Electrochemical Analysis of Parallel Connected Lithium-Ion Batteries
Authors: Huang Chih-Sheng , Lin Yang-Shan , Hsieh Wan-Lin , Chen Kuo-Ching 
Published: February 2020

Over the past years Lithium-ion battery (LIB) has been widely used in the electric devices such as notebooks, cell phones, and electric vehicles. For the demand of electric vehicles, LIB cells would be connected in series and parallel to achieve a high voltage and capacity. However, connected batteries will cause some negative effects on the battery pack performance. For example, battery pack with mismatching internal resistance of cells in parallel connection will induce unbalance temperature and cause cycle life reduction [1]. Therefore, the development of an effective simulation model, which is able to predict the performance of cell pack, to help the electric vehicle designers to know the pack performance in advance is fundamentally needed.
In the previous work [2], we have developed an electrochemical model to simulate the behavior of battery in series connection and have discussed about the effect on cell pack with uneven temperature distribution. However, due to possible difference in the internal resistance for each cell, current flowing through one cell is different from the other cell when the two cells are parallel connected. To deal with the issue of parallel connection of LIBs, Guo and White [3] have presented an initialization step, called self-balancing, to satisfy the constraint of equal open circuit voltage (OCV), which required that the cells in parallel connection should have the same voltage at any time. In the current study, we propose an alternative model, which combines the porous electrode theory and an algorithm. This algorithm helps us to calculate and distribute the instant current through each cell in the configuration of parallel connection.
Figure 1 presents the discharge curves for a single 5Ah lithium manganese dioxide (LiMnO2) cell and three identical 5Ah LiMnO2 parallel-connected cells. The capacity of the cell pack is three times greater than the single cell. The amount of current through each cell is shown in figure 2, in which a slight fluctuation of the three currents can be observed. The small fluctuation could come from the small change in the slope of discharge curve around the capacity 10.5Ah, which leads to a deviation from a uniform current distribution in the pack. When the value of the current changes sharply, the discharge of the cell pack will stop. In order to discuss the aging effect on the performance of cell pack, we consider a particular scenario in which three cells are parallel connected with different initial state of charge (SOC) of 100, 90 and 80, respectively. Figure 3 displays the simulation result, which shows that the battery pack composed of different degree of aging cells presents a poorer performance than the pack with three identical cells.

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Figure 1