The coupling between a natural-convection thermosyphon loop and a thermal storage device is analyzed numerically for a charging process in which energy is added to the system at a fixed rate. Since energy accumulates in the storage component, the driving buoyant force is continually altered and the behavior is inherently transient. The undesirable consequence is a less than uniform charge of the storage volume. Relevant dimensionless parameters are identified which enable improvement in the shape of the charge profile. The results indicate that a more uniform charge profile is obtained by targeting the frictional losses to a flow regime most strongly proportional to flow rate and by decreasing the relative vertical height over which heat input occurs.