The impedance response of LiCoO2 broken vertical bar C battery cells is exploited to develop a single-frequency (500 Hz) diagnostic tool to detect overcharge abuse. The impedance data are reversible at all states-of-charge during normal charge/discharge operations (2.8-4.2 V), with little change in Z' or Z '' at 500 Hz even after 200 cycles. However, a single 4.6 V overcharge shows clear differentiation in the impedance values at 500 Hz, which is responsive to the cell passivation layers, with the Z' and Z '' first decreasing and then irreversibly increasing with cycling. Thus, this single-frequency impedance method provides a simple means of monitoring battery health. (C) 2012 The Electrochemical Society. [DOI:10.1149/2.014204esl] All rights reserved.