Traditional battery energy storage systems (BESS) are based on the series/parallel connections of big amounts of cells. However, as the cell to cell imbalances tend to rise over time, the cycle life of the b. ••Modular and traditional battery systems' reliability analysis••. The penetration of renewable energy sources into the main electrical grid has dramatically increased in the last two decades. Fluctuations in electricity generation due t. 2.1. Reliability model of a BESSIn order to evaluate the BESSs' reliability, it is necessary to deeply analyse the failure rate of each of the components. All these items are consid. After analysing the design characteristics and the reliability estimation methodology in 2.1 Reliability model of a BESS, 2.2 Factor importance analysis methodology, this Section presents. In view of the difficulty for defining the design factors of a BESS, a reliability analysis method including a factorial regression has been developed. By using this strategy, a fa.