The influence of series and parallel connection of lithium batteries on the performance of battery packs

The influence of series and parallel connection of lithium batteries on the performance of battery packs

Due to the voltage and capacity constraints of individual batteries, in order to meet the high voltage and large capacity requirements of electrical equipment and energy storage systems, lithium batteries are usually used in series, parallel, or series parallel hybrid ways. However, inconsistency is inevitable in the manufacturing process of lithium batteries, and this degree of inconsistency will gradually be amplified with the extension of usage time and the increase of cycle life. The typical parameters of lithium batteries, such as AC internal resistance, DC internal resistance, voltage, etc., will also vary under different states of charge. Under the barrel effect of lithium batteries, the strong remain strong, while the weak weaken until the entire energy storage system fails and is retired. The series and parallel connection of lithium batteries have a significant impact on the performance of the battery system

The influence of parallel connection on the performance of lithium battery packs

In the actual parallel grouping application of lithium batteries, the parallel branch may experience current imbalance during operation due to the influence of battery consistency. The current of the parallel branch is also affected by the parameters of this branch and other branches.

When lithium batteries are connected in parallel, when the capacity and initial state of the individual batteries are consistent, the internal resistance of the batteries will cause a relatively stable unbalanced current in the parallel branch platform period, resulting in inconsistent changes in the SOC of the parallel branch. Due to the sharp change in the polarization internal resistance of the batteries at the end of the battery state, the parallel branch will have a larger unbalanced current at the charging end. When screening in parallel groups, the distribution of Ohmic resistance and Polarization resistance of the battery can be analyzed to reduce the unbalanced current value at the end of branch charging. In the actual use of parallel battery packs, the inconsistency of various parameters of branch batteries often exists simultaneously, and the distribution of branch current after parallel grouping is affected by various inconsistent parameters.

The influence of series connection on the performance of lithium battery packs

The operation of equipment or energy storage systems requires a certain voltage, and the platform voltage of lithium batteries varies depending on the positive and negative electrode materials. For example, for lithium batteries with graphite as the negative electrode material, when lithium iron phosphate is used as the positive electrode material, the platform voltage is 3.2V; When using ternary materials 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 a single battery cannot meet the requirements of equipment and system usage, and it needs to be connected in series to achieve the rated working voltage.

Similarly, due to the inconsistency in the production and manufacturing process of individual batteries, when batteries are used in series, the SOC of individual batteries is inconsistent, resulting in different parameters of individual batteries. With the increase of usage time and cycle times, the capacity degradation and aging degree of each individual battery vary, and in severe cases, it can lead to overcharging or over discharging of some batteries. The existence of a battery management system will greatly alleviate the problem of battery inconsistency.

Series to parallel grouping of batteries is relatively simple. The working current of series battery packs is constant, and the working current of individual batteries is the same. Independent operation does not have mutual coupling effects. The voltage of individual batteries in series battery packs is easy to measure and is commonly used to evaluate the consistency of battery packs.

The influence of series parallel hybrid connection on the performance of lithium battery packs

In pure electric vehicles and grid energy storage applications, single cell batteries are connected in series to meet voltage requirements and in parallel to meet capacity requirements. The series parallel connection method often exists simultaneously. The typical series parallel connection methods for battery packs include first parallel connection followed by series connection, first series connection followed by parallel connection, and of course, there are also more complex topology structures of series parallel mixing. The electric buses for the Beijing Olympics will be connected in parallel first and then in series, and the energy storage system will generally be connected in series first and then in parallel.

The problems of overcharging, over discharging, over temperature, and over current of individual batteries that occur during the use of series parallel battery packs have greatly shortened the service life of grouped batteries and even led to malignant accidents such as combustion and explosion. The shortened service life and reduced safety of grouped power lithium batteries have become the key constraints on their promotion, application, and industrial development. The battery screening process and battery management system are key to improving the performance of series parallel battery packs.

Due to the inconsistency in the capacity, initial SOC, internal resistance, and polarization of individual batteries in series battery packs, the battery management system is required to detect the voltage of individual batteries and communicate with the charging and discharging equipment during the charging and discharging process to prevent overcharging or discharging of some individual batteries. Under good battery management conditions, series battery packs should avoid abuse during use, such as high current rates and high environmental temperatures. Series battery packs will not experience a faster decline in lifespan than individual batteries due to being connected in groups. However, the short board effect of some battery performance will reduce the capacity utilization rate of series battery packs, which can be improved through battery management systems with balancing functions.

In parallel battery packs, the branch current is affected by the coupling of branch battery parameters. The differences in branch battery capacity, initial SOC internal resistance, and polarization after grouping can cause differences in branch current conditions. Although the parameters of most single parallel branch batteries are relatively consistent, the average current ratio during the entire charging and discharging process is not significantly different from the applied current ratio of the parallel battery pack. However, the current difference formed in the SOC range at both ends of the charging and discharging battery voltage platform is significant. Another significant influencing factor is that parallel battery packs generate current circulation due to dynamic current conditions (acceleration, braking, and idle processes) in actual operating conditions. The circulation also damages the battery pack's lifespan to a certain extent during charging and discharging.

The battery topology structure of series first and then parallel is beneficial for detecting and managing individual batteries in the system. The more series batteries there are in the parallel branch connection method, the closer the parameters of the entire branch battery, such as internal resistance and polarization, are to the integer multiple of the average parameters of a unified batch of batteries. The capacity difference and initial SOC difference of the parallel branch become the main factors leading to parallel current imbalance. The normal distribution of parameters for the same batch of batteries in each branch of the series parallel battery pack significantly reduces the current imbalance of the entire series parallel battery pack.

However, referring to relevant literature on the composition of electric vehicle batteries, from the perspective of the reliability of battery pack connections, the development trend of battery voltage inconsistency, and the impact of battery pack performance, it is better to connect in parallel and then in series than in series and then in parallel. Among them, in terms of system connection reliability, the reliability of first in parallel and then in series is higher than that of individual batteries, while the reliability of first in series and then in parallel is lower than that of individual batteries; In terms of battery voltage inconsistency, the voltage distribution of the first in series and then in series grouping method is more concentrated, and there are no batteries with low voltage. However, the voltage of the battery pack using the first in series and then in series grouping method is generally low, with a large voltage distribution range.

From the above, it can be seen that series after parallel and parallel after series have their own advantages and disadvantages. In different application conditions, it is advisable to choose the appropriate grouping method based on usage, battery system capacity, charging and discharging conditions, etc.
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