There may be the following reasons for the slow charging of mobile phones:
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Slow charging when the charger or data cable is not plugged in or the interface is not clean: When charging, the charger, data cable, and mobile phone charging interface are not in contact, which may lead to slow charging, unable to enter the fast charge or unable to charge, please plug and unplug the charger several times. Check whether the connectors and jacks are clean. If dust is found, it is recommended to wipe it with alcohol or clean it with a brush. If alcohol wipe, need to wait for alcohol to dry, and then re-charge to confirm.
It's possible the system's self-protection is on.
Because the phone is fast charging at the beginning, it is easy to cause the phone to heat up, so when the power reaches more than 30%, it will reduce the speed, in order to maintain the temperature, it will slow down.