Abstract: A battery energy storage system, comprising one or more rechargeable battery packs, each including a plurality of battery cells; and a battery management system (BMS) including a first circuitry configured for battery pack protection management, and a second circuitry configured for capacity management and to perform an auxiliary service while performing the capacity management. The first circuitry monitors an operating parameter of at least one cell of the plurality of cells to detect an occurrence of the operating parameter being outside a predetermined safe operating range. The second circuitry monitors cell voltage balancing of a plurality of cells of the battery pack to detect an occurrence of a cell being in a state of unbalance, and further monitors proper operation of the first circuitry. In response to detection of a faulty operation of the first circuitry, the second circuitry is configured to control the battery pack protection management.