In the smart grid field, renewable power plants coupled with electrical storage systems are becoming a promising challenge to optimize the exploitation of renewable sources in order to maximize self-consumption, thus minimizing the exchange with the grid. Sodium/Nickel chloride batteries are considered optimal storage systems, due to their limited environmental impact, high reliability and specific energy, and reduced maintenance. The present paper describes a mathematical model elaborated to calculate the electrical parameters, during the discharge phase, of a Sodium/Nickel chloride galvanic cell contained within an electrical storage system designed for smart grid applications. Some results of the model have been compared with values achieved in experimental tests carried out on the FZSoNick electrical storage batteries of the Genoa University Smart Polygeneration Microgrid.