NOVELTY - Battery management system comprises: a controller (102) comprising a processing device configured to transmit and receive control signal, and downstream and upstream communication links; first sub-controller connected to the communication links, where the first sub-controller is configured to receive first power unit control signal from the controller; and second sub-controller connected to first sub-controller and configured to receive second power unit control signal from the first sub-controller.    USE - The battery management system is useful for complex battery system, hybrid vehicle, computing device, electric vehicle or power supply to a residence.    ADVANTAGE - The battery management system: exhibits flexibility, ease of expansion and cell replacement; is independent from cell or pack voltages, charge and discharge currents, system capacity or battery chemistry; is utilized in an environment or device where large voltage and stored electricity is needed; allows removal, replacement, addition and/or exchange of power units with minimal disruption to the system and association of any number of power units and battery management sub-controllers with the controller without programming or configuring the controller; is incorporated into varying battery pack configurations without minimal customizing to the components; actively controls power supplied to each battery management system sub-controller, disables power to the battery management system sub-controllers, when the system is in an idle state or a fault state and communicates across large potential differences of the power units of the battery system; and balances the battery modules, thus providing best performance.    DETAILED DESCRIPTION - Battery management system comprises: a controller (102) comprising a processing device configured to transmit and receive at least one control signal for configuring a battery system, a downstream communication link and an upstream communication link; a first sub-controller connected to the downstream and upstream communication links, where the first sub-controller is configured to receive at least one first power unit control signal from the controller on the downstream communication link and, configures first many of battery cells, in response to the first power unit control signals; and a second sub-controller connected to the first sub-controller and configured to receive at least one second power unit control signal from the first sub-controller and, configures second many battery cells in response to the second power unit control signals. INDEPENDENT CLAIMS are also included for:    (1) controlling (I) the battery system having many power units, comprising: receiving the first control signal from the controller on the first downstream communication link at a first management control sub-controller associated with a first power unit (104) of the battery system, where the first management control sub-controller is connected to the controller by the first downstream communication link; configuring at least one setting of the first power unit in response to the received first control signal; receiving at least one performance indicator from the first power unit; storing the at least one performance indicator of the first power unit in a computer-readable storage device associated with the first management control sub-controller; and transmitting a second control signal on a second downstream communication link to a second management control sub-controller associated with the second power unit (106) of the battery, where the second control signal is substantially similar to the first control signal; and    (2) management sub-controller of the power unit of the battery system comprising: a processor; a downstream communication receiver configured to receive at least one control signal from the downstream communication link; an upstream communication transmitter configured to transmit at least one control signal on the upstream communication link, where the downstream communication receiver and upstream communication transmitter are electrically isolated from the corresponding downstream link and upstream link; a computer-readable device with instructions stored on it, cause the processor to perform the operations, when executed; obtaining at least one performance indicator of the power unit of the battery system; storing the at least one performance indicator in the computer-readable device; and transmitting the at least one performance indicator to the upstream communication transmitter for transmission on the upstream communication link.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic block diagram illustrating complex battery system architecture, including the battery management system.    Battery system architecture (100)    Controller (102)    Power unit 1 (104)    Power unit 2 (106)    Power unit X (108)