Power-backup capabilities are provided by implementing a variety of different methods, systems and devices. According to one such implementation, a data storage device stores data in response to data accesses under the control of a memory control circuit. A solid-state memory circuit and a volatile caching memory circuit provide the memory control circuit with access to a set of common data. A power-reservoir circuit includes two or more capacitor cells that respectively hold charge to provide operating power to the data storage device to permit transfer of the data from the volatile memory circuit to the solid-state memory circuit in the event of a power loss. A detection circuit is connected to a center tap between the capacitor cells and uses the tap to detect characteristics of the cells relative to one another, and to provide an output that can be used to characterize the cells' electrical characteristics relative to one another.