**3.1 Measuring method of capacitance and internal resistance**

Before and after the voltage hold test, charging and discharging were performed at a constant current of 6 A and a voltage of 2.5 V using a stabilized DC power supply (PAN60-6A) and an electronic load device (PLZ603WH) by Kikusui Electronics Corp. Using the energy conversion method, the capacitance was calculated based on the obtained charge and discharge waveforms, and the internal resistance was calculated based on the voltage drop.
