A battery energy system consists of networks of cells. This paper considers the coupling among the cells that are connected in parallel. Cells are designed to react uniformly in response to charging and discharging. We study the possible occurrence of symmetry breaking when small perturbation leads to further deviation from uniform responses. By including coupling among the cells to the existing battery models, we formulate a dynamical system governing the rate of change of thermal and electrical quantities of the cells. Limited numerical solution using fourth-order Runge-Kutta method was carried out, and stability criteria are obtained from algebraic analysis. The results show that a much more stringent effort in thermal management is required to prevent thermal runaway initiated by cells interacting with each other due to asymmetric perturbations. The increased heat transfer between the two cells serves to offset the undesirable effect of the parallel connection, but the stabilizing effect could be greatly compromised if the cell incorporates three parallel units as in some commercial Li-ion battery cells. Copyright (c) 2013 John Wiley & Sons, Ltd.