The lithium/iodine-polyvinylpyridine battery, first implanted 20 years ago, has become the power source of choice for the cardiac pacemaker. Over the last 20 years, improvements in cell chemistry, cell design, and modeling of cell performance have been made. Cells today exhibit an energy density over three times as great as cells produced in 1972. Well over 2 million pacemakers have been implanted with this chemistry, and the system has exhibited excellent reliability.