Overcharging of rechargeable lithium batteries leads to irreversible damages to the cell materials and causes safety problems. For multi-cell battery packs designed for high voltage applications (such as EV's and HEV's), it is essential to incorporate an efficient overcharge protection mechanism to permit current to flow through or around a damaged cell so that the rest of the cells can continue to charge normally. Although current methods provide a certain level of protection, the high cost and added weight and volume of external devices, and the poor high-rate performance of internal redox shuttles limit their applicability. An alternative approach of overcharge protection by using electroactive polymers was developed at LBNL. Electroactive polymers have the ability to switch rapidly between conductive and insulating states due to changes in their electronic structure brought about by oxidation or reduction, with intermediate conductivities dependent on state of charge. When incorporated in the lithium cell configuration, they can provide overcharge protection by means of a reversible, self-actuating, low resistance internal shunt that allows overcharge currents to pass through a cell while maintaining its discharge capacity. Fundamental studies of conducting polymer behavior and demonstrations of their effectiveness in providing overcharge protection for lithium batteries at substantial current densities will be addressed.