An in-line rapid impedance measurement device and methods of performing in-line rapid impedance spectroscopy including a switch network operable under a switching algorithm useful to rapidly perform targeted impedance measurements to assess state of health, defect or failure of battery packs or modules having a plurality of electrochemical cells interconnected in series or parallel and to isolate individual cells within a pack or module to distinguish between non-repeatable anomalous impedance measurements from repeatable deviant impedance measurements and identify the source of deviant impedance measurements.