In this work, we present a set of thermal characterization experiments of charged prismatic polymer lithium-ion battery (PLB) comparatively with those of a lithium-ion battery (LIB). These cells at different state of charge (SOC) were tested inside an accelerated rate calorimeter (ARC) to determine the onset-of-thermal runaway (OTR) temperatures. In addition, the thermally activated components of these cells were followed by monitoring the impedance (at 1 kHz) and the open-circuit voltage (OCV) as a function of temperature. An increase in the impedance was observed at around 133 degreesC corresponding to the polyethylene separator shutdown. Above 140 degreesC, the OCV dropped to zero indicating an internal short-circuit due the separator meltdown suggesting that the pinholes created in the separator at meltdown are large enough to create an internal short-circuit. (C) 2003 Elsevier Science B.V. All rights reserved.