SOH is the lifetime state of the battery and is primarily used as a parameter to estimate the battery lifetime scale. SOH is defined as 100% for the initial unused battery, and when the battery capacity reaches 80% of the initial capacity owing to the continuous charging and discharging of the battery, that is, when SOH reaches 80%, the battery life is defined as the end of the battery [4]. Therefore, reliability of the battery can be increased through the prediction and estimation of the life of the battery, and safety accidents can be prevented.

However, owing to the complex chemical reaction process of the battery, it is difficult to accurately predict SOH; consequently, related ongoing studies are being conducted. Among the methods for estimating SOH, there is a method for estimating SOH based on the consumed capacity of the battery, and it can be expressed as follows [5].

In Equation (1), SOH is represented as a value that is obtained by subtracting the amount of change in the SOH (Equation (2)) from the initial lifetime state, SOH(0) (i.e., 100%). The parameter N indicates the number of charging and discharging cycles to the end of life of the battery, Q_new is the capacity of the initial battery, and P_b is the amount of power used. The above SOH measurement method is calculated by only considering the capacity of the battery; therefore, modeling of the battery is significantly important.

The battery used for power supply of the electric device consists of series or parallel connections of several battery cells, and the energy stored in each cell is converted into electrical energy through oxidation and reduction reactions.

As the load current increases, the total charge of the battery decreases. Then, the ratio of the load current to the battery capacity is called the discharge rate. The discharge time of the battery varies according to the discharge rate. As the discharge rate increases, the discharge time becomes faster. In addition, there is a difference in battery usage time in the case of continuous discharge and intermittent discharge, where a pause period is provided in the middle of discharge, while discharging batteries of the same capacity.

When a period of resting is provided during the discharge process, the battery voltage, which was lowered from the existing continuous discharge, increases during the period of rest, resulting in an increase in the capacity of the battery. This is called the recovery effect of the battery.

The reason for the recovery effect is that the electrolyte that exists far from the electrode can stably transfer electrons to the nearby electrolyte during the rest time when the current is not drawn from the battery; consequently, more charges can be drawn from the battery [6].

Pulse control methods include pulse width modulation (PWM) and pulse amplitude modulation (PAM). The PWM method does not change the size of the smoothed DC voltage but changes the output time of the voltage on the pulse to change the equivalent voltage. PAM achieves constant width and cycle of the pulse and changes only its amplitude according to the signal wave.

In this study, pulse discharge was performed using PWM method; moreover, based on the voltage value that was outputted through pulse width control, the output was maintained as a high value for a certain ratio and for the remaining period, the output was a low value to produce a pulse waveform output.

Recently, active studies performed battery discharge in the form of currents such as pulse, triangular, and trapezoidal waves, and the discharge characteristics of batteries were analyzed [7]. Pulse discharge method is a method of discharging pulsed currents at discharge. It was determined that lithium-ion batteries increase the efficiency by approximately 1.3% and discharge capacity by 1.32% when compared to a method with conventional CC discharge [2].

In this paper, based on the pulse discharge study, the influence of the discharge current shape on the characteristics of the battery was analyzed, and a basic study of the power system design was conducted to improve the battery lifetime.

Total harmonic distortion (THD) is a measurement of the harmonic distortion present in a signal, defined as the consensus ratio of the forces of all harmonic components to the forces of the basic frequency. It is formulated as follows.

In wireless communication, devices with low THD tend to cause less unintended interference with other electronic devices. As harmonic distortion tends to broaden the frequency spectrum of output emission from the device by adding multiple signals of the input frequency, devices with high THD are not suitable for applications such as spectral sharing and spectral sensing. Low THD in the power system implies low peak current, low heat generation, low electromagnetic emission, and low core loss of the motor.

The power system circuit proposed in this paper is illustrated in Figure 1. Figure 1 shows a part of the circuit that can inject any waveform into the battery and a circuit with a step-up converter on the load side. Discharge is performed in the form of a waveform injected into the battery. C block is inserted for the repetition of charging and discharging, and the circuit is designed to charge when the SOC value of the battery reaches 0.3 and discharge when the SOC reaches 0.8. The charge and discharge current amount for charging and discharging had a root mean square (RMS) value of 2 A of discharge current

Figure 1: 
Proposed circuits of power system

Existing battery charging/discharging circuits primarily use step-down converters to lower voltages to drive electrical devices. These devices are already applied to the pulse discharge based on the principle of the step-down converter itself; moreover, it is difficult to study the characteristics of the battery according to the discharge waveform. Therefore, in this study, a discharge circuit was designed using a step-up converter. In addition, the output voltage was maintained constant through feedback control for smooth SOH measurement.

The simulation algorithm is illustrated in Figure 2. Simulation starts from the initial SOC value of 0.8 (80%) of the battery, and the current SOC is checked to start the charging or discharging process. After the start of charging or discharging, the voltage and current of the battery are measured at each sampling time to calculate the power used in Equation (1). After the energy capacity is estimated by using the calculated power, the SOH reduction amount is measured and the current SOH is measured. This is repeated until SOH reaches 80%. When the SOH reaches 80%, the simulation is completed and the results appear in graphs.

Figure 2: 
Flow chart for Simulation

Figure 3 illustrates a circuit for measuring the SOH of the battery; moreover, the SOH measurement algorithm expressed in Equation (1) is designed as a circuit. The voltage and current of the battery are measured and the power value is calculated and then integrated. Then, the integrated value is divided by “2,” “N,” and “Q_new” and the SOH variation is calculated. The SOH variation was subtracted from the initial SOH value and represented as a percentage value.

Figure 3: 
Configuration circuits for state-of-health (SOH) measurement