# Research on Equalization Strategy of Lithium Battery Pack Based on Multi-Layer Circuit
## 2 Related Work  
Passive and active balancing, often known as dissipative and non-dissipative balancing, are the two basic approaches for balancing lithium batteries [16; 17]. Passive balancing links resistors in parallel at both ends of the battery uses the battery with the lowest power in the battery pack as the benchmark and uses resistance discharge to consume the power of the high SOC battery. There is no energy transfer between the batteries in this case. This approach offers the benefits of a simple structure and inexpensive cost. However, it also has significant drawbacks, including constant energy loss, battery life shortening owing to continual heating, and low equalization efficiency. Battery active equalization technology uses the current shuttle of capacitance or inductance to transfer the charge in the high charge battery to the low charge battery [18; 19; 20]. By designing a specific energy converter, the energy is redistributed. Compared with the passive equalization method, the active equalization technology makes the most of all the energy stored in the battery. It wastes almost no energy and maximizes the available capacity of the battery. Due to the advantages of active equalization, researchers have proposed a variety of active equalization circuits in recent years. Reference [21] proposed an adjacent battery equalization circuit based on inductance, which has the advantages of low cost and small volume. However, for large-scale series batteries, the energy transfer path is too long, the energy dissipation is large, and the equalization time is long. Reference [22] proposed a multi-layer equalization circuit based on inductance, which improves the low equalization efficiency of adjacent equalization circuits. Nevertheless, when the multi-layer equalizer equalizes simultaneously, the current flowing through the battery exceeds the limit. Reference [23] proposed an equalization circuit based on a transformer. The number of transformers is the same as that of batteries, and the equalization efficiency is higher. However, when the number of batteries changes, the equalization circuit must be redesigned, and the size of the equalizer is significantly increased due to too many transformers. Reference [24] proposed an adjacent battery equalization circuit based on the Cuk equalizer, making the equalization strategy and system more simple. For large-scale series batteries, the energy transfer path is too long, and the equalization process takes a long time. Reference [25] proposed a capacitor-based equalization circuit, which realizes the equalization of the battery pack through the MOSFET switch and capacitor. The circuit has the advantages of low cost, small volume, and simple control, but it has low equalization efficiency, excessive switching loss, and cannot be fully balanced. References [26; 27] proposed a variety of equalization circuits based on capacitance, which changed the structure of the equalization circuit and significantly improved the equalization efficiency. However, with the increase in the number of batteries, the equalization efficiency accordingly decreased. The performance comparison of different equalization circuits is shown in Table 1.  
Based on the active equalization circuit, researchers have proposed a variety of equalization strategies. Reference [28] proposed a control strategy based on minimum energy dissipation, which can reduce energy consumption but increase the equilibrium time. Reference [29] proposed a two-level equalization strategy, which can make the multi-level equalization circuit equalize at the same time and improve the equalization speed, but it will lead to excessive battery current. Reference [30] proposed an equalization circuit based on inductance and transformer; the series battery uses single inductance equalization, and the parallel circuit uses transformer equalization.