It can be used.
The charging speed should only reach 18 watts charging head, can provide such a quantity that some people say this 120 watts, this is basically can not be achieved, can only achieve the latter.
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You can't use an 18-watt charger on a 120-watt fast charger. The power is all wrong. You can't even charge it. Our phones don't even charge.
In general, as long as the charging voltage is consistent, it is theoretically possible to use an 18W charger to charge a 120W mobile phone, but the charging current will be much smaller, and if it is full, the charging time will be much longer.