Page 420 - Battery Reference Book

Comparison of lithium-iodine and nickel-cadmium cells in CMOS-RAM applications 3%/7
life, be hermetically sealed to prevent leakage, have Nickel-cadmium batteries are secondary systems
proved high quality and reliability and possess high while lithium systems are usually primary devices. This
safety characteristics. Nickel-cadmium and lithium- means that, when nickel-cadmium batteries are used, a
based batteries are two commercially available prod- more complicated circuit is required. This translates to
ucts which offer the most potential for a CMOS-RAM higher part, labour and board costs. In addition, limited
back-up power source. nickel-cadmium battery capacity between charges can
Selecting between nickel-cadmium and lithium bat- affect the duration of continuous data retention time.
teries for CMOS-RAM back-up depends on specific With a lithium cell, the total capacity may be used in
differences between the various battery systems in rela- one longer, continuous standby cycle.
tion to the products being designed. Both systems have In the past, nickel-cadmium batteries have been a
proved reliability, but there are fundamental differ- favourite choice as a back-up power source. Contrary
ences which become significant when the two bat- to their popularity, they are far from the ideal battery
tery types are considered for use in battery back-up for long-term memory back-up because of their narrow
applications. These differences include voltage gener- operating temperatures, short replacement time. high
ating capability, operating life, temperature range and replacement labour costs, recharging requirements and
recharging. potential leakage problems. These shortcomings are
Nickel-cadmium batteries require more than one not encountered with lithium-based batteries.
cell to provide power above 2.0V, which is the com- Lithium functions as the anode in the battery's
mon minimum data retention voltage of CMOS-RAM chemical system. When considering using lithium bat-
devices. In most cases, nickel-cadmium batteries also teries, it is crucial to take a look at the material
take up relatively large amounts of valuable board used for the cathode. There are a number of cath-
space. Lithium batteries, on the other hand, are cap- ode and depolarizer materials used in conjunction
able of delivering more than 2.0 V of power in a single with the lithium metal anode to make up the generic
small unit. term 'lithium batteries'. These materials, which include
Generally a iithium battery will last the life of manganese dioxide, sulphur dioxide, carbon fluoride,
the memory in which the battery is used. For long- thionyl chloride and lead iodide, greatly influence
term applications, nickel-cadmium batteries may need the properties and characteristics of lithium batteries
replacement in 3-5 years, causing service and battery (Table 38.2).
hardware costs to become factors. One currently available lithium system, which seems
Most lithium systems operate over temperature to meet most of the requirements for an ideal CMOS-
ranges similar to those of nickel-cadmium systems RAM back-up power source, is the lithium-iodine
(-40 to f60"C). However, lithium systems, capable battery. The lithium-iodine battery has long life char-
of operating tlxoughout a temperature range of acteristics, and is designed for wave soldering and
-55 to +125"C, are also available. printed circuit board mounting.

Table 38.2 Compal4son of commercially available lithium batteries
-
System Lithium- Lithium- Lithium- Lithium- Lithium- Lithium-
iodine manganese sulphur carbon thionyl lead
P2VP dioxide dioxide Jluoride chloride iodide
Energy density
(W h/cm3 )+ 0.60 0.44 0.45 0.33 0.66 0.47
Seal Hermetic Crimp Crimp or Crimp Crimp or Hermetic
hermetic hermetic
Voltage (V) 2.8 2.9 2.9 2.8 3.6 1.9
Electrolyte Solid salt Liquid Liquid Liquid Liquid Solid salt
organic organic organic organic
Self-discharge rate Low Moderate Moderate Moderate Moderate Low
Internal resistance High Moderate Moderate Moderate Moderate High
Separator Not required* Plastic Plastic Plastic Plastic Lii§
Relative life cycle costs Low Low to Low to Low to Low to Low to
(5+ years) moderate moderate moderate moderate moderate
* Poly-Zvinylpyridine
'Energy density based on small sealed celk
:Chemical reaction of lithium anode and iodine. PZVP cathode forms its own self-healing lithium iodide separator
$Added during fabrication process