This paper describes the reliability indices analysis of a battery electric energy storage consisting of multiple battery submodules for an electrical search and rescue helicopter. The whole system reliability assessment is based on the reliability evaluation of system components including individual battery cells. Regarding the high requirements to reliability and fault tolerance, including the strict limitations on the installation space and weight of the whole system, the accurate assessment and correct choice of the type and design features of such electric energy storage is extremely important. Considering the task of minimizing the weight and size of a storage device, in terms of a system approach, an optimal number of phases of the traction motor integrated with multilevel power inverter has been discussed and recommended.