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30w fast charging 27w mobile phones should be no problem, and the charging head with large power can fully meet the charging needs of small power mobile phones. However, whether it can achieve fast charging, not only depends on the power, but also depends on whether the fast charging protocol matches. At present, the commonly used fast charge protocols are as follows, usb pd, Qualcomm qc, Mediatek pe, oppo Fast charge and Huawei Fast charge and so on. Some chargers support more than one fast charging protocol.
The 30w fast charge cable can be used, which is compatible, but the 30w fast charge can only play the 27w fast charge power supported by the mobile phone.