A back-up power supply system for use in conjunction with a primary electrical power source, the system comprising: a primary power sensor configured to detect a characteristic of primary electrical power provided by the primary electrical power source to a load, the primary power sensor being configured to output a threshold signal, the threshold signal indicating a discharge mode or a charge mode based on the characteristic detected; a system controller; a battery system comprising a plurality of rechargeable battery cells connectable in series, each cell comprising: (i) a cell controller; (ii) at least one ready state sensor coupled to the cell controller for sensing one or more conditions of the cell for determining if the rechargeable battery cell is in a ready charge mode for charging, and if the rechargeable battery cell is in a ready discharge mode for discharging; (iii) a cell bypass switch coupled to the cell controller, the cell bypass switch being switchable between a normal state for enabling said cell to be electrically coupled in said series and a bypass state bypassing the cell within the series; and (iv) a power switch switchable between a closed state electrically coupling the battery cell within the series to communicate power between the battery and the load and an open state electrically decoupling the battery cell from the series; wherein the system controller is coupled to each cell controller and each cell controller is configured to receive communication signals from the system controller; wherein the primary power sensor is electrically coupled in parallel to the system controller and to each cell for transmitting the threshold signal directly thereto, thereby avoiding passing the transmission of the threshold signal to the cells through the system controller; wherein each cell is configured to switch its respective power switch to said closed state in response to receiving the threshold signal; wherein each cell controller is configured to switch said bypass switch between said bypass state and said normal state in response to said at least one ready state sensor. a primary power sensor configured to detect a characteristic of primary electrical power provided by the primary electrical power source to a load, the primary power sensor being configured to output a threshold signal, the threshold signal indicating a discharge mode or a charge mode based on the characteristic detected; a system controller; a battery system comprising a plurality of rechargeable battery cells connectable in series, each cell comprising: (i) a cell controller; (ii) at least one ready state sensor coupled to the cell controller for sensing one or more conditions of the cell for determining if the rechargeable battery cell is in a ready charge mode for charging, and if the rechargeable battery cell is in a ready discharge mode for discharging; (iii) a cell bypass switch coupled to the cell controller, the cell bypass switch being switchable between a normal state for enabling said cell to be electrically coupled in said series and a bypass state bypassing the cell within the series; and (iv) a power switch switchable between a closed state electrically coupling the battery cell within the series to communicate power between the battery and the load and an open state electrically decoupling the battery cell from the series; (i) a cell controller; (ii) at least one ready state sensor coupled to the cell controller for sensing one or more conditions of the cell for determining if the rechargeable battery cell is in a ready charge mode for charging, and if the rechargeable battery cell is in a ready discharge mode for discharging; (iii) a cell bypass switch coupled to the cell controller, the cell bypass switch being switchable between a normal state for enabling said cell to be electrically coupled in said series and a bypass state bypassing the cell within the series; and (iv) a power switch switchable between a closed state electrically coupling the battery cell within the series to communicate power between the battery and the load and an open state electrically decoupling the battery cell from the series; wherein the system controller is coupled to each cell controller and each cell controller is configured to receive communication signals from the system controller; wherein the primary power sensor is electrically coupled in parallel to the system controller and to each cell for transmitting the threshold signal directly thereto, thereby avoiding passing the transmission of the threshold signal to the cells through the system controller; wherein each cell is configured to switch its respective power switch to said closed state in response to receiving the threshold signal; wherein each cell controller is configured to switch said bypass switch between said bypass state and said normal state in response to said at least one ready state sensor.