Also called deferred-action batteries, reserve batteries are special purpose primary batteries usually designed for emergency use. The electrolyte is usually stored separately from the electrodes which remain in a dry inactive state. The battery is only activated when it is actually needed by introducing the electrolyte into the active cell area. This has the double benefit of avoiding deterioration of the active materials during storage and at the same time it eliminates the loss of capacity due to self discharge until the battery is called into use. They can thus be stored for 10 years or more yet provide full power in an instant when it is required.
Examples are Ampoule batteries Thermal Batteries and Water-activated batteries
Ampoule batteries store the electrolyte in a separate ampoule which is incorporated into the battery case. When the battery is needed, the ampoule is broken open allowing the electrolyte to enter the cell. This technique has been used for military fuses as well as marine applications.
The electrolyte in Thermal batteries is solid and inactive at normal ambient temperatures. It only becomes active in molten form at high temperatures by the application of heat from an external source. These batteries are use almost exclusively for military applications.
In Water-activated batteries the electrolyte is usually water or seawater as the name implies.
An added advantage is that the water does not have to be transported around with the cell since
it can be obtained local when it is required. These batteries have many marine and military applications and find extensive use with the emergency services.