined by the worst-performing monomer in the battery. Therefore, the consistency of the battery pack plays an extremely important role in its performance and cycle life. The impact of consistency on battery life will be discussed and ways to improve consistency will be suggested.
1. The impact of consistency on the life of the power battery pack
The consistency of lithium-ion power batteries mainly includes voltage consistency, capacity consistency and internal resistance consistency. As the power battery usage time increases, the degree of inconsistency will gradually increase. The most intuitive reflection is that the degree of inconsistency of the cell voltages in the battery pack increases. There are two main reasons for the poor consistency of lithium-ion power batteries: one is the production and manufacturing reasons. Due to slight differences in electrode plate thickness, chemical activity, microporosity, etc., there are some differences in parameters such as internal resistance and capacity of single cells. The second is the inconsistency in the use process. Lithium-ion power batteries have complex operating conditions, long-term work under harsh conditions such as high-rate charge and discharge current, vibration, etc., coupled with the structural layout of the battery system and the design of the heat dissipation system, which lead to the temperature, self-discharge degree, and electrolyte activity of each battery cell. There are differences, and the inconsistency of lithium-ion power battery packs gradually increases as the number of uses increases. It can be seen that the inconsistency of lithium-ion power batteries is inevitable. Figure 1 shows the causes of battery pack inconsistency and its transmission process.
As the inconsistency of the power battery pack increases, the performance and life of the battery pack are seriously affected. Scholars from various countries have already made some research results on the impact of inconsistency on the life of power batteries. Wang Zhenpo et al.  proposed a formula for calculating the remaining capacity of the battery pack after n times of use under the influence of inconsistency:
In the formula, C(n) represents the remaining capacity of the battery pack after n times of use; f(△C) represents the maximum value of the damage coefficient of the battery charge and discharge capacity during each charge and discharge process, which is a positive number less than 1; N represents the battery pack The service life of the battery pack; P indicates the specified capacity decay percentage at the end of the battery pack life; C0 indicates the initial capacity of the battery pack.
If f(△C) takes the maximum value of 0.999, the end of battery life is defined as the capacity decay of 20%. Assuming that there are three single cells with cycle life of 300 times, 600 times, and 1200 times, respectively, according to the formula, the cycle life when they are used in groups can be calculated as shown in Table 1.
|sample||Monomer cycle life (times)||Battery pack cycle life (times)|
According to the calculation results, the cycle life of the lithium-ion power battery pack is much lower than that of the corresponding monomer. Due to the inconsistency, the cycle life of the single cell is doubled, and the life of the battery pack can only be improved by dozens of times. If the battery pack is not repaired and maintained in time, the life of the battery pack can only reach a fraction of the life of the single unit. The lithium-ion power battery used in the demonstration operation of Beijing’s public transport has a single cell life of more than 1,000 times. The capacity of the power battery system applied to the vehicle will be seriously attenuated after 150 times of charging and discharging, and the capacity of some cells has been lower than 80% of rated capacity.
2. Measures to improve battery consistency
The cycle life of the battery pack is increased by increasing the cycle life of the lithium-ion power battery cells, which is ineffective and expensive. By optimizing the charging and discharging method of the battery pack, reducing the inconsistency caused by charging and discharging, and regularly repairing and maintaining the battery pack during use, the electrical performance of the lithium-ion power battery pack can be effectively guaranteed and the service life of the battery pack can be improved. Combined with the research on the life characteristics of lithium-ion power batteries for vehicles and the actual use of vehicles, the following measures can be taken to prevent the expansion of inconsistencies in the use of battery systems.
(1) To ensure the delivery quality of lithium-ion power battery cells, the initial voltage of each cell needs to be consistent, and the same batch of cells must be correlated with voltage, internal resistance and other data before leaving the factory to ensure the same batch of cells. performance as consistent as possible. The battery cells of the same batch, the same specification, and the same type must be selected when assembling the battery.
(2) Adopt practical battery balancing system and energy management system. At present, the most effective and practical equalization method is to equalize the voltage of each cell during the charging process of the battery pack, so that the cell voltage is as consistent as possible, and equalization management is realized from the source. Charging is terminated when the cell voltage reaches the charge cut-off voltage. Charge equalization is to use an active or passive equalization method to make the voltage of each cell consistent before charging is terminated, and the passive equalization method is currently used more. The principle of passive balancing is shown in Figure 2. Each cell is connected to a load resistor and controlled by a switch. According to the result of the cell voltage detection in the battery system, the balance management system closes the switch connected to the cell with the faster voltage rise during charging, thereby maintaining the consistency of the cell voltage and improving the electrical performance of the entire battery pack. Passive equalization is carried out by means of heat dissipation, the discharge current is generally controlled at about 0.1A, and the charging equalization takes several hours to complete. Active charge equalization requires an energy storage element (capacitor, magnetic field, etc.) to transfer energy between cells. The equalizing current is large and the power consumption is small, and no special cooling measures are required, which is beneficial to improve the consistency of the battery pack. However, this method has a complex structure and high cost.
The purpose of the battery management system (BMS) is to avoid premature failure of battery cells due to excessive use, so that the main electrical performance of the battery pack can reach and maintain the performance level of poor cells. Its main task is to prevent overcharge and overdischarge. , which provides status information such as voltage, current, temperature, and remaining power. Using thermal resistance, semiconductor refrigeration device for temperature control, etc., and controlling the charging and discharging state of the power battery pack through BMS can effectively increase the cruising range of the vehicle, prolong the service life of the battery system, and at the same time ensure the safety and reliability of the battery pack during use. Sex is important.
(3) Strengthen the maintenance and maintenance of the battery pack during use. During the use of the battery pack, it is necessary to avoid the contamination of the battery poles by water and dust as much as possible, to ensure a good working environment for the battery pack, and to avoid excessive use as much as possible. The power battery pack should be maintained regularly, and the cells with poor performance should be replaced or adjusted in time through the analysis of parameters such as the voltage of each single cell of the battery pack. The power battery pack is charged with a small current at regular intervals to promote its balance and performance recovery.