Abstract: An energy storage system for supporting dual electrical functions of a vehicle includes an energy storage unit having a plurality of energy storage modules connected in series, a plurality of sensing units for sensing state of charges of the plurality of energy storage modules, and a pair of primary voltage terminals. The series connected plurality of energy storage modules is connectable across the pair of primary voltage terminals during a key-on state of the vehicle to supply energy storage power at a first voltage level to support primary electrical functions of the vehicle. The energy storage system is further configured to select a subset of the plurality of energy storage modules during a key-off state of the vehicle to connect across a pair of secondary voltage terminals using a switch network to supply energy storage power at a second voltage level.