## 1 Introduction  
In recent years, eco-friendly technologies have become increasingly important in addressing greenhouse gas issues, and clean energy sources are being adopted to reduce greenhouse gas emissions. Currently, popular rechargeable batteries include lead-acid, nickel-metal hydride (Ni-MH), nickel-cadmium (Ni-Cd), and lithium-ion batteries. These batteries are widely employed in various types of electric vehicles, such as forklifts, pallet trucks, stackers, and other mobile equipment. Conventionally, lead-acid batteries have been used to supply driving power for electrified vehicles. However, lead-acid batteries are characterized by their bulky and heavy structure, while Ni-Cd batteries raise environmental concerns due to cadmium contamination. Among the various types of secondary batteries, lithium-ion batteries exhibit the highest power and energy densities, making them ideal candidates to replace other rechargeable batteries in demanding applications, particularly those requiring large-capacity battery packs [1, 2, 3, 4, 5]. A solid-state lithium battery utilizes a solid electrolyte instead of the liquid or gel-based electrolytes used in conventional lithium-ion batteries. By employing solid materialssuch as ceramics, glass, and sulfides as electrolytes, the flammability issues associated with liquid electrolytes are eliminated, thereby reducing the risk of thermal runaway. Moreover, compared to conventional lithium batteries, the solid-state lithium batteries offer several advantages, including faster charging speeds, a wider operating temperature range, higher energy density, longer cycle life, and slower capacity degradation [6; 7]. Whether in liquid or solid-state form, lithium-ion batteries are increasingly adopted in portable electronic devices and electric vehicles for both backup power and powertrain applications, due to their high power and energy density. In large-capacity battery packs, a dedicated power supply is required to charge the lithium-ion battery pack efficiently, as rapid full charging is often desired. For this reason, the typical lithium-ion battery chargers operate in two output modes: constant-current (CC) and constant-voltage (CV). Under the low state-of-charge conditions, the battery charger is operated in constant-current mode to supply a uniform charging current, thereby enabling rapid charging. As the battery voltage of the lithium-ion battery pack approaches the predetermined full-charge voltage, the charger transitions to constant-voltage mode, maintaining this voltage while the charging current gradually decreases. The fuzzy logic is a computational approach designed to manage uncertainty and ambiguity by emulating human reasoning and decision making. It employs a set of fuzzy rules to perform inference and control tasks without relying on precise mathematical models. The fuzzy logic is most commonly applied in fuzzy control systems, which incorporate the processes of fuzzification, fuzzy inference, and defuzzification. Fuzzy logic has found practical applications in various fields, including home appliances, autonomous driving and vehicle controls, industrial automation systems, robotics, and intelligent systems. In [8], fuzzy control was applied to a battery charger, where a fuzzy proportional-integral-derivative control strategy was employed to enhance the control performance of the charger. In addition, the genetic algorithms were used to optimize the membership functions and fuzzy rules of the fuzzy controller. In [9], the fuzzy logic was employed to enhance the charger's control performance by improving transient response speed and reducing inductor current ripple during steady-state operation. In this study, the battery charger integrated power factor correction (PFC) with a current-doubler rectifier and synchronous rectification. The PFC stage rectified the AC input power into a regulated DC voltage of approximately 400 V and reshaped the input current waveform to closely follow a sinusoidal reference, thereby achieving a near-unity power factor.