sustaining
Generally speaking, the wattage difference is not large enough to charge, but the wattage difference is large, it is easy to break,
So the 18-watt flash charger can use the 20-watt charger, so you can use it and it will be fine, because they are not much different.
Yes, the charging current is determined by the charging agreement between the charging bank and the charging head.
This is a problem of fast charging and slow charging, fast charging may be able to complete the charge in an hour, slow charging may take several hours.
unsupported
Fast charge will only be backward compatible, not upward compatible!