Experimental Results and Analysis of Wireless Charging Technology for Equipment Under Seawater

Abstract

A constant current constant voltage charging scheme based on a single-ended primary inductive converter is proposed to address the key issues of wireless power transfer (WPT) technology for charging devices in seawater environments. The scheme can effectively adapt to the complex transmission conditions of a WPT system in a seawater environment by using the advantages of single-ended primary inductor converter (SEPIC) topology, such as adjustable voltage, wide input range, and the same polarity as output; its regulating effect on charging current and voltage is modeled and analyzed. An underwater experimental platform is built to test the charging performance of the system under different transmission distances, radial offsets, and deflection angles (1 A is set for the constant current stage and 5 V for the constant voltage stage). The experimental results show that when the distance is 2 cm, the maximum fluctuation amplitude of the current is 0.04 A. When the transmission distance is increased to 6 cm, and a radial offset of 5 cm is introduced, the fluctuation amplitude increases to 0.13 A. Under the condition of dynamic charging, the maximum fluctuation range of current is 0.15 A, and the fluctuation rate reaches 16.7%. It shows that the system has good applicability and application prospects in seawater environments.

1. Introduction

With the continuous development of marine science and technology, the demand for marine exploration, seabed resource development, and environmental monitoring is increasing. In the marine environment, numerical equipment such as underwater robots, detectors, and sensors needs to work underwater for a long time. This equipment usually undertakes tasks such as data acquisition, real-time monitoring, and complex operations, which require a long-term and stable power supply. However, due to the particularity of the deep-sea environment, the traditional power supply mode faces many restrictions. For example, the battery has limited energy and needs to be replaced frequently, which will not only interrupt the continuous operation of the equipment but also lead to the interruption of tasks and the reduction of work efficiency. In addition, the high corrosivity in the marine environment and complicated water flow conditions make the traditional online charging method face serious security risks, such as loopholes, electric shocks, and other risk problems. In order to solve the above problems and improve the working efficiency of underwater equipment, wireless charging technology in seawater environments came into being [1,2]. Wireless power transmission transmits electric energy through indirect contact, which is especially suitable for the power supply demand of underwater equipment. When it is applied to the power supply of underwater equipment, it can improve the continuous operation ability of the equipment, ensure the stability and safety of the energy supply, and facilitate the orderly progress of underwater operation [3,4,5,6].

Several major WPT systems [7] are shown in

Table 1. Four main WPT systems.

Wireless Charging Mode	Operating Principle	Advantage	Disadvantage	Applicable Scene
Magnetic induction	Generation of induced current at the receiving end by electromagnetic induction between the primary and secondary coils	Higher short-range transmission efficiency and higher charging power	Shorter transmission distances, need for transmitter and receiver alignment	Smartphones, small appliances
Magnetic resonance	The transmitter and receiver each form a resonant circuit to achieve wireless energy transmission through magnetic field resonance at the same resonant frequency.	Longer transmission distances and moderate conversion efficiencies	Higher electromagnetic interference, complex system design	Medical equipment, industrial robots
Microwave radiation	Through the high-frequency microwave (GHz) propagation in space, the receiving end, through the rectifier antenna to convert microwave energy into direct current	Long transmission distances (meters to kilometers)	Low conversion efficiency, directional antenna required	Drones, satellites
Capacitive coupling	Capacitive coupling between the two electrode plates through a high-frequency electric field to achieve energy transfer	Higher resistance to horizontal misalignment, simple structure	Low transmission power and high environmental impact	Sensors, Wearables

, among which is the microwave radiation WPT, which has serious signal attenuation and energy loss in a seawater medium, resulting in extremely low transmission efficiency, and thus is not applicable to underwater energy supply scenarios [8]. The electric field coupling WPT is suitable for small power applications but has high requirements on the electrical parameters of the medium, and in a seawater environment with high salinity and high electrical conductivity, the energy transmission is limited, and it is not suitable for the power supply of high-power underwater equipment [8]. Electromagnetic induction WPT relies on the precise alignment of the transmitter and receiver, and the transmission distance is shorter, which is more limited in the complex marine environment. In contrast, magnetically coupled resonant wireless power transfer (MCR-WPT) combines high transmission efficiency, moderate transmission distance, and low sensitivity to the dielectric environment and can adapt to the electromagnetic parameter changes of seawater within a certain range [9]. Therefore, MCR-WPT has become the focus of underwater wireless charging technology research and application.

Compared with an air environment, seawater as the transmission medium of WPT has its particularity. The high conductivity of seawater makes the WPT system produce an eddy current effect in seawater [7], which causes part of electric energy to be consumed in the form of an eddy current and has some impact on wireless power transmission. Eddy current loss is positively correlated with the square of the electric field strength, the volume of seawater between the two coils, and the conductivity of the seawater [10]. Among them, the electric field strength is affected by the current in the primary and secondary coils, while the volume of seawater between the two coils increases with the transmission distance. Therefore, when the transmission distance increases, the volume of seawater between the two coils expands, and the propagation path of the electromagnetic field lengthens, resulting in a decrease in the mutual inductance of the system. The decrease in mutual inductance causes the reflected impedance of the secondary side to the primary side to decrease, which drives the current in the primary coil to increase to maintain the transmitted power. Due to the increase in current, the electric field intensity of the primary side radiation will also increase, and the higher conductivity of seawater will further aggravate the eddy current loss and reduce the overall transmission efficiency of the system. Therefore, in the underwater WPT system, the increase in transmission distance not only affects the mutual inductance coupling characteristics but also increases the eddy current loss, which adversely affects the efficiency and stability of the system. An experimental study of wireless power transmission (WPT) with a frequency of 85 KHz and a transmission distance of 50 mm was carried out in salt water, deionized water, and air environments in the literature [11]. The experimental results show that the transmission efficiency of the WPT system in deionized water is basically the same as that in the air environment, while in the saltwater environment, its transmission efficiency is about 5% lower than that in air. This is mainly due to the eddy current loss caused by the higher conductivity of salt water, which loses part of the power and reduces the transmission efficiency.

However, although there is an eddy current loss, which reduces the transmission efficiency, the WPT system also shows advantages in the transmission of seawater media. In the literature [12], the transmission characteristics of a medium-range underwater wireless power transmission (WPT) system with an asymmetric structure are investigated, and the reduction of the balance resistance on the primary side can effectively reduce the power loss and thus improve the overall transmission efficiency of the system. The maximum transmission efficiency of the system reaches 15.3% at an operating frequency of 410 kHz and a transmission distance of 50 cm. A dual-transmitter-single-receiver wireless power transmission (WPT) system is proposed in the literature [13], and its transmission characteristics in seawater environments are experimentally investigated. It was found that the transmission efficiency of the system was four times higher than that of the air environment at a transmission distance of 25 cm. In addition, compared with the traditional single-transmitter-single-receiver WPT system, the dual-transmitter-single-receiver structure can effectively enhance the magnetic field coupling strength and improve the energy transfer capability of the system, resulting in an 11% increase in the transmission efficiency. Studies in the literature [14] have found that WPT systems in seawater media have significant superiority in terms of stability of transmission over medium distances compared to WPT systems in air environments. When the coil spacing changes in a certain range, the fluctuation of load voltage is small, showing strong stability. In addition, the study also found that the load voltage remained almost unchanged when the radial deviation of the two coils was within 15 cm. These characteristics are not possessed by the WPT system in air, which shows that the WPT system in a seawater medium can effectively overcome the influence of spatial positioning error on transmission performance in a certain range, which is helpful in improving the adaptability of the WPT system in seawater environment and is the advantage of the wireless charging system in seawater.

Significant contributions have been made by scholars to improve the transmission efficiency and stability of WPT systems. In the literature [15], an arc-shaped magnetic coupler is proposed and utilizes iron-based nanocrystalline alloy soft magnetic material as the magnetic core, which can transmit 3 KW of power with a DC-DC efficiency of 91.9% at the point of forward maximum output power. The method starts from the perspective of enhancing magnetic field coupling and utilizes the higher permeability and higher saturation flux density characteristics of iron-based nanocrystalline alloy soft magnetic material to guide the magnetic field so that the main magnetic flux is more concentrated in the receiving coil, reducing leakage loss and improving coupling efficiency. Meanwhile, its lower hysteresis loss and eddy current loss can reduce energy loss under high-frequency conditions and improve the efficiency of WPT. This method is useful for improving the charging efficiency of subsequent wireless charging systems. A transmitter design based on a star-shaped coil matrix was proposed in [16], in which two layers of interleaved square coils were adopted in the transmitting array to enhance the electromagnetic field coverage and improve the rotational tolerance of the receiver, thereby increasing the stability of the system. In addition, by optimizing the coil current direction and phase control, the blind area of power transmission is eliminated so that the receiving coil can obtain uniform electromagnetic power distribution in different orientations. A novel hybrid transmitter composed of conical and planar spiral coils was proposed in [17], which improved alignment accuracy and enhanced transmission stability, with output power variation limited to 5.7%. In the literature [18], a parameter optimization method based on chaotic sparrow search algorithm is proposed for the problem of low transmission efficiency of MCR-WPT, which introduces a chaotic learning strategy to optimize the sparrow search algorithm and optimizes the parameters such as operating frequency, coupling coefficient and load impedance with this algorithm, so that the optimized transmission efficiency of the MCR-WPT system can reach up to 91%. In the literature [19], a constant voltage control strategy based on active impedance matching is proposed for the application of WPT technology in power transmission to solve the problem of output voltage instability due to the change of load impedance. The optimal load impedance is determined by adjusting the phase shift and duty cycle, thus achieving impedance matching and improving the efficiency of the whole system.

In charging management, common control strategies include variable frequency control, hybrid compensation topology, and DC-DC conversion. Among them, frequency conversion control is a technology that achieves constant current-constant voltage output by adjusting the frequency of alternating current output by inverter [20]. This method has the advantage of simple implementation, but its disadvantages are also obvious, especially the influence on the resonance characteristics of the system. Frequency conversion control requires high-frequency control of constant current and constant voltage and is highly dependent on the stability of resonance parameters. A wireless charging system based on the modulation depth control of the SPWM inverter circuit to adapt to the load change is proposed in the literature [21], which can achieve CC and CV charging when the load changes. Monitoring the load voltage and current in real time to judge the load change and adjust the modulation depth of the SPWM inverter circuit to achieve CC or CV output can be applied to device charging in an air environment. A bilateral LCC underwater WPT system employing a primary-side and secondary-side phase-shifting strategy was proposed in [22], which increases the phase difference between the coil currents to reduce the synthesized induced electric field amplitude at various points between the coils. This reduction leads to a lower proportion of eddy current loss in the total energy loss. When the phase difference of the coil currents reaches 116°, the system achieves a maximum transmission efficiency of 91.5%, representing a 0.8% improvement in energy transfer efficiency compared to a system without the phase-shifting strategy. In order to achieve control of the current phase difference in the two coils, this method needs to add inverter circuits in both the primary and secondary sides so that the circuit structure will be more complicated, which increases the size and cost of the system. It relies on an efficient phase detection and adjustment mechanism to ensure the precise matching of the current phases between the primary and secondary sides; otherwise, additional power loss may be introduced, and it also puts higher requirements on the synchronization and stability of the subsequent charging control. The literature [23] achieves output current or voltage regulation by implementing frequency control in the inverter. This method needs to monitor the output change of the secondary side in real time and transmit the load current and voltage signals to the primary side control circuit for adjustment through wireless communication. This method relies heavily on communication between primary levels and has strong limitations. Wireless signals may be interfered with in complex environments, which will reduce the control accuracy of the system. In addition, the particularity of a seawater environment further aggravates this problem, and the resonant frequency of the WPT system in seawater will shift [24], which may lead to the instability of resonant frequency control, thus leading to the decline of system transmission efficiency. On the whole, this method is not suitable for wireless charging systems under seawater.

As for the hybrid compensation topology, it is a technical means to comprehensively utilize the advantages of different topologies and learn from each other’s strong points. By combining various compensation networks and switching, each topology can give full play to its functional characteristics in a specific mode [25]. For example, the S-S topology is used to achieve a load-independent constant current output characteristic, and the S-LCL topology is used to achieve a constant voltage output. However, the output characteristics of a specific topology are closely related to the system parameters, which limits the freedom of output current or voltage and makes it difficult to satisfy diverse charging requirements. A wireless charging system based on a novel multidirectional magnetic coupler was proposed in [26] for group AUV applications. The transmitter coil, consisting of two oppositely wound spiral rings, generates a multidirectional magnetic field to supply power to multiple arc-shaped receiver coils positioned in different orientations. When powering four AUVs, the system achieves an efficiency of 92.25% at a power level of 200 W. However, this system only implements constant voltage (CV) charging and relies on communication between the primary and secondary sides. The switching topology based on LCC-S hybrid compensation for CC-CV charging was proposed in [27]. The scheme transmits the control signal of the secondary side to the primary side through Bluetooth communication, so as to achieve a switch of charging mode. Although this approach improves the flexibility and performance of the system to a certain extent by combining the advantages of LCC and S topologies, the method also requires communication between the primary stages and is not suitable for wireless charging systems under seawater.

DC-DC conversion is a technology that converts one DC voltage into another DC voltage [28], and its basic principle is to achieve efficient energy transfer through the cooperation of high-frequency switching modulation and energy storage elements. In order to meet the precise control requirements of voltage and current in different application scenarios, DC-DC conversion usually adopts a closed-loop control strategy, which detects the output current and voltage in real time, feeds the measured parameters back to the control circuit, and dynamically adjusts the switching frequency and duty ratio based on the preset target value, so as to accurately adjust the output current and voltage. In the literature [29], a wireless charging system based on an arc coil and secondary side control is proposed and designed. The arc coil can adapt to the curved shape of an autonomous underwater vehicle (AUV) and has high magnetic coupling characteristics (coupling coefficient 0.53), thus improving the power transmission efficiency. At the receiving end, the system uses a Buck converter to achieve effective regulation of charging current and voltage. The experiment was carried out in the air environment; the transmission distance was 8 mm, the measured charging power was 599.3 W, and the efficiency of the whole machine reached 88.3%. However, since the Buck converter only has the function of buck reduction, this control strategy is only applicable to the situation where the input voltage is higher than the output voltage, and its applicability has certain limitations for the application scenarios with large voltage variation ranges. The constant voltage output characteristics of the secondary side DC-DC converter in the wireless power transfer system were systematically studied in the literature [30]. The principles of constant voltage output and the impact of the boost converter on the performance of the WPT system were analyzed in depth, optimizing the WPT system’s performance with a transmission efficiency of up to 92.32%. The research results show that the reasonable selection and design of a DC-DC converter can not only effectively keep the output voltage of the WPT system constant but also improve the overall transmission efficiency of the WPT system.

In response to the above issues, this paper designs a CC-CV charging system that is suitable for seawater environments based on the magnetically coupled resonant radio energy transmission system of S-S topology and the charging control strategy constructed using SEPIC topology. In order to verify the adaptability of the system in a complex seawater environment, an underwater experimental platform was built, and its charging performance was tested and evaluated under different transmission distances (2, 4, and 6 cm), radial offset (5 cm), and deflection angle (15°, 30°). The experimental results show that the scheme can effectively achieve CC-CV charging and has strong adaptability to the environment, which provides strong support for the optimization and application of wireless charging systems in seawater environments.

2. Control Strategy Analysis

2.1. DC-DC Conversion Topology

As shown in Figure 1, in the wireless energy transmission system under a seawater environment, effective regulation of output current and voltage can be achieved by introducing a DC-DC converter at the receiving end so as to achieve the function of constant current and constant voltage charging. This method does not need to rely on real-time communication between the initial stages and has a high degree of system independence. This design can effectively avoid the effects of restricted communication and complex electromagnetic interference in the marine environment, which is suitable for the wireless charging needs of devices in the seawater environment.

In the seawater environment, the WPT system is often influenced by many complicated factors of the external environment, such as water flow, impurities, and the deviation of transmission distance and position. These factors may cause the power and voltage received by the secondary side to fluctuate to a certain extent, which will adversely affect the subsequent stability process. In order to overcome these problems and ensure that the system can achieve stable and reliable charging, the DC-DC converter on the secondary side should have the following key functions:

(1)

Input voltage range applicability: The voltage received at the secondary side of a WPT system under seawater may fluctuate widely, so the DC-DC converter needs to be able to adapt to a wide input voltage range;

(2)

Stabilized output: Even if the input voltage fluctuates, the DC-DC converter needs to be able to maintain a stable output by regulating the output current or voltage to ensure that the load battery can be safely charged;

(3)

High-efficiency conversion: Since the efficiency of wireless energy transfer in seawater environments may be low, the use of a high-efficiency DC-DC converter can minimize energy loss.

Different types of DC-DC converters have their own unique performance characteristics.

Table 2. Six basic DC-DC converters.

DC-DC	Advantage	Disadvantage	Applicable Scene
Boost	Output voltage higher than input voltage	Higher input current	Conversion of low voltage to higher voltage
Buck	High efficiency simple circuit	Output voltage cannot be higher than input voltage	Conversion of high voltage to lower voltage
Buck-Boost	Output voltage higher or lower than input voltage	Complicated circuit, output voltage polarity opposite to input	Applications with large input voltage fluctuations, such as battery-powered systems
Cuk	Output voltage higher or lower than input voltage, voltage stabilization	Complex structure low efficiency	Battery-powered systems or applications requiring voltage stabilization
Flyback	Multiple outputs	Lower efficiency and higher EMI at lighter loads and higher frequencies	Power systems requiring multiple outputs or isolation
SEPIC	Wide input voltage range, higher or lower than the output voltage, input and output voltage polarity is the same	Complex circuit Higher cost	Applications requiring stabilized output voltage

compares six basic DC-DC converters. In view of the need that the DC-DC converter on the secondary side of the WPT system can adapt to a wide input voltage range to flexibly adjust the charging current and voltage, and considering that the SEPIC circuit has the advantages of step-down voltage and the same polarity of input and output voltages, it is suitable for load battery power supply systems. Therefore, in this study, SEPIC topology is selected as the charging circuit of the back end of the WPT system.

2.2. System Topology and Control Analysis

In this study, the circuit of the wireless charging system consists of three parts: a wireless power transmission module, a rectifier filter module, and a charging control module based on SEPIC topology. As shown in Figure 2, the wireless power transmission module adopts an S-S topology, which has the advantages of a simple structure, strong load capacity, and long transmission distance. The rectifier and filter module select a full-bridge rectifier circuit, whose main function is to rectify and filter the high-frequency alternating current received by the secondary coil so as to provide stable direct current for the subsequent SEPIC circuit. Finally, through the combination of SEPIC topology and PWM controller, the output current and voltage can be adjusted to meet the constant current and constant voltage charging requirements of the system.

The impedance of the secondary side of the wireless charging system is as follows:

$$Z_{\mathrm{s}}=j\omega L_s+{\displaystyle \frac{1}{j\omega C_s}}+Z_{Z-SEPIC}$$

(1)

where $\omega$ is the working angular frequency of the system and $Z_{Z-SEPIC}$ is the total equivalent impedance of the rectifier filter circuit and SEPIC circuit.

When the system operates at the resonant frequency $f_0$, the system operates at angular frequency $\omega =2\pi f_0$. The reflection impedance $Z_{ref}$ of the secondary circuit to the primary circuit is as follows:

$$Z_{ref}={\displaystyle \frac{{\left(\omega M\right)}^2}{Z_s}}={\displaystyle \frac{{\left(\omega M\right)}^2}{Z_{Z-SEPIC}}}$$

(2)

where $M$ is the mutual inductance between the primary coils, and its equivalent impedance is zero when the system works at the resonant frequency.

The relationship between the input impedance of the SEPIC circuit and the load impedance $R_L$ is as follows:

$$Z_{SEPIC}={\displaystyle \frac{V_{in}}{R_{in}}}={\left({\displaystyle \frac{1-D}{D}}\right)}^2\cdot R_L$$

(3)

where $D$ is the duty cycle of the SEPIC circuit and $R_L$ is the actual load impedance. By adjusting the duty ratio of the SEPIC circuit to $D$, the input impedance can be effectively changed, and then the equivalent impedance of the rectifier and filter circuit can be affected.

The total equivalent impedance $Z_{Z-SEPIC}$ of the rectifier filter circuit and SEPIC circuit can be expressed as follows:

$$Z_{Z-SEPIC}={\displaystyle \frac{8}{{\pi }^2}}\cdot {\left({\displaystyle \frac{1-D}{D}}\right)}^2\cdot R_L$$

(4)

The input impedance $Z_{in}$ on the primary side is as follows:

$$Z_{in}=R_1+Z_{ref}$$

(5)

Then, the current $I_{in}$ in the primary coil is as follows:

$$I_{in}={\displaystyle \frac{U_{in}}{Z_{in}}}={\displaystyle \frac{U_{in}}{R_1+Z_{ref}}}={\displaystyle \frac{U_{in}}{R_1+\frac{{\left(\omega M\right)}^2}{Z_{Z-SEPIC}}}}$$

(6)

The input voltage $U_r$ after rectification and filtering on the secondary side is as follows:

$$U_r=j\omega M{I}_{in}={\displaystyle \frac{j\omega M{U}_{in}\cdot Z_{Z-SEPIC}}{R_1+\frac{{\left(\omega M\right)}^2}{Z_{Z-SEPIC}}}}$$

(7)

$U_r$ is rectified and filtered to obtain a DC output voltage $U_{DC}$. An ideal rectification case is as follows:

$$U_{DC}={\displaystyle \frac{\pi }{2\sqrt{2}}}\cdot U_s$$

(8)

The relationship between the output voltage $U_L$ of the SEPIC circuit and the input voltage $U_{DC}$ and the duty ratio $D$ is as follows:

$$U_L={\displaystyle \frac{D}{1-D}}\cdot U_{DC}$$

(9)

Substituting $U_{DC}$ into the above equation gives the following:

$$U_L={\displaystyle \frac{\pi }{2\sqrt{2}}}\cdot {\displaystyle \frac{j\omega M{U}_{in}\cdot Z_{Z-SEPIC}}{\left[R_1\cdot Z_{Z-SEPIC}+{\left(\omega M\right)}^2\right]}}\cdot {\displaystyle \frac{D}{\left(1-D\right)}}$$

(10)

The output voltage $U_L$ and output current $I_L$ are obtained:

$$U_L={\displaystyle \frac{2\sqrt{2}}{\pi }}\cdot {\displaystyle \frac{j\omega M{U}_{in}\cdot R_L}{\left[R_1\cdot Z_{Z-SEPIC}+{\left(\omega M\right)}^2\right]}}\cdot {\displaystyle \frac{1-D}{D}}$$

(11)

$$I_L={\displaystyle \frac{2\sqrt{2}}{\pi }}\cdot {\displaystyle \frac{j\omega M{U}_{in}}{\left[R_1\cdot Z_{Z-SEPIC}+{\left(\omega M\right)}^2\right]}}\cdot {\displaystyle \frac{1-D}{D}}$$

(12)

It can be seen from Equations (11) and (12) that the purpose of adjusting the output charging current and voltage can be achieved by adjusting the PWM duty ratio of the SEPIC converter. The double closed-loop control of constant current and constant voltage is shown in Figure 3. The two loops work together to provide closed-loop control of current and voltage during the constant current and constant voltage charging stages, respectively. In the early charging stage, the system runs in constant current mode, the current loop dominates, and the voltage loop outputs voltage in the late charging stage. When the output voltage is close to the set constant voltage value, the system switches to constant voltage mode. At this time, the voltage loop dominates, and the current loop monitors the output current. Figure 4 shows the schematic diagram of constant current and constant voltage charging.

2.3. CC-CV Controller

In this study, the LT1512 chip is selected as the wireless CC-CV charge controller, combined with the SEPIC topology design, to achieve stable and accurate battery charging control. As shown in Figure 5 for the LT1512 chip external circuit, in addition to the traditional voltage feedback node, it also has a current detection feedback circuit for precise control of the output current of the charger with SEPIC topology. The current detection circuit takes ground as the reference potential and is completely isolated from the battery itself, thus effectively simplifying battery switching operation and system grounding issues. This design not only enhances system stability but also improves the reliability of the charging process. In addition, the topology allows for charging operations even when the input voltage is lower than the battery voltage, up to 30 V. This makes it ideal for charging control in the back end of a WPT system, which is able to cope with a wide range of power transmission environments and system stability requirements. Figure 6 shows the charging control board made for the wireless charging system.

3. Experimental Research

3.1. Experimental Platforms

In order to verify the charging performance of the system in seawater, an underwater experimental platform is built, as shown in Figure 7. The wireless charging system mainly includes two parts: transmitter and receiver. The transmit end consists of a signal generator, a pow amplifier, a primary compensation circuit based on an S-S topology, and a transmitting coil. The receiver consists of a secondary compensation circuit based on an S-S topology, receiving coil, rectifier circuit, and SEPIC converter. The transmitting and receiving coils are both 17-turn planar spiral coils wound by AWG16 wires with a diameter of 2.1 mm. The rectifier and filter circuit adopt a C3D06060 diode and 470 μF aluminum electrolytic capacitor. The SEPIC controller selects the LT1512 chip, which is responsible for sampling the output current and voltage and achieving the control of constant current and constant voltage charging. The water tank (size: 120 cm × 70 cm × 70 cm) used in the experiment is filled with seawater dissolved by water and sea salt, with a salinity of 3.5%. The conductivity of seawater is measured by a conductivity meter (Lichen Bangxi: LCDDB 1A, Shanghai, China) and adjusted to simulate the actual seawater environment. The measured conductivity is about 3.41 S/m, and the detailed system parameters are listed in

Table 3. The parameters of the WPT system.

Symbol	Quantity	Value
$L_p$	Primary coil inductance/μH	60.54
$L_s$	Secondary coil inductance/μH	60.15
$C_P$	Primary capacitance/nF	44.25
$C_s$	Secondary capacitance/nF	44.32
$R_1$	Sampling resistance/Ω	4
$r_1$	Inner diameter of the coil/cm	12
$r_2$	Outer diameter of the coil/cm	20

. The connections of the experimental setup are shown in Figure 8.

3.2. Experimental Results and Analysis

3.2.1. Determination of Mutual Inductance and Optimum Operating Frequency

In this experiment, the primary and secondary coils were firstly submerged in seawater coaxially and in parallel, the transmission distance was changed (spacing: 2, 4, and 6 cm), and an LCR measuring instrument was used to measure the mutual inductance between the two coils in seawater environment, as shown in

Table 4. Mutual inductance between two coils with different distances.

Distance/cm	2	4	6
${\mathrm{M}}_{\mathrm{s}\mathrm{e}\mathrm{a}}$/μH	23.75	17	14.24

. There is a significant difference between the mutual inductance values for transmission distances of 2 cm and 6 cm, indicating that the effect of transmission distance on the mutual inductance of the coils is more pronounced in the case of close transmissions. Subsequently, a radial deviation of 5 cm was introduced at transmission distances of 2 cm and 6 cm, respectively, and the measured mutual inductance values were 21.35 μH and 13.52 μH, respectively, which showed a smaller change in mutual inductance compared with no radial deviation. To further analyze the effect of the deflection angle, the mutual inductance was measured to be 24.35 μH for a transmission distance of 2 cm and a deflection angle of 15°, and 13.73 μH for a transmission distance of 6 cm and a deflection angle of 30°. The results show that the mutual inductance between the coupling coils in seawater environments has a lower sensitivity to a certain range of radial deviation and deflection angle. This characteristic shows the adaptability and advantage of WPT in a seawater environment.

The input signal to the wireless power transmission system is provided by a signal generator (Tektronix: AFG3102), which is capable of generating high-frequency sinusoidal signals with a wide and adjustable frequency range. The generated sine wave signal is amplified by a 50 W power amplifier and fed into the primary circuit of the WPT system. Since the resonant frequency of the WPT system is shifted in seawater, it is difficult to maintain the system in the optimal transmission state. Therefore, the optimal operating frequencies of the system at different transmission distances (2, 4, and 6 cm) were first determined by sweeping experiments as 96 KHz, 94 KHz, and 90 KHz, respectively.

3.2.2. Charging Performance of the System Under Different Conditions

Subsequently, the constant current and constant voltage values of charging were set, where the charging current in the constant current charging stage was set to $I_L$ = 1 A and the charging voltage in the constant voltage stage was set to $U_L$ = 5 V. The charging performance of the system at different distances was tested, and the changes in the charging current $I_L$ and charging voltage $U_L$ with load resistance $R_L$ were recorded in the experiment. As shown in Figure 9, the charging current can be stabilized at 1 A in the constant current stage at the initial stage of charging. In the constant voltage stage, the charging voltage can be maintained at 5 V. At the same time, the system can achieve the correct switching between constant current and constant voltage, and the system can achieve the function of wireless charging.

Taking a short distance of 2 cm and a long distance of 6 cm as examples, the transmission efficiency of the WPT system is calculated as shown in Figure 10. When the transmission distance is 2 cm, the transmission efficiency ranges from 60% to 70%, while at a transmission distance of 6 cm, the transmission efficiency range decreases from 32% to 44%, and this significant decrease indicates that the transmission distance has a significant effect on the transmission efficiency of the system. This trend is consistent with the pattern of variation of coil mutual inductance values with transmission distance shown in Table 4. With the increase in transmission distance, the mutual inductance between coils decreases significantly, which leads to a decrease in energy coupling efficiency.

Under the conditions of transmission distance of 2 cm and 6 cm, respectively, a radial offset of 5 cm is introduced to test the anti-offset performance of the system. As shown in Figure 11, under the condition of a transmission distance of 2 cm, the system has a better adaptability to the radial offset, and the constant current phase shows high stability, with the maximum fluctuation of the current of 0.05 A. Under the condition of transmission distance of 6 cm, the radial offset slightly weakened the coupling strength of the system, with a small fluctuation of the constant current phase and a current fluctuation of 13.1%, and the overall performance was good. It shows that the system is adaptable to the radial offset under the condition of short-distance transmission.

Next, the deflection angle is introduced to test the charging performance at a transmission distance of 2 cm and a deflection angle of 15° and at a transmission distance of 6 cm and a deflection angle of 30°, as shown in Figure 12. The charging performance of the WPT system has a better adaptation to the deflection angle of 15° under the condition of the transmission distance of 2 cm, and the maximum fluctuation of the current is 0.05 A. The system performance is slightly degraded under the condition of the transmission distance of 6 cm, and the current in the constant current phase fluctuates slightly, with a current fluctuation rate of 11.3%. This suggests that in long-distance transmission scenarios, coil design needs to be optimized, or compensation strategies need to be employed to reduce the impact of the deflection angle on the system performance.

From the above measurements, it can be concluded that the wireless charging system can achieve the function of constant current and constant voltage charging under the conditions of radial offset of 5 cm and deflection angles of 15° and 30° at 2 cm and 6 cm transmission distances. This is mainly due to the insensitivity of the coupling coil of the WPT system to a certain degree of radial offset and deflection angle in seawater environments, as well as the advantages of combining the small fluctuation of mutual inductance in the proximity transmission with the wide input range characteristics of the SEPIC topology.

According to the above measurement results, the wireless charging system can stably achieve the function of constant current and constant voltage charging when the transmission distance is 2 cm and 6 cm, the radial offset is 5 cm, and the deflection angles are 15 and 30. This is mainly due to the insensitivity of the coupling coils of the WPT system in seawater environments to a certain degree of radial offset and deflection angle, as well as to the advantages of combining small fluctuations of mutual sensing in close transmissions with the wide input range characteristics of the SEPIC topology.

In order to further verify the performance of the system, the charging situation is tested when the transmission distance is changed (2~6) cm, the radial offset is (0~5) cm, and the deflection angle is 0~30 during the charging process, as shown in Figure 13. The wireless charging system can be used under the conditions of the transmission distance being (2~6) cm, the radial offset being (0~5) cm, and the deflection angle being 0~30°. However, with the increase in transmission distance and the change of radial offset and deflection angle, the system performance decreases slightly, the fluctuation in the constant current stage increases slightly, and the volatility of the constant current phase increases slightly, with the maximum fluctuation of the current being 0.15 A with a fluctuation rate of 16.7%.

The above experimental results indicate that the wireless charging system is capable of achieving constant current and constant voltage charging functionality, but it is subject to slight fluctuations due to the influence of transmission distance. This suggests that, in practical applications, optimizing the coupling coil design or introducing compensation techniques, along with refining the SEPIC charging control, is necessary to enhance the system’s adaptability to dynamic changes, thereby ensuring the stability and reliability of charging performance in complex environments.

Considering the accuracy error of the measuring instruments (assumed to be 1%), a deviation analysis of the optimal operating frequency was performed with a ±1 kHz variation, and the change in transmission efficiency was measured to assess the system’s sensitivity to measurement errors. As shown in Figure 14, when the transmission distance is 2 cm, the transmission efficiency of the testing system at 95 kHz and 97 kHz frequencies shows a similar trend to that at 96 kHz, with a maximum deviation rate of 4.4%. When the transmission distance is 6 cm, the transmission efficiency at 89 kHz and 91 kHz frequencies follows the same trend as at 90 kHz, with a maximum deviation rate of 6.8%. This result indicates that despite slight frequency deviations, the experimental data still exhibit high consistency, validating the repeatability and reliability of the experimental results.

3.3. Heat Generation and Improvement of the System

Figure 15 illustrates the overall thermal distribution of the wireless charging system, including the power amplifier, rectifier, SEPIC charging management module, and coupling coils measured by a thermal infrared imager (CHAUVIN ARNOUX: CA75). As observed in the thermal imaging results, the power amplifier on the left side of Figure 15a exhibits the highest temperature rise, indicating significant power losses during high-frequency operation, primarily due to switching and conduction losses. Additionally, the heat concentration in the rectifier is mainly observed around the four diodes, suggesting that forward conduction losses and reverse recovery losses significantly impact the overall system efficiency. Similarly, the diodes in the SEPIC charging management module show noticeable heat accumulation. No localized overheating was observed in the coils, mainly due to the good thermal conductivity of seawater, which facilitates rapid heat dissipation and prevents the formation of localized hot spots.

The power losses of the modules are obtained by measuring their input and output voltage drops and currents, and the percentage of the power losses of each module in the total losses is shown in Figure 16, where the power losses of the rectifier account for about 13%, and the SEPIC charge control module accounts for about 7%. In the future, more efficient power components, synchronous rectification, and the replacement of conventional diodes with MOSFETs can be selected to reduce reverse recovery losses and improve rectification efficiency.

3.4. Comparison of Results and Exploration of Applications

Several studies and applications of DC-DC converters of WPT systems in

Table 5. Application of DC-DC conversion topology in WPT system.

Reference	DC-DC	Frequency	Out Power	Distance	Maximum Efficiency	Working Scenario
[31]	Buck	50 KHz	659.8 W	3 cm	86.3%	Seawater
[32]	Buck	80 KHz	410 W	100 mm	80%	Air
[33]	Boost	100 KHz	1.6 KW	10 cm	91.9%	Air
[34]	Boost	65 MHz	30.77 mW	/	86%	Human body
This work	SEPIC	96 KHz	5 W	2 cm~6 cm	70%	Seawater

, literature [31] is based on an inductive wireless power transmission (WPT) system with a Buck topology. When tested in seawater, the transmission efficiency was 86.3% at a transmission distance of 3 cm. However, due to the loosely coupled design of the system, the power loss is higher at longer transmission distances, leading to some limitations in system performance. Literature [32] also uses a Buck topology, but only constant current control is implemented. The system is able to provide 410 W of output power at a transmission distance of 100 mm, and the transmission efficiency fluctuates between 70% and 80%. This scheme is suitable for charging devices in air environments, but due to the fluctuation of transmission efficiency, it is less adaptable over long distances or in different operating environments. Literature [33], on the other hand, adopts Boost topology and introduces adaptive dynamic composite robust control (ADRC) into the secondary-side closed-loop control to achieve constant current output. The method is verified by simulation tests to verify its theoretical feasibility, but further experimental verification is still needed to confirm its effectiveness in practical applications. Literature [34] presents an 11-stage regulated charge pump circuit based on a DC-DC boost converter, where the system is able to step up the output voltage to 20 V at an input voltage of 3.5 V at a frequency of 65 MHz, with a transfer efficiency of up to 86%. The technology is particularly suitable for implantable biomedical devices with high energy efficiency and application prospects.

The wireless charging system proposed in this study is verified by prototype testing in a laboratory environment, and the charging performance under different transmission distances, radial offsets, and deflection angles are all able to achieve the constant current constant voltage charging function, thus verifying the feasibility of the system. In the future, in the real underwater application environment, further optimization and improvement are still needed to better adapt to the complexity of the real seawater environment. The following aspects can be mainly considered:

(1)

Optimization of the magnetic coupling mechanism: In a seawater environment, the dynamic action of ocean current may cause the relative position of the coil to deviate greatly, which can be achieved by using a U-lock structure or another mechanical alignment mechanism, thus reducing the influence of attitude change on the system performance;

(2)

Application of high-performance core materials: High permeability and high saturation flux density core materials are selected to improve flux guiding capability and reduce leakage loss, thus enhancing the overall transmission efficiency of the system;

(3)

Coil encapsulation and enhancement of environmental suitability: The use of corrosion-resistant, low-loss, and excellent insulating encapsulation materials, such as epoxy resin and methyl methacrylate MMA resin to encapsulate the coil in order to provide effective physical protection can effectively reduce the deterioration of the coil to protect the coil [35]. In addition, the multilayer composite encapsulation strategy can further improve the durability and adaptability of the system. For example, the outer layer adopts a superhydrophobic coating to reduce water adherence and lower fluid resistance; the middle layer adopts a high-strength polymer material to enhance abrasion resistance and mechanical strength; and the inner layer employs a low-loss insulating coating to minimize the impact on the electromagnetic coupling characteristics, which enhances the overall transmission efficiency and environmental adaptability of the system;

(4)

Power stage optimization: Practical applications require higher charging currents and voltages, which requires the design of higher-performance charge controllers to meet higher power outputs. SEPIC-LLC cascade topology or digital control technology is used to adapt to the demand for higher output voltage and current;

(5)

Optimize feedback mechanism: The underwater feedback mechanism is crucial to the stability of the WPT system. Optical communication is seriously affected by water absorption and scattering; signal attenuation is fast, and high alignment is required, which makes it difficult to adapt to dynamic underwater environments [36]. In contrast, acoustic communication is more stable under short-range and low-frequency conditions and is a potentially feasible solution for the transmission of feedback information between the primary and secondary sides underwater [37]. In addition, adaptive control based on energy parameters achieves communication-free feedback control by real-time monitoring of input voltage, current, and other parameters, reducing the dependence on traditional communication links. However, this method relies on high-precision sensors and still faces real-time and stability challenges in complex underwater environments.

4. Conclusions

The transmission characteristics of WPT in seawater and the charging control strategies are explored systematically. A charging control strategy adaptable to the seawater environment is proposed, which achieves the functionality of constant current and constant voltage charging.

In the experiment, the constant current and constant voltage charging performance under different transmission distances, radial offsets, and deflection angles are analyzed emphatically. The results show that the system has good charging stability and reliability under different transmission conditions. The maximum fluctuation of the current is only 0.04 A at a distance of 2 cm, and the maximum fluctuation of the current is 0.05 A when adding a radial offset of 5 cm and a deflection angle of 15°, respectively. The maximum fluctuation of the current is 0.08 A at a distance of 6 cm, and the maximum fluctuation of the current is increased to 0.13 A and 0.12 A when adding a radial offset of 5 cm and a deflection angle of 30°, respectively. It is further found that when the transmission distance, radial offset, and deflection angle are dynamically adjusted, the system still has a strong adaptive ability, and the maximum fluctuation of the current in the constant current phase is 0.15 A, and the maximum fluctuation of the voltage in the constant voltage phase is only 0.15 V, which is able to meet the charging requirements of the complex seawater environment.

In addition, the experiment also found that the heating of the system is mainly concentrated in the rectifier diode area, and the power loss is high. In the future, power devices with low conduction loss and high-temperature resistance can be used in practical applications to reduce the heat loss of the system and improve the overall charging efficiency and reliability.

Further future research can be carried out in the following areas:

(1)

Improve transmission distance and efficiency: optimize magnetic coupling structures, such as multi-layer windings, asymmetric structures, or coupling-compensated topologies, to enhance magnetic field utilization and reduce leakage losses. Adopt materials with high permeability and high saturation flux density to improve flux guiding capability and reduce transmission losses due to core saturation;

(2)

Enhancement of dynamic adaptability: intelligent control algorithms can be introduced to achieve real-time adjustment of transmission parameters during the charging process to adapt to more complex underwater environments;

(3)

Expanding multi-device wireless charging capability: researching multi-emitter and multi-receiver cooperative charging technology to support wireless energy sharing in underwater AUV group operations.

Through these optimizations and innovations, future underwater wireless charging systems will further enhance applicability and provide more efficient and safe energy security for applications such as deep-sea exploration, ocean monitoring, and underwater robotics.