To solve the above issues, the usage of an equalizer could keep the battery module balance, improve energy utilization, and further prolong the life span of the battery module [ 13, 14, 15].

Therefore, the designer can decide the use of more layers of capacitors to decrease the equalization time. Moreover, despite the literature not mentioning it, the same principle can be applied to add more layers of capacitors. In the same mode (half a switching cycle), the polarity of the primary and secondary side currents i 1 and i 2 of the transformer changes, that is, the battery is charged and discharged respectively. Therefore, a single voltage loop equalization control strategy can be obtained, as shown in Figure 5.

The proposed control method does not require a battery-equivalent circuit model or complicated calculations.

The balancing procedures of three cases using the three control strategies started with the same initial voltage, and they stopped when the voltage difference between cells was below 50 mV. BECs are categorized in a passive or active equalizer depending on if it dissipates energy [ 39, 40]. Even if the voltage difference between groups is very small, as long as a certain shift ratio is guaranteed, there is still a large inter-group equilibrium current.

It uses the terminal voltage of the high SOC and the low SOC of adjacent batteries to obtain the optimal proportion of the conduction time and the cut-off time in a switching period, which is used to maximize the balancing current to shorten the equilibrium time and improve overall performance. The main disadvantage of this configuration is the low efficiency and that it must avoid damaged cells. Z. Low cost battery equalizer using buck-boost and series LC converter with synchronous phase-shift control. S. Cryogenic-Energy-Storage-Based Optimized Green Growth of an Integrated and Sustainable Energy System. It shows the behavior of the current and its dependence on the difference in voltage between adjacent cells.

The control and structure are simple and easy to implement, but the balancing current will decrease as the voltages among cells become closer, resulting in a prolonged equilibrium time [ 23, 24, 25]. The energy flow is step-by-step among Lithium-ion-battery when an equalizer based on the buck-boost converter is adopted, resulting in a long energy transmission path and low equalization efficiency. V. The extensive equalization time is because the current is not controlled, and after a peak in the current, it decreases to zero. In addition, the circuit operated in discontinuous conduction mode (DCM) enables the controller to calculate an accurate average current by integrating the area of the inductor current waveform over one switching cycle. Since a converter-based equalizer uses magnetic components such as transformers or inductances as energy storage components to transfer energy, it can be designed to transfer energy from a cell to a module or from a module to a cell.However, the membership functions and the rule table need to be designed to comply with the system specifications, and the calculation of the function is more complex so it is difficult to implement by a low-cost MCU with a simple calculation. Moreover, the possible transferences are divided into cell-to-cell (C2C), string-to-cell (S2C), cell-to-string (C2S), pack-to-cell (P2C), cell-to-pack (C2P), string-to-string (S2S) and layer-based [ 22, 40, 46]. A Switch-Reduced Multicell-to-Multicell Battery Equalizer Based on Full-Bridge Bipolar-Resonant LC Converter. To save equilibrium time, the current-based inductive topology is prioritized in this application [ 27, 28, 29]. If the voltage or state of charge (SOC) of a cell is higher than that of other cells, the corresponding resistor releases the excess energy through the switch tube to keep the SOC of all cells consistent [ 10].