A device that transform the energy created by a chemical reaction into electrical energy. Typically, a battery lasts for two to five years. Its lifespan will depend on the type of watch, its size and the amount of energy required for the different functions. A Chronograph will consume more energy than a watch that shows only hours, minutes and seconds.
Some watches have a power reserve indicator for the battery: when the seconds hand jumps every five seconds, the battery has almost lost its charge and needs replacing.
Special lithium-iodine batteries have a theoretical lifespan of 10 years.