Usually we confirm the battery condition by discharging the battery. In this way, we can handle the actual capacity, but it is very time-consuming. After related research, we found that the internal resistance of the battery will increase because the battery capacity will decrease with the increase in the use time. If we regularly monitor the internal resistance of the battery and compare it to the initial internal resistance, it will be easy to detect changes in the battery's performance and identify the faulty battery.