## 2 Hardware Description of the Proposed Battery Diagnostic System  
Figure 1 depicts the circuit architecture of the proposed diagnostic system, which consists of a modified class E resonant circuit, a micro-controller (MCU: dsPIC33 FJ64 GS606), a USB(universal serial bus) UART (universal asynchronous receiver/transmitter) module (FT-232), an auxiliary power circuit, a signal (voltage, current, and temperature) capturing circuit, and a DC (direct current) level offset circuit. In this study, the modified class E resonant circuit is a key part of the proposed electronic load, which can provide a high-frequency sinusoidal current via the resonant technique in order to simulate the practical work conditions for catching the related characteristics of the battery module. The MCU shown in Figure 1 not only provides the gating signal of the MOSFET (metal-oxide-semiconductor field-effect transistor) in the class E resonant circuit but also receives all the sensed signals by the capturing circuit and then communicates with the human-machine interface (HMI) via the USB UART module (FT-232). The HMI is used to preset test conditions and display  
the tested results, including battery module parameters, battery protection conditions, baud rate, and SOC/SOH. The most important feature of the proposed high-frequency AC load is that the energy extracted from the battery module during the diagnostic process can be recycled. When the diagnostic process begins, the battery module is connected to the modified class E resonant circuit to discharge with a resonant-type current. During the diagnostic process interval, the capturing circuit can detect some dynamic characteristic parameters of the battery module to calculate the corresponding SOC/SOH. Moreover, the proposed AC electronic load can provide sinusoidal load current, and the direction of the load current drawn from the battery module will be reversed when the positive semi-cycle ends. After the sinusoidal load current reverses into the negative semi-cycle, the original load current can charge the battery module along the original path based on the resonant characteristic of the modified class E resonant circuit. In other words, if all the component impedances can be ignored, there is almost no energy loss during the battery module test process. As shown in Figure 1, the modified class E resonant circuit plays a critical role within the proposed system. In practice, the proposed high-frequency AC electronic load can be used to generate a periodic load current which has wide slope variations from positive to negative infinity for the battery module test. Furthermore, the sinusoidal load is capable of supplying a dynamic load current within the positive semi-cycle for diagnosing battery characteristics and recycling the extracted energy within the negative semi-cycle. In order to provide such a load current, a modified class E topology is adopted as the main circuit of the sinusoidal load. The MCU is the control core of the system which can output the corresponding PWM (pulse width modulation) signal to drive the modified class E resonant circuit according to the test conditions, where the test conditions are preset in the HMI. Then, both the capturing values and the calculated values such as SOC/SOH will be displayed in real-time. The high-frequency AC electronic load can generate a bidirectional sinusoidal load current. The wide slope variation characteristics of the sinusoidal waveform can be used to estimate the dynamic parameters of the battery module; hence, the \(\Delta V\) and \(\Delta I\) from the battery module can be obtained in the test operation and the dynamic impedance value (\(\Delta V\)/\(\Delta I\)) can be calculated. The dynamic impedance value can be used to estimate the SOC/SOH of the battery [26]. This method is different from the traditional EIS (electrochemical impedance spectroscopy) technique [31] because it does not need to specify an operating frequency or complicated circuit model.