Linear Power Supply Design Thermal Control
Put a linear 7812 regulator to make 12V for the fan. Running it directly from 18V can exceed fan ratings.
However, otherwise the design makes thermally no sense. If the 5V regulator provides 1A charging current (5 watts), it needs to dissipate 12-13 watts as heat when doing so.
If the junction temperature is allowed to go up to 125 °C, and the thermal resistance of the regulator between junction and case is 3°C/W, the case temperature must be held below 86 °C at all times. It really does not sound practical.