Page 421 - Battery Reference Book

38/8 Lithium primary batteries
Table 38.3 CMOS memory back-up parameters for Nicad and lithium comparison

Item ldeal Nicad Lithiurn+

Voltage (V) 2.0 1.2 2.8
Low current capability Required Yes Yes
Shelf life 10 years 1 montht 95% of capacity retained after 10 years
Self-discharge Low High Low
Operating life (years) Life of memory 3-5* 10
Energy density (W h/cm3) Higher the better 0.09 0.33 or better
Temperature range ("C) Mil Spec -40 to +60§ -55 to $125
Sealing Hermetic Hermetickrimp Hermetickrimp
Recharge circuit - Required Not required
*This is a summary of all commercially available lithium systems
'Between charges
'Exceptions claimed
$Available from limited manufacturers, most are -40 to +60"C

When current is not being drawn from a adapting a back-up system. More specifically, addi-
lithium-iodine battery, as is often the case in memory tional consideration should be given to current drains
back-up applications, self-discharge takes place. Self- and voltages of components, selection of RAM devices
discharge is a natural process which consumes active for minimal current and voltage drain properties, selec-
battery materials and, hence, battery capacity. Capacity tion of diodes for optimal voltage drop and verification
losses at 25°C vary from less than 5% over IO years for of data integrity after power-up. All of these factors
lithium-iodine batteries to more than 25% per month must be considered in combination with battery spec-
for some secondary batteries. ifications.
A high equipment storage temperature will increase The connecting circuitry should constantly monitor
self-discharge and must be considered when selecting system voltage to avoid causing errors in the software
a battery for back-up applications. Lithium-iodine because of inadequate power. As long as the voltage
batteries stored at an average temperature of 50°C remains above a specified level, usually around 2.9 V,
or less will hold a charge for more than 10 years. the system's power retains the memory. It is when the
On the other hand, a nickel-cadmium battery has voltage drops below this specified point that a smooth
a shelf life of about 1 month before recharging is transition to data retention power is needed.
required and is limited to 300-500 chargehecharge Recent research efforts have resulted in the develop-
cycles (Table 38.3). ment of an interface circuit that will dependably mon-
An advantage of the lithium-iodine battery is itor voltage availability and switch between external
its temperature performance. Not only does the power and battery back-up as needed. The circuit and
battery operate over a wide temperature range the lithium battery have been incorporated into module
(-55 to +125"C), but it has a high impedance at low form. One module design even contains a 16 K RAM.
temperatures and low impedance at high temperatures. Modules are designed to be mounted on a printed cir-
Therefore, as the power requirements of the memory cuit board, thus providing system designers with a
chip decrease with a decrease in temperature, the black-box method of meeting the need for non-volatile
lithium-iodine battery reacts similarly by providing memory.
less current to the chip and vice versa.
Hazardous incidents have been experienced with
some lithium systems, particularly those using sulphur
dioxide and thionyl chloride cathodes. These incidents 38.12 Lithium-iron disulphide primary
generally occur at later stages in battery life, under cells
reverse current conditions, during voltage reversal and
while operating at high temperatures. Safety incidents To date only one cell the AA size 3.7 Ah and possibly
have not been experienced with lithium-iodine cells, the AAA size cell seem to be available on the market.
which is why they power 90% of the cardiac pace- Output and demand for these cells is still poor despite
makers presently in use. earlier promise due in part to the high manufacturing
In theory, devising a workable memory-battery costs and small production rates.
connecting circuit sounds simple, but in actual practice Eveready have announced their intention to produce
it is a major obstacle to adapting the battery back-up AA type (IECR6) high rate cells for photographic use.
concept. The battery, the memory, and other circuit Lithium-iron disulphide cells have also been used in
component specifications must be considered when paging applications.