Abnormal, Apple 12 charging will heat up the reason, the battery in the charging state, this circuit is responsible for energy distribution, especially while charging this scenario, because in addition to charge management, but also to dynamically allocate energy to different components, such as processors, speakers, radio frequency, modules, etc., the load becomes larger, the chip will heat up. Excessive temperature may cause aging or even damage of electronic components.
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When the temperature is high, other components will cooperate to cool down. For example, the processor will slow down, the brightness of the screen will be lowered, and it may even remind you of the overheat protection and ask you to pause using the phone.
Probably because the background program is not closed