The equipment or energy storage system requires a certain voltage. The platform voltage of lithium batteries varies according to the positive and negative electrode materials. For example, for lithium batteries with graphite as the negative electrode material, when the positive electrode material is lithium iron phosphate, the platform voltage is 3.2V ; When the ternary material is selected as the positive electrode material, the platform voltage is 3.7V; when the negative electrode material becomes lithium titanate, the platform voltage will change with the change of the positive electrode material. The voltage of the single battery cannot meet the use requirements of the equipment and the system, and it needs to be connected in series to achieve the rated working voltage.

Similarly, due to the inconsistency in the manufacturing process of single cells, the SOC of single cells is inconsistent when batteries are used in series, resulting in different parameters of single cells. With the increase of use time and cycle times, the capacity of each single cell decreases. And the degree of aging is different, and it will cause some batteries to be overcharged or overdischarged. The existence of the battery management system will alleviate the problem of battery inconsistency to a great extent.

Batteries in series are simpler than those in parallel. The working current of the series-connected battery is constant, the working current of the single-cell battery is the same, and the independent operation has no mutual coupling effect.

Influence of series-parallel hybrid on the performance of lithium battery pack:

In pure electric vehicles and grid energy storage applications, single cells are connected in series to meet the voltage requirements and parallel to meet the capacity requirements, and the series and parallel connection methods often exist at the same time. The typical series-parallel method of battery packs includes parallel-connection first, series-connection first, and series-parallel connection first. Of course, there are also more complex topologies of series-parallel hybrid. The electric bus of the Beijing Olympic Games adopts the method of first-parallel and then series, and the energy storage system generally adopts the connection method of first-serial and then-parallel.

The problems of overcharge, overdischarge, overtemperature and overcurrent of the battery cells during the use of the series-parallel battery pack have greatly shortened the service life of the battery pack, and even caused vicious accidents such as burning and explosion, and shortened the service life of the power lithium battery pack.  security decline has become the key to restrict its popularization and application and industrial development. The battery screening process and battery management system are the keys to improve the performance of series-parallel battery packs.

Due to the inconsistency of single battery capacity, initial SOC, internal resistance, and polarization in the series battery pack, the battery management system needs to detect the single battery voltage and communicate with the charging and discharging equipment during the charging and discharging process to prevent overcharging of some single cells. Or over-discharge, under good battery management conditions, the series battery pack should avoid abuse during use, such as high current rate, high ambient temperature, etc., the series battery pack will not cause the life of the battery to decline faster than the single battery due to the connection into groups. However, the short-board effect of some battery performance will reduce the capacity utilization rate of the series battery pack, which can be improved by the battery management system with equalization function.

In a parallel battery pack, since the branch current is affected by the coupling of the parameters of the branch battery, the differences in the capacity, initial SOC internal resistance and polarization of the branch batteries after grouping will cause differences in the branch current conditions. Although the parameters of the batteries are relatively consistent, the average current rate of the entire charge and discharge process is not much different from the applied current rate of the parallel battery pack, but the currents formed in the SOC interval between the two ends of the battery voltage platform of the charge and discharge are quite different. Another significant influencing factor is that the parallel battery pack produces a current circulation due to the dynamic current conditions (acceleration, braking and idling process) in the actual working conditions.

The battery topology structure of series first and then parallel is conducive to the detection and management of each single battery of the system. In the series-before-parallel connection method, the more the number of series batteries in the parallel branch, the more the battery parameters of the whole branch, such as internal resistance and polarization, are closer to the integer multiple of the average value of the battery parameters of the unified batch. The capacity difference of the parallel branch is different from the initial The SOC difference becomes the main factor causing the parallel current unbalance. The battery parameters of the same batch are normally distributed in each branch in series and then in parallel, which significantly reduces the current imbalance of the entire series-parallel battery pack.

However, referring to the literature on the composition of electric vehicle battery packs, from the perspectives of the reliability of battery pack connections, the development trend of battery voltage inconsistency and the impact of battery pack performance, the connection method in parallel first and then in series is better than the connection method in series first and then parallel. Among them, in terms of system connection reliability, the first parallel and then the series is higher than the single battery, while the first series and then the parallel reliability is lower than the single battery; in terms of battery voltage inconsistency, the voltage distribution of the group mode of the first parallel and then the series is higher than that of the single battery. Concentration, there is no battery with too low voltage, and the battery pack using the series-parallel method is generally low in voltage, and the voltage distribution range is large.