# Research on Equalization Strategy of Lithium Battery Pack Based on Multi-Layer Circuit
## 1 Introduction  
People are focusing more and more attention to the storage and utilization of clean energy as energy demand and pollution grow. Lithium batteries are widely employed in various energy storage devices due to their high energy density, extended cycle life, low natural discharge rate, and lack of memory effect [1; 2; 3; 4; 5]. For instance, Lithium batteries perform a vital part in smartphones, electric vehicles, and other mobile devices [6; 7; 8; 9]. However, the battery voltage is limited by the material properties of lithium. The voltage of a single lithium battery is typically 2.5-4.2 V [10], which falls short of the high voltage demand required in practical applications. Therefore, in order to provide the required high voltage, lithium-ion batteries are typically coupled in series as a battery pack [11]. Due to the influence of factors such as manufacturing process differences and internal impedance, the battery is often unstable [12]; influence factors such as the use environment aggravate the inconsistency, thereby reducing the energy utilization rate of the lithium battery pack, and even causing safety problems in severe cases. For example, the accident of the energy storage power station in Arizona, USA in 2019 resulted in the casualties of firefighters; the fire of the Fengtai energy storage power station in Beijing in 2021 resulted in the sacrifice of two firefighters; between 2018 and 2020 South Korea had nearly 30 energy storage accidents.  
A battery pack is fabricated of multiple batteries connected in series, each with the same charge and discharge current [13]. When a cell in the battery pack is fullycharged/discharged, the connection to the external power source/load must be discontinued to prevent overcharge/over-discharge of the Li-ion battery. Otherwise, the remaining cells in the battery pack that are not being fully charged/discharged lower the battery pack's real capacity [14; 15]. As a result, a balanced system must be built to balance and control the lithium battery, improve consistency between the single batteries in the lithium battery pack, extend the battery pack's cycle life, and increase safety during battery pack operation.  
In this study, a multi-objective optimization model is proposed for the balance of series lithium batteries. This model considers factors such as balance time, external current, and battery current. The model aims to optimize the equalization current and ensure that the battery current is within safe range, and ultimately achieve the goal of reducing excessive battery heating and realizing safe, fast charging and discharging of the battery pack. The following are the main contributions of this study:  
1. The proposed equalization strategy considers the equalization of the battery in the group from a broad perspective, takes all equalization speed and current constraints into account, and optimizes the equalization current. The strategy can not only ensure the equalization speed of the battery in the group but also that the battery charge and discharge current are within the allowable range and improve the safety of the equalization process;
2. A layered battery equalization method is proposed, which reduces the calculation difficulty of the equalization current by layered equalization of the batteries in the group and calculates the equalization current in real-time according to the state of the batteries in the group. The method makes the proposed equalization strategy easier to implement and more suitable for large-scale applications.