Batteries for implants must possess characteristics such as safety, reliability, high volumetric energy density, low self-discharge, and long duration of service, which represent essential commitments from manufacturers. The state of discharge must be indicated. In the primary batteries, lithium metal anodes are used. The cathode systems include iodine, manganese oxide, carbon monofluoride, silver vanadium oxide, and crossbreed or hybrid cathodes. This choice of batteries caters to the power levels required by implantable devices, which are spread over a broad range of current values from microampere to ampere levels. Limited battery life is a major impediment to the development of advanced medical implant devices, e.g., when a pacemaker battery runs out, it has to be replaced by surgery. With progressive shrinkage of implant size, more emphasis is laid on building smaller, longer-lasting batteries. Applications involving high power usage rates such as neurostimulators working at milliwatt powers employ secondary rechargeable batteries to achieve longer life span with reduced size.