Lithium-ion batteries are being developed for nonimplantable and implantable medical devices. The high voltage, energy density and unique characteristics of this battery system are, in some cases, an enabling technology for the medical device. In other cases, the lithium-ion system provides additional convenience to the patients who use these devices. Especially for implantable medical devices, lithium-ion batteries must be designed and manufactured to the highest standards of quality and reliability. Safety characterization of various cylindrical and prismatic cells has been performed under both electrical and mechanical abuse conditions. Cycle testing at various charge and discharge rates indicates that the lithium-ion battery can function as a safe and effective power source in medical devices.