This paper proposes a new battery management system (BMS) for a series string of battery cells. The proposed control structure is based on a duality of the adaptive droop control theory used in grid / microgrid connected sources. The presented BMS control structure yields higher reliability compared to the conventional ones due to its decentralization features and its communication-less ability. The adaptive droop control duality is achieved by adjusting the DC/DC converter virtual admittance value according to the associated battery State Of Charge (SOC). Thus, balanced power sharing can be achieved between the connected battery cells during both charging and discharging conditions. The proposed BMS architecture and control structure are validated both using two case study results and experimentally.