A continuum-scale model is presented to explain how electroactive polymers such as polythiophene can be used to provide overcharge protection for lithium-ion batteries. The model shows how the cell is transformed upon overcharge from a battery to a resistor with a resistivity that varies with position across the separator. Upon discharge or open circuit, self discharge transforms the resistor back into a battery, and normal cycling can be resumed. A simplified model yields a design equation that shows how the potential at which the cell shorts depends on the current density, separator thickness, and the variation of the electronic conductivity and the oxidation potential of the polymer with a degree of oxidation. The effect of these parameters on the transient and steady-state behavior of the short is explored. The shorting voltage is independent of the choice of positive electrode and scales with the potential of the charged negative electrode. Kinetic effects on the transient behavior and thermal effects are also considered. (C) 2004 The Electrochemical Society.