# Active Equalization of Lithium-Ion Battery Based on Reconfigurable Topology
## 1 Introduction  
The number of electric vehicles (EVs) has increased rapidly to reduce atmospheric pollution in recent years. However, power batteries on EVs are suggested to be replaced when their capacities decline to less than 80% of the initial capacity to ensure the range of EVs. The number of retired power batteries in China exceeded 200,000 tons (about 26 GWh) in 2020 and is expected to rise to about 780,000 tons (about 134 GWh) in 2025, according to data from China Automotive Technology and Research Center. It will cause much waste and heavily harm the environment if these retired batteries are scrapped directly [1]. Retired batteries can be used in energy storage systems in power grids or microgrids when their capacities are greater than 60% of the initial states [2]. The energy storage systems can not only provide energy transmission between interconnected power grids but also provide continuous power to local power grids when faults occur, thus improving the robustness of the power system when confronting extreme weather or sudden faults [3]. Lithium-ion batteries are usually connected in series and parallel to meet the voltage and power requirements of loads because of the low voltage and capacity of one single battery [4]. The origination of inconsistency of lithium-ion batteries can usually be separated into the production process and the usage process [5]. The former can be caused by variances in the raw materials and manufacturing equipment and procedures, which will lead to variances in physical volume, internal impedance, and self-discharge rate [6]. The self-discharge of lithium-ion batteries means that the charge will gradually decrease even if no load is connected, which will accumulate a lot over a long time and cause significant imbalances in SOCs [7; 8]. The latter mainly originate from environmental differences during battery usage and storage. The recoverable power and capacity can be reduced significantlywhen these batteries are operated or stored at temperatures above 50 \({}^{\circ}\)C due to multiple factors, including loss of lithium from repair of solid electrolyte interphase (SEI) on negative electrode [9]. Charging at low temperatures may result in lithium deposition and even the growth of dendrites [10]. Additionally, the history of working in high temperatures would aggregate the battery's self-discharge rate permanently [11]. The battery with the lowest capacity determines the amount of electricity that can be discharged from the series battery group. The lowest-capacity battery may be overcharged during charging, which may lead to overheating and even cause an explosion [12]. These problems are more obvious in the secondary utilization of retired batteries since the inconsistency among cells is aggravated by the scattered sources, different production conditions, performance degradation on vehicles, etc. The equalization technique is essential to eliminate the influence of more discrete voltage, internal resistance, and capacity to ensure the available capacity and safety of the battery pack. The equalization methods of lithium-ion batteries can be divided into active methods and passive methods. Passive methods use resistors connected in parallel with the batteries to dissipate excess electricity to balance the battery pack [13]. This kind of method has a simple structure and low cost, but it wastes energy and raises the temperature in high-power situations. Active methods mainly use capacitors, inductors, transformers, and converters as energy transfer devices [14; 15]. In the simplest active equalization circuit, the balancing current can only be transmitted between adjacent battery cells. In that case, if the battery cell with a high imbalance appears at one end of the battery pack, the energy to be equalized will be transmitted over a longer distance in the pack, which will affect the equalization speed and efficiency [16]. Ref.