The inclusion of a battery system for a diesel mechanical short sea ship was investigated. The main benefits of the battery were assumed to emerge from shaving thruster generated power peaks, rather than starting additional generating sets to accommodate the power demand and additionally from replacing a diesel engine as a reserve power source. To support the analysis, an auxiliary engine power output dataset of a roll-on/roll-off passenger ferry operating in the Baltic Sea was acquired. Required capacity for the battery system was derived by considering power availability requirements and battery safety margins for performance deterioration. A multi-period mixed-integer linear programming model was constructed to derive a globally optimal power management strategy for the auxiliary engines and the battery, with the goal of minimizing the battery installation total cost. The battery system was found to reduce fuel oil consumption by 257.5 tons annually due to improved auxiliary engine efficiency alone. Furthermore, the battery system total cost advantage was found to vary from -€0.61 to €2.82 million during the ten-year investment period, depending on fuel oil and battery system costs applied in the modeling. For the studied case ship, the hybrid electric topology was concluded to be economically feasible.