Several types of battery have evolved for ordnance and missile applications where the equipment is to be stored, complete with its battery, for a long period. Typically, the battery must be capable of rapid activation and must provide high power for a comparatively short time. In a reserve cell the electrolyte is kept separate from the rest of the battery so that it is inert. Very long shelf lives - up to 20 years - are then possible, but the battery can spring into life almost instantly when activated.
- Spin-activated lead-acid: Spin-activated lead-acid reserve batteries, for discharges lasting about 4 min, are used extensively in artillery for powering electronic fuses and sensors. The electrolyte is often fluoroboric acid, contained in an ampoule in the centre of the cylindrical cell stack, which is cut open by a rotary cutter or crushed by a weight when the shell is fired. The electrolyte then wets the cell stack via the centrifugal force of the shell spinning. Full voltage is normally attained within a few tens of milliseconds. The battery usually consists of a stack of bipolar electrodes, if high voltage is needed, or a set of alternate anodes and cathodes connected in parallel to give a high current, or a combination of the two.
- Lithium-thionyl chloride: Special lithium-thionyl chloride reserve batteries have been developed for artillery-delivered minelets or communications jammers, and for projectiles and sub-munitions that must operate while being slowed by parachute. The batteries are usually spin-activated and they must work for some time after impact on the ground . Once the electrolyte has spun into the cell stack, absorbent separators retain it so that the battery can still operate after spinning has ceased. One advantage of the reserve construction for lithium cells is that no passivating layer forms on the anode surface during storage, thus allowing very high rates of discharge with no voltage delay.
- Silver-zinc: Automatically activated silver-zinc (zinc/silver ) reserve batteries are suitable for discharges of 20 sec or longer. The cells are assembled dry with the potassium hydroxide electrolyte stored in a separate compartment. Activation usually entails igniting a propellant material that, in turn, forces the electrolyte into the cell stack, often heating the electrolyte in the process to improve battery performance. One type of silver-zinc single-cell reserve battery has a built-in DC/DC converter in order to raise the low voltage of the cell to a more useful, stabilized level. Silver-zinc reserve batteries can deliver many kW for several minutes and have been used for combat torpedoes and for many types of guided missile.
