A power reserve apparatus is disclosed. In one embodiment, the power reserve apparatus comprises a first module including a first set of battery cells and a first inter-cell balance adjustment unit configured to use passive balancing to reduce voltage variance among the first set of battery cells. The power reserve apparatus also comprises a second module including a second set of battery cells and a second inter-cell balance adjustment unit configured to use passive balancing to reduce voltage variance among the second set of battery cells. The power reserve apparatus further comprises an inter-module balance adjustment unit configured to use active balancing to reduce voltage variance among the first and second modules.