Page 419 - Battery Reference Book

3816 Lithium primary batteries
0.5 mm steps. Many advantages will become apparent back-up 65 kbytes of CMOS-RAM. In addition, these
to the design engineer. cells withstand a considerable amount of overloading,
The lithium-iodine cell is an example of a highly and even dead shorts, without suffering damage.
reliable lithium power source, proven in the pacemaker According to Catalyst Research Corporation, in the
industry. It is used in security, space and defence sys- case of a continuous dead short, the battery cells are
tems and, more recently, CMOS-RAM data retention protected by the formation of a film, which raises the
applications. It is available in several sizes, suitable internal impedance, thereby limiting current. This self-
for watches (LCD and analogue stepping motor) and limiting effect makes lithium-iodine safer than those
for low-current LCD calculators. Life spans as long as lower impedance chemistries that heat up and, in the
10 years are possible in watches. In calculators, 5000 h larger sizes, even explode when shorted.
operating times can be obtained over a 10-year period; Another important characteristic of a lithium-iodine
for example, a 0.27 Ah battery on a 50 mA drain would cell is its sloping discharge curve over time; the volt-
deliver 5000 h of service over a 10-year period. age drops approximately 0.1 V every few years over
More recently, Catalyst Research Corporation have a 10-year period for a current drain of a few micro-
issued a preliminary specification sheet for a 7.5Ah amperes. Many designers, however, might not consider
oil well logging battery (model 407220, Military spec- this characteristic a disadvantage; it provides an easy
ification D-cell). This is housed in a 38 mm x 57 mm means of monitoring the remaining life.
circular container and is hermetically welded with a
glass-metal compression seal. It has a voltage of 2.8 V
(open circuit 2.8 V), a maximum operating tempera- 38.1 1 Comparison of lithium-iodine
ture of 150°C and a self-discharge rate of about 5% in and nickel-cadmium cells in
10 years at 25°C. The prototype D-34 cell maintains its CMOS-RAM applications
voltage of 2.8 V during 6 weeks continuous operation
at lOmA constant current at 130°C. Applications of CMOS-RAM (complementary metal
Also available is the model CRC Li D 2.8 D-type oxide random access memory) are growing at a rapid
cell (2.8V) with a capacity of 14Ah at 25°C and pace. Manufacturers are using these memories with
an operating temperature of -20 to +50"C. The bat- increasing frequency in industrial process controls and
tery weighs 80g and has a height of 33.8mm and a communications, energy management and security sys-
width of 57.1 mm. This cell suffers a capacity loss tems, consumer electronics, data loggers and computer
of less than 5% in 10 years at 25°C and can with- peripherals such as teleprinters and word processing
stand substantial shock, vibration and short-circuiting equipment, among others. Advantages of CMOS-RAM
at temperatures up to 150°C without venting, leaking, memories include relatively high data volumes, rela-
swelling or exploding. The cell continues operating at tively fast reading and writing, easy reprogramming,
2.8 V (25°C) for 18 months (1 mA) and 2.7-2.8 V for low power requirements for memory operation and
about 8 months (2mA). retention, comparatively low cost, and immunity to
Lithium-iodine batteries in the 35- 1000 mA h high noise.
capacity range are available for printed circuit board Unfortunately, CMOS-RAM memories suffer one
applications. major drawback-they are susceptible to memory
Of all the lithium compositions, lithium-iodine is losses during power fluctuations or outages. This
considered by its inventor, Catalyst Research Cor- memory volatility dictates the need for some type of
poration, to be a completely solid-state cell (although back-up power source.
the electrolyte is purported to have a 'tar-like', rather This could be accomplished by backing up the
than rock-hard, consistency). entire system with a large uninterruptible power supply
The lithium-iodine combination has been used for which, in many cases, would be impractical because of
most implanted cardiac pacemakers since 1972. It thus the small size of many systems. A much more logical
has a believable reliability history, at least at human alternative is to selectively protect the individual mem-
body temperature. Because of its critical main use, ory chip or array of memory chips with an on-board
the lithium-iodine cell comes in a hermetically sealed battery power supply.
(laser-welded) nickel or steel container, which makes Selective battery back-up of a CMOS-RAM pro-
it more expensive than other types ($5-8 for the vides numerous benefits, including space savings, easy
200-350 mA h coin-shaped cells). Yet the high quality inspection and servicing, long-life expectancy and reli-
of the chemical processing and encapsulation makes it ability. Most importantly, a battery-backed CMOS-
a good choice where very long shelf life (10 years and RAM serves as a cost-effective non-volatile memory
beyond) is the objective. system.
Besides its higher cost, the main disadvantage of The ideal battery for CMOS-RAM back-up would
the lithium-iodine system is that the current output be printed circuit board mountable and wave solder-
is small; a maximum of only 50pA is recommended able, provide data retention of the life of the memory,
at room temperature. Even with this current level, retain data over a wide temperature range, not require
however, one of these cells has enough capacity to recharging, not require replacement during memory