Various aspects of the present disclosure are directed to monitoring battery cells. In accordance with various embodiments, an energy storage cell apparatus includes a current injection circuit that separately inject current into individual ones of a plurality of battery cells that store energy, and an impedance-detection circuit detects an impedance characteristic of each of the plurality of battery cells in response to the injected current. A filter circuit receives impedance data regarding the detected impedance characteristic and separates low-frequency components of the impedance data from high-frequency components of the impedance data. A memory circuit stores data corresponding to high-bandwidth data including both the low-frequency components and the high-frequency components, and an access circuit provides the low-frequency components for the plurality of battery cells to a battery pack controller. The access circuit further accesses and provides the stored high-bandwidth data for a subset of the individual battery cells to the battery pack controller based upon available bandwidth for communicating the high-bandwidth data.