A multi-parameter controlled pinch test was developed to study the occurrence of internal short circuits in Li-ion and Li-ion-polymer cells. By tuning the control parameters (i.e., cell voltage as well as pinching area, load, and speed), the pinch test can reproducibly create an internal short between a cell jelly-roll's inner layer electrodes as small as 1-mm wide. This recreates conditions similar to those that may occur during service. In this paper we demonstrate the use of the pinch test as a means to assess design and manufacturing changes in Li-ion-polymer cells on their thermal stability and to identify features or characteristics that lower risk of potential thermal events created by internal short circuits.