In this work the development, tests and thermodinamic numerical simulations on a flat cell of an innovative Sodium-metal-halide battery has been presented: a prototype of a single cell was designed, realized and eventually extensively tested at CNR-ITAE. The following performances were recorded during the test stage: 76% energy efficiency and 84% faradic efficiency. Similarly to tubular cells commercially available, the flat cells constituting the battery have to be placed into an insulated box equipped with a proper heating system because of their high working temperatures. To this aim, a numerical thermal analysis by using the software COMSOL MULTIPHYSICS has been carried out to evaluate the best solution to guarantee a reduced thermal loss and reasonable warming up time.