Secondary battery modules require efficient thermal management to maintain their optimal operating temperature, especially with high temperature batteries. In order to obtain spontaneous and efficient spatial temperature equalization, natural convection in the interior of the module needs to be carefully designed. In this study, average Nusselt numbers induced by natural convection are calculated in various hypothetical enclosure geometries, i.e., triangular and rectangular enclosures, in order to formulate a time-efficient transient equivalent thermal network model. A vertical cylinder with multiple lateral baffles is placed in each enclosure representing a planar high temperature battery stack. Based on the configuration, average Nusselt numbers have been obtained by varying the Rayleigh number and the radial space gap between the baffle and enclosure. The results show that the Nusselt number remains constant at low Rayleigh numbers (conduction-dominated regime) whereas it increases with the Rayleigh number at high Rayleigh numbers (convection-dominated regime). Regarding the enclosure geometries, the triangular enclosure has advantages in convection heat transfer. Using the average Nusselt number correlation with the Rayleigh number and space gap, a transient thermal network model has been developed that enables substantial calculation time reduction. Calculation with the developed model is 25 times faster than the fully elliptic three-dimensional calculation for corresponding configurations in good agreement with temporal heat flux variations on the cylinder surface.