Materials commonly used for both the can and header are 304, 304L, and 321
Corrosion Resistant Steel (CRES). Each of these alloys can be deep drawn to
form the case. The most common alloy used is 304 because of its weldability,
relatively low cost, ease of manufacture, strength, and low permeability. 304
is available in a wide range of sizes of plate, drawn seamless tubing, welded
and drawn tubing, and strip. Seamless tubing is recommended when tubing is
used for the case, although the cost is higher and some sizes may not be
available. Hermetic seal integrity must be checked after the base is welded to
the tube. Alternatives to 304 include:
304L Stainless Steel : 304L stainless steel can be used when improved
corrosion resistance is required. The cost is slightly higher than 304 and
greater attention must be given to cleanliness of parts and subsequent welding
processes. 304L stainless steel may not be available in some sizes.
Titanium: Titanium has been suggested for consideration as a battery
enclosure material when a strong, lightweight material is needed to minimize
battery weight. There are several concerns that must be addressed before using
titanium, among which are high cost and availability in shapes and sizes
needed. The most significant concern, however, is that glass-to-titanium
hermetic seal technology is still in development. Although good seals have
been made, the quality of the process remains to be proven. Welding of
titanium to 304L headers would require a filler material and careful attention
to the welding process.
Cold Rolled Steel : Cold rolled steel has been used extensively. However,
its major disadvantage is that it must be plated to prevent corrosion. The
toxicity of plating materials should be considered if cold rolled steel is
Care must be taken in the selection of materials used in the header, case,
mounting brackets, and connectors so all materials are compatible to prevent
Selection Of Activating Devices
The selection of an activating device depends on the
type and amount of external energy available for activation. Cost, ease of
manufacture, and performance should also be considered when choosing an
activating device. For descriptions of the types of activating devices, see
"Battery Activation Methods" in Section
Terminal & Header Configuration
Solder terminal configurations have, in the past, been the most common
connections found on thermal batteries. Header designs should prevent
terminals from interfering with the weld groove and other terminals.
There are two general types of plug-in connectors. One type of plug-in
connector is an integral part of the battery header. The other type of
connector is attached to a cable or harness and joined to the solder terminal
Connectors offer several advantages:
They provide better protection of the pins during
battery shipping and handling.
The connections are "fool-proof" usually they will
only interconnect with the system in one orientation.
They provide a sealed electrical connection to the system.
The major drawbacks to using plug-in connectors
Other documents are available to describe
glass-to-metal seal integrity for high reliability applications.
A header with glass-to-metal sealed electrical
terminals is welded onto the can to form a hermetic enclosure. The header
thickness is typically 0.125 inch. The fit of the header into the can is
important in obtaining high-quality hermetic welds. Header design
considerations should preclude:
Terminals too close to the weld groove
Terminals too close to each other
Tight location tolerances on terminal
The connections made within a thermal battery between electrodes, leads,
lead strips, and header terminals are either spot welded or crimped. Soldering
is avoided inside a thermal battery because of the extreme heat generated.
Solder will melt between +175 and +290șC. The inside of a thermal battery can
reach temperatures of well over 480șC. In applications where solder
connections must be made, only high temperature solder may be used. After the
solder connection is made, potting is applied to the inside of the header to
protect the solder connection from the high internal temperatures of the
Spot welds are used to make most of the connections within the battery
because of their strength and durability. Lead strip to header terminals, lead
strip to lead strip, and electrode to lead strip are examples of spot weld
connections. Spot welds are required to conform to MIL-W-6858. Crimp
connections are made between two wires or between a wire and a lead strip when
the conductor material cannot be spot welded.
Electrical isolation must prevent undesired contact of components within a
battery which could result in an electrical short circuit. Electrical
isolation inside a thermal battery is accomplished by using sheets of an
electrical insulator such as mica or isomica to insulate the inside of
headers, cell stack containers, and electrical leads.
The battery thermal insulator also provides some
isolation between the cell stack and the container/header assembly (see Figure 8-3 ).
Glass sealed terminals insure electrical isolation between voltage sections
as well as isolation of the activation circuit. When a squib or a match is
used for activation, the electrical circuit is embedded in potting material
which acts as an electrical insulator.
Series and Parallel Connections
Thermal batteries are built with cells stacked in series. A battery may
provide two or more voltage sections. This can be accomplished by having sep-
arate cell stacks for each voltage output, or by inserting voltage taps along
one common stack of cells.
Separate cell stacks can be used for complete
electrical isolation between separate voltage sections -- see Figure 8-6A. If
it is not possible to use separate cell stacks because of volume or weight
considerations, then a single cell stack with one or more voltage taps must be
used. In this case, complete electrical isolation will not be possible because
the only insulation between adjacent sections is the electrolyte material--
see Figure 8-6B.
Although the electrolyte is a good insulator at room temperature, it does
not provide the isolation that mica or isomica provides during operation. This
should be considered when specifying isolation limits between sections in a
Identification & Labeling
Thermal batteries, like most other types of batteries,
need to be properly identified. MIL-STD-130 and MIL-STD-1285 detail labeling
standards. Serial numbers, lot numbers, part numbers, manufacturer's name, and
date of manufacture are usually etched on the outside of the battery
container. The recommended method of applying labeling information is to use
the surface conversion method (LectroEtch). Normal battery labels with the
printed information on one side and adhesive on the other side are not
recommended because thermal battery container walls become extremely hot
during operation. Label adhesive may lose its ability to stick to the
container wall, resulting in the possible loss of the label.
Thermal batteries may require an indicator to show whether or not the
battery has been activated. This can be accomplished by using either a heat
sensitive paint or adhesive dot. In both cases there is a color change
associated with a change in temperature. Typically, the temperature at which
the color change occurs is between +105 and +120șC.
There are a variety of color changes of heat sensitive paint available from
manufacturers. This paint is recommended instead of the adhesive dots for the
same reason that battery labels are etched rather than using adhesive backed
A label should be included on the battery which gives information on the
activation state of the battery. That is, which color of the paint or dot
indicates an activated battery and which other color indicates an unactivated
battery. Such a label might read "PINK -- UNACTIVATED; PURPLE --
All tolerances should be as practical as possible. Because of variations in
raw materials, tolerances on the active components (anode, cathode) must be
broad enough to allow adjustments that are necessary to meet battery operating