Reserve battery

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.

These batteries are used in radiosondes, missiles, projectile and bomb fuzes, and various weapon systems.

While not advertised as reserve batteries, the principle is illustrated by the sale of "dry charged" car batteries where the electrolyte is added at the time of sale. Another example is zinc-air batteries where the cell is sealed until use: a tab is removed to admit air and activate the cell.

Reserve batteries may be activated by addition of water, by adding electrolyte, by introducing a gas into the cell that is either the active cathode material or part of the electrolyte, or by heating a solid electrolyte to a temperature at which it becomes conductive. The missing element of the battery can be added before use in several ways. The battery can have water or electrolyte added manually, the battery can be activated when the system is dropped into the water (such as in a sonobuoy), or electrolyte can be stored in a capsule within the battery and released by mechanical means, an electrical trigger, or by spin or shock. A molten-electrolyte battery is activated by igniting a pyrotechnic heat source. The battery delivers current for a short time (seconds to a few minutes), but some thermal batteries can be stored ten years or more without deterioration. Reserve batteries remain uncommon in civilian applications because of their higher cost and relatively short life after activation.

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Wikipedia