# A Novel Inductor-Based Non-Dissipative Equalizer
## 1 Introduction  
Currently, lithium batteries are widely used in electric vehicles, hybrid electrical vehicles and stationary storage systems. Due to the limited voltage of a single cell, a lot of cells have to be connected in parallel and in series to provide the required voltage and capacity. However, after multiple charge and discharge cycles, battery cells in the battery pack may suffer from imbalance resulting from aging, internal impedence, manufacturing inconsistencies and differences in operating environment [1]. This imbalance will intensify over time, which greatly degrades the available capacity and battery life, or even leads to overcharge and deep discharge and thus creates potentially dangerous situations [2,3]. Battery equalization helps the battery strings alleviate the imbalance, release as much energy as possible and extend battery life. Therefore, the equalization for battery strings needs to be realized. Battery equalization is a technology for keeping lithium batteries in the same state to avoid deterioration of lithium batteries and safety hazards. It can be categorized into dissipative equalization and non-dissipative equalization. Dissipative equalization appears earlier, so it is more mature, simpler and widely used. However, dissipative equalization consumes energy and generates heat, which reduces the efficiency of the system and increases the difficulty of battery thermal management. The non-dissipative equalization has a higher energy utilization rate and lower heat production than dissipative equalization, but the structure is more complex and the cost is higher [4]. At present, there are a large number of studies on non-dissipative equalization circuits, in which the number of energy storage elements and the speed are two key considerations of the equalization circuit. Non-dissipative equalization circuits can be divided into the following two types: one uses only one energy storage element, such as the single switched-inductor equalizer (SSIE) proposed in [5], as shown in Figure 1, and the equalization circuit in [6]. This kind of topology is simple and low-cost, but it is hard to realize ideal equalization speeds. The other topology has fast equalization speeds, butthe number of energy storage components used in the equalization circuits are close to the number of batteries, resulting in an oversized circuit and excessive cost. As shown in Figure 2, a parallel architecture equalizer (PAE) based on buck-boost converters for battery strings in [7] is one of the most representative circuits. A systematic comparison has been done in [7], which proves that the PAE has an extraordinarily fast equalization speed. Equalizers for comparison include PAEs based on buck-boost converters, traditional inductor-based adjacent equalizers (IBAE) [8], parallel architecture equalizers based on a multi-wind transformer (PAEBMWT) [9] and double-tiered switched-capacitor equalizers (DTSCE) [10]. The equalization circuits in [1; 2; 3; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19] also suffer from the same problem of oversized circuits and excessive cost. In summary, it is difficult to reduce the number of energy storage elements while maintaining a good equalization speed. This paper improved the MSIE proposed in [11] by using MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) to replace some of the inductors and proposed a novel inductor-based non-dissipative equalizer (NIBNDE). The NIBNDE can significantly reduce the number of energy storage elements and keep a good balancing speed. With the same number of batteries, the NIBNDE uses fewer energy storage elements than the equalization circuits in [1; 2; 3; 7; 18; 19]. Figure 1: Single switched-inductor equalizer proposed in [5]. Figure 2: The parallel architecture equalizer proposed in [7]. The equalization principle and the key parameters of the NIBNDE are presented in Section 2. Section 3 describes the simulation results and the comparison between NIBNDE, SSIE and PAE. Section 4 compares the experimental results with the simulation results.