Design methodology of the compact size charging system for emergency uses is investigated in this paper. The system consists of a gearing unit, generator, charger and battery cells. The system electrical model has been introduced and a 3-phase axial flux surface mounted PM generator with concentrated winding is picked as a generation unit. The basic equations needed for the generator design are presented as well as general considerations and constraints for the designs. After validating the design equations using Finite Element Analysis, sizing equations are used to design different machines. The resulted designs are compared based on main characteristics including output power, delivered power to the battery, and generator efficiency. Finally, the performances of valid designs in the system model are analyzed on the wide speed range. The proposed methodology could be used for portable power generation units design for applications such as rescuing, power system outage, camping, so on.