The lifeline of a smartphone is a battery. All contacts with friends and family are social media, and if you run out of battery, you won't be able to get in touch. Especially if you pay for trains and buses on your smartphone.
If that happens, the battery-eater culprit is of concern. On Android, you can check apps that consume a lot of battery on the “Battery” screen of the “Settings” app, so if you can identify apps that are likely to be problematic, you can take measures such as killing them.
However, system apps (processes) cannot be inadvertently taken care of, regardless of general apps. “Mobile network standby” (“cell standby” depending on the version of Android OS) is a representative example, and it may occupy most of the power consumption, but it cannot be forcibly terminated because it is an app that manages mobile communications.
Although it is a mobile network standby, it has a role to secure the connection to the mobile phone line, and when it is not connected, it repeatedly searches for connectable base stations. If you stay in a place where 3G / 4G radio waves are difficult to reach, that is, a place where the number of antennas is small, the battery consumption will increase accordingly.
Note that this problem basically occurs with 3G lines, and it is said that this problem does not occur with SIM cards that support “VoLTE”, which uses voice communication for data communications. In overseas specifications, the search target line / prioritized connection line may not be set for Japan. In this case, the mobile network standby consumes power wastefully. Because it is a problem involving both the terminal and the SIM card, it may be more practical to take measures such as switching to airplane mode if necessary if both cannot be replaced immediately.