## 3 Results and Discussion
On the contrary, for the case of 6C pulse current charging, 723.58 mAh was charged by the pulse charging in the same cycle, recording about 95% of the charging capacity of the 10 th cycle, and 546.1 mAh was charged before reaching the limit voltage of 4.2 V. As explained above, the pulse current charging on the lithium-ion battery caused relatively little increase in resistance and decrease in capacity of it, and the results support the aforementioned deterioration mitigation. It was confirmed that the charging proceeds very stably by repeated voltage drop due to the pulse current even after the 4.2 V limit voltage, which is explained again by comparing the charging time in Figure 6.  
As mentioned above, in the current research, the charge and discharge test of the commercial cylindrical battery was designed so that the amount of charging was limited to 3 Wh. This amount of current applied per hour is considered to allow the same capacity and to maintain the SOC range theoretically. The comparison of charging time in Figure 6 helps infer the effectiveness of the charging method on the battery. In the case of the 3C constant current charging on the battery, it appeared that the electrochemical insertion and desorption reactions of lithium are limited due to the deterioration caused by various factors, as discussed above. In addition, it showed that the charging time increased quite  
Figure 5: Charging start voltage and capacity in accordance with charging methods. Figure 6: End voltage and elapsed time for each method. rapidly as the cycle increased [13]. Specifically, it took about 10 min 30 s for the initial cycling of the battery, but it drastically increased after the 100 th cycle and took about 72 min for the 200 th charging process. After that, the battery reached a state where it was no longer possible to charge it in the 213 th cycle. On the contrary, in the case of 6C pulse current charging, the initial charging took 6 min 20 s, while the 400 th charging took approximately 8 min 20 s. These values for the charging process revealed that they did not deviate significantly from the theoretical time required to charge 30% of the full capacity in an ideal environment, which was estimated to be around 6 min. Since there was only about a 0.3 s/cycle increase in the charging time up to the 400 th cycle, it could be interpreted as the pulse current charging was very effective, not only for fast charging but also for maintaining the power rate while keeping the stable battery operation. The end voltage of the charging increased from 3.91 to 4.21 V at 3C and converged to 4.2 V as the number of cycles increased, because it was the fixed value as charge limit voltage. The end voltage of 3C constant current charging reached the charge limit voltage before 100 cycles, displaying a very steep slope. On the other hand, in the 6C pulse current charging method, the end voltage gradually increased. The extrapolated lines of the ending voltage after reaching 4.2 V implied the slope of 3C constant current charging was steeper than that of 6C pulse current charging because the internal resistance due to battery degradation was larger under the 3C constant current charging case. Finally, EIS measurements were performed after every 50 cycles for both cases to compare the changes of internal impedance, as presented in Figure 7a,b. After the end of first cycle (the first charging and discharging), the impedance spectra were similar. However, in the case of 3C constant current charging, the impedance curves, which were measured at the end of 1, 50, 100, and 150 cycles, showed great increase in their values depending on the number of cycles. The equivalent circuit model [14] constructed based on the result from the EIS measurement can be expressed as the electrolyte resistance (R\({}_{\text{s}}\)), the first semicircle (R\({}_{1}\) and C\({}_{1}\)), the second semicircle (R\({}_{2}\) and C\({}_{2}\)), and the Warburg impedance (W\({}_{0}\)).