## 1 Introduction  
The trend toward more electric mobility has demanded the need for high voltage, high efficiency, and long-life battery systems. Thus, the importance of battery balancing technology is growing [1]. There exist many battery balancing schemes that are broadly grouped into either passive or active schemes. All these schemes have their own advantages and disadvantages, and, hence, it is upon the user to decide on which scheme will best work for them [2]. The balancing method should consider balancing speed, efficiency, complexity, and cost according to the intended use [3]. According to the Association for Unmanned Vehicle Systems International (AUVSI), agricultural drones will account for approximately 80% of the drone market in the future. The main disadvantage of current commercially available agricultural drones is that they only support short flights of about 15-20 min. We employ rechargeable secondary batteries with larger capacities compared to the batteries used in hobbyist drones to overcome the short flight time. Examples include lead-acid, nickel-cadmium (Ni-Cd), nickel-metal hydride (Ni-MH), and lithium-ion (Li-ion) batteries. Between them, Li-ion batteries have a number of advantages, including a higher power density by three times compared to lead-acid batteries, high energy density, and low self-discharge rate [4], all of which are properties important to support longer flight times. Drone batteries for agricultural use require large capacities and typically consist of a monolithic battery module or a pack of several cells connected in a series/parallel manner. However, the inter-cell voltages in this configuration can become unbalanced due to the electrochemical side reactions that occur when charging/discharging and the impedance differences between the battery cells. Moreover, overcharging the battery causes the active material to evaporate, which increases the pressure inside the battery and also increases the corresponding risk of an explosion. Similarly, over-discharging damages the gap structure in lithium-ion batteries, thereby reducing the battery lifetime. Thus, to prevent excessive overcharge/discharge and uneven cell voltages, a cell balancing system is essential [5]. We proposed a battery cell balancing algorithm using a fuzzy controller to solve the DoD (Depth of Discharge) of a high-power drone battery pack. The charging efficiency improvement of the battery pack through the fuzzy control, which has been studied recently, is the same, but it is applied to the drone to control the output through the improvement of the DoD. Conventional cell balancing techniques include passive balancing methods using resistors and active balancing methods using inductors, capacitors, and power change devices. Passive balancing techniques dissipate the energy of a relatively high state-of-charge (SOC) battery cell as heat [6]. These techniques require a simple circuit configuration and are inexpensive; however, when the energy is dissipated using resistance, these methods generate a considerable amount of heat and are relatively slow. By contrast, active balancing techniques transfer energy from cells with a high SOC to cells with a low SOC, and the corresponding circuit configuration is more complicated than that in the passive balancing technique. However, the balancing process is fast and efficient [7]. Because the capacity of the battery in the proposed design is large, a converter is needed to stabilize the voltage of the inverter. This type of converter is typically configured as a bidirectional DC/DC converter with a proportional-integral (PI) controller; however, PI controllers are better suited for linear systems, while the proposed system is nonlinear. This issue occurs because the mathematical model of the power electronic system and battery depends on the SOC, temperature, and C-rate [8]. In this paper, we propose a battery cell balancing algorithm, a bidirectional DC/DC converter, and a fuzzy controller to solve the balance problem.