Shapes & Configurations
Thermal batteries are generally cylindrical in configuration because of
mechanical and thermodynamic considerations; however, other shapes are
feasible (toroidal, rectangular). The cylindrical shape minimizes heat losses
and is also the most practical and economical to manufacture.
Length versus Diameter
To minimize heat losses, the length of the battery should be close to the
diameter of the unit. This reduces the surface area.
Cell Stack Design
The voltage and current output of a thermal battery can be changed by
varying the number of cells and by varying the cell area, respectively. Each
cell produces a voltage that varies from 1.6 volts to 3.3 volts depending on
the electrochemical system and/or the current loading. The effective area can
be varied by changing either the individual cell plate size or by electrically
connecting cells in parallel. The ampere-hour capacity of the cell can be
varied by adjusting the amount of material in the cell.
Multiple voltage outputs can be readily provided by connecting the required
number of cells in series, however the output voltage must be a multiple of
discrete cell voltages. The multiple voltage output sections can be drawn from
either a common group of cells or from an isolated group of cells. An
electrically isolated group of cells may be required to eliminate transient
voltages (cross-talk) between voltage taps. It is also possible to use a fast
activating and a high current carrying electrochemical system in the same
battery and thus obtain the desired performance characteristics of each from a
Battery Activation Methods
Batteries may be activated by electric squibs,
percussion primers, or inertially activated devices.
Electric squibs are pyrotechnic devices containing one (or two) bridgewire
and pyrotechnic material. Upon application of an electric current, the
bridgewire ignites the pyrotechnic material, thereby expelling hot gases and
particles which ignite the integral pyrotechnic heat source in the battery.
There are two general types of electro-pyrotechnic devices used in thermal
batteries, igniters and electric matches.
Igniters: Igniters are contained in a metal or ceramic enclosure and most
of them require a minimum activation current of 3.5 amperes, have no-fire
limits of 1 ampere or 1 watt (whichever is greater), and a bridgewire
resistance of 1 ohm. There are single and dual bridgewire igniters. The
maximum test current that should be applied to the bridgewire circuit is 20
milliamperes. Repeated testing or exceeding the specified test current may
desensitize the igniters. Requirements for the battery to meet MIL-STD-1512
and/or MIL-I-23659 dictate the use of igniters which are qualified to these
Electric Matches : The electric match is a bridgewire connected to two (or
three) leads and covered by a pyrotechnic material. It is not contained in a
metal or ceramic enclosure and requires a minimum activation current of 500
milliamps to 5 amps depending on the bridgewire. The bridgewire resistance of
electric matches ranges from 0.1 to 5.0 ohms. The maximum test current that
should be applied to the bridgewire circuit is 20 milliamperes. Repeated
testing or exceeding the specified test current may also desensitize these
devices. As a recommendation, the applied activation current should not exceed
the specified all-fire current by a factor of three. This practice will
prevent burning through the bridgewire before the pyrotechnic material is
Typically, igniters cost four (4) to ten (10) times as much as electric
Primers are pyrotechnic devices which are activated by the application of
an impact from a mechanical striking device. Typically, a primer requires 28
to 40 inch-ounces of energy to activate. The striking mechanism may be
designed as part of the battery or it may designed as a separate
The design of the striking pin is very critical for proper initiation. The
typical firing pin should have a spherical tip with 0.025 to 0.040 inch
radius. It should be designed so it strikes the primer in the center but does
not puncture the primer. If the primer is punctured, electrical performance of
the battery may be effected. The WW42C1 and C2 manufactured by Olin
Corporation are commonly used primers in thermal batteries.
The primer is the least expensive of the activating devices. However, the
battery case must be constructed to accommodate the primer holder, which will
add extra costs in welding and machining.
Inertially Activated Devices
An inertia activated device, more commonly known as an inertial starter, is
a mechanism which initiates a pyrotechnic reaction when exposed to the
appropriate all-fire shock environment. These devices can be classified
according to several criteria.
The first of these criteria is the pyrotechnic device used to provide
output. The two devices which have been used in thermal batteries are a
friction operated two (2) component match and a percussion primer.
The second criteria for classification is the kinematic design of the
inertial starter mechanism. The most often used mechanism is that of the White
starter. This design is based on a dual, cascaded spring-mass system which
makes the device sensitive to both duration and amplitude of the G force.
Other designs using high G forces and a shear mechanism are also
It is possible to use more than one activating device inside a thermal
battery. For example, two squibs or matches which are connected in parallel
either internally or externally are sometimes used to increase the reliability
of activation. A thermal battery may incorporate more than one type of
activation device (e.g. an inertial activator and a squib).
Thermal batteries can be designed with either integral mounting provisions
that are permanently attached to the battery or the mounting can be part of
the next higher assembly or system. In either case, special consideration must
be given to the interface between battery and system.
High Surface Temperature
The activated life of a thermal battery
depends on maintaining internal battery temperature above the electrolyte
melting point. Therefore, the battery should be mounted in a way that
minimizes its heat loss to the surrounding surface. This must be taken into
consideration when designing mounting provisions for batteries with operating
lives of more than one minute.
The external surface temperature of an activated thermal battery is
typically 230 - 600ºC. Care must be taken that electronics and other thermally
sensitive parts are protected. Proximity to explosive material must be taken
into consideration. Batteries can be designed with lower surface temperature
by increasing the amount and/or changing the type of insulating material used
internally and externally. However, this may require added volume. More
efficient insulation material is also more expensive.
Typically, the external surface of the battery reaches
its maximum temperature 2 to 10 minutes after activation. This may occur after
the useful life of this device and not cause any problem. Figure 8-4 shows a
typical case temperature profile for a thermal battery. When a hang fire or
misfire of the device is possible, the battery will still be mounted in the
system when it reaches its maximum surface temperature. This should be
considered if any heat sensitive components are to be reused or if explosives
and/or combustible materials are located near the battery. The same cautions
should be applied to explosive atmospheres.
Effect of Dynamic Environment on Battery
For optimum operating performance, the battery should be mounted so G
loading and vibration is applied along the direction of the battery
longitudinal axis. For high spin environments, on-center spin has a less
severe effect than off- center spin.
The battery should be solidly mounted so it does not move in the mounting
when subjected to shock, vibration, acceleration, or spin.
Environmental Effects on Battery Performance
Thermal batteries are typically designed to withstand high G loading and
vibration levels before and after activation without affecting electrical
Electromagnetic Interference (EMI), Electromagnetic
Radiation (EMR), and Hazards of Electromagnetic Radiation to Ordnance
Thermal batteries are designed with a metallic outer
enclosure and therefore are not affected by electromagnetic environments.
However, any external wiring can function as an antenna and induce electric
currents into the electrical igniter circuit. This may lead to premature
activation of the battery. To minimize the effect of induced currents, the
ignition circuit can be designed to incorporate filters. Mechanical activation
systems are not affected by electromagnetic fields.
HERO and other EMR requirements are usually specified
for system level assemblies. These requirements are not placed on the
batteries as components.
Thermal batteries are not significantly affected by
nuclear radiation. Contact Sandia National Laboratories in Albuquerque, New
Mexico for further information on this topic.
Operating Temperature Environment
Thermal batteries are designed to operate over a wide
temperature range, typically -54ºC to +74ºC. If required, they can be designed
to operate over a wider temperature range. In general, the battery will give
optimum operating life at room ambient conditions. The lower the operating
temperature, the longer the activation delay will be. All thermal batteries
are designed for the specific environments in which they will be required to
operate. Figure 8-5 depicts battery activation delay and operating life
versus operating temperature.