for the following reasons
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1, the charger voltage is insufficient, the charging is slow, the battery is not fully charged.
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2, the battery deteriorates, the battery use time is more than two years, the battery internal resistance is smaller than the large capacity, the charging terminal voltage is full but the battery voltage is not full, the display power becomes less.
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3, the battery internal resistance produces voltage drop, charging will not be fully full, if the battery use time is short, that is, the battery capacity becomes smaller and the internal resistance becomes larger.
Because of the charging protection function, with the increase of power, the input current slowly decreases, and the sufficient power per unit time will be less and less. So you will find that at the beginning of the charge, the charge increases very quickly, but it takes a long time to charge from 99 to 100. At this point, if you pull it off and plug it in, you can let the phone think that it has just started to charge and it is quickly filled. But it's not safe.