Because the charge protection is on. It is also possible that the data line does not match the current and voltage.
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The charging protective film refers to the transparent solar cell produced by Ubiquitious Energy of the United States. Attach the device to the screen of an iPad or phone and charge it while you continue to use it, giving you near-unlimited battery life outdoors.
This situation is normal, due to the charge management function of Apple's MagSafe external battery. Both the MagSafe battery case and the iPhone can heat up when charging because they're left plugged in for a long time. Therefore, in order to extend the service life of the battery, the charging may be limited to more than 80%, and the power may be automatically cut off when it is charged to 80%.