A reserve battery, also called stand-by battery, is a primary battery where part is isolated until the battery needs to be used. When long storage is required, reserve batteries are often used, since the active chemicals of the cell are segregated until needed, thus reducing self-discharge.
A reserve battery is distinguished from a backup battery, in that a reserve battery is inert until it is activated, while a backup battery is already functional, even if it is not delivering current.
Some reserve batteries are:
- Aluminium battery, a variant of zinc-air battery where aluminium and air are used
- Silver-zinc battery, often found in old missiles
- Thermal battery, a class of battery types with molten salt as an electrolyte. The battery is in an inert state until the electrolyte melts through heating. Common in military applications, especially for missiles and proximity fuzes Applications include V-2 rockets, Sidewinders, Patriots, TOWs, Tomahawks, many nuclear weapons, and other single-use applications.
- Water-activated battery, a class of batteries where the electrolyte is water that has to be added.