15w fast charge 18w charger can not, this is because the rated voltage and rated power of 15w fast charge than the rated voltage and rated power of 18w charger are smaller, when the high voltage of 18w charger is used to charge 15w fast charge, the 15w fast charge will explode, so 15w fast charge 18w charger can not be used. You should try to use a charger that matches the 15w fast charge.
It is not recommended that the power difference also has an effect on the mobile phone