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.