Page 310 - Battery Reference Book

Calcium anode-based thermal batteries 27R

Heat pellet (+) K2Cr20,/LiCI-KCI
impregnated fibreglass
DEB pellet tape cathode
(cathode and Vacuum-
electrolyte) / deposited
I/ calcium
- - Calcium , laver
--I -.
anode at paper pad
Current
collector (-)
Iron dumbbell
Figure 27.8 Typical thermal battery design (Courtesy of Catalyst
Research Corporation) Figure 27.9 Cell cross-section of a Catalyst Research cal-
cium-calcium chromate thermal cell (Courtesy of Catalyst
Research Corporation)
Significant advantages come from the use of
the molten salt electrolytes, usually lithium chlor-
ide-potassium chloride eutectic (61 mol% lithium strip, which is folded around the heat pad. The anode
chloride), which has a melting point of 352°C. First, a is a vacuum-deposited calcium layer on the steel base,
wider span of operating temperatures can be used than which forms a bimetal assembly.
in other types of system without serious degradation of The EAP-6273 12-V battery (see Table 27.1) by
performance. Using the internal heat source to raise the Eagle Picher was specifically designed to provide
temperature of the electrolyte above its melting point, emergency starting power for military vehicles and air-
a range of ambient temperatures from -40 to +70"C craft, and provides a good example of an application
is common and wider ranges of -55 to +100"C can of thermal batteries.
also be accommodated. Secondly, a high cell voltage, The basic design of this battery is eight parallel cell
up to 3.0Vkel1, can be obtained. A wide voltage band stacks of 152.4mm diameter. The cell area of 181 cm2
can be used in this battery system since neither oxida- per cell for the eight sections results in 1449cm'
tion nor reduction of the eutectic alkali-metal chloride of active area. At a discharge rate of 0.7A/cm2, the
electrolyte occurs over the range of 3.0 V, unlike aque- EAP-6273 could produce 1OOOA. This rate is well
ous electrolyte systems or even molten alkali metal within the normal operating limits of the standard cal-
sulphates. Furthermore, conductivity of molten alkali cium-calcium chromate system. The cell is fabricated
electrolytes is considerably greater than that of aque- in one piece using the standard two-layer (DEB) con-
ous electrolytes. Lithium chloride-potassium chloride figuration and is 152.4 mm in diameter, 2.2 mm thick,
eutectic has a specific conductance of 2.1/(acm) at and contains a 12.7mm centre hole. Each cell was
900°C as against 0.24Qcm) for saturated aqueous pressed at 18.1 x 104kg or about 1000kg/cm2.
sodium chloride and 0.039/(Q cm) for 0. IN hydrochlo- The header is of standard 304 stainless steel 9.5 mm
ric acid at 15°C. Thus significantly higher currents for thick. Because of the high current, four terminals for
limited periods can be carried in molten salt batteries each negative and positive lead are used. At the 800 A
compared with other systems. level, each terminal would carry 200A. The battery is
A typical design of a calcium chromate thermal connected externally by paralleling the four positive
cell produced by Catalyst Research Corporation and the negative terminals at the load connections.
(Figure 27.9) consists of a folded bimetal dumb-bell The battery case is assembled from 321 stainless
and pad of pyrotechnic paper. The catholyte disc steel welded tubing with a 9.5 mm steel plate welded
consists of glass fibre tape impregnated with a molten in the bottom. The burst strength of this container was
mixture of lithium chloride, potassium chloride and calculated to be in excess of 454 kg.
potassium dichromate. The tape acts as a binding Because of the total height of the cell stacks (approx-
matrix for the molten salt when the battery is active. imately 356 mm) the stack was assembled in two separ-
The pyrotechnic paper or 'heat paper' supplies the ate sections, each containing four parallel stacks of
thermal energy to elevate the thermal battery cell to cells. These are wrapped and inserted into the case sep-
operating temperatures. The pad consists of a ceramic arately. The case is lined with 12 mm of thermoflex and
fibre paper, which acts as a bindedcarrier for a slurry 12 mm of asbestos in the bottom. Each battery section
of zirconium metal fuel and barium chromate oxidant. is wrapped with 3.8mm of glass cloth. Asbestos for
The paper is extremely fast burning, making it ideal for the top of the battery and the header insulation are
a fast activation cell. Because the remaining ash of the added to obtain a closing pressure of 10-22 kg. The
heat pad is electrically non-conductive, it is necessary total battery weights 20.4 kg and is 406.4 mm tall and
to provide a means for series intercell connection. 177.8 mm in diameter.
The intercell connection is provided, along with the The load desired is 0.024 Q, capable of dissipating
calcium anode, by a dumb-bell shaped piece of steel 25kW. A 15.8mm outside diameter stainless steel