DURACELL.DEC–SC
Choosing the right battery to power the portable product
Attention to design criteria is key to obtaining an optimum combination
of performance and cost
BY JOHN COSTELLO Duracell Inc. New Products and Technology Div.
Bethel, CT
As sales of portable electronic devices continue to climb, designers are
recognizing at the onset of the design phase the importance of considering
battery options to achieve more cost-efficient, user-friendly, and
technically reliable applications. In choosing the most effective power
source, it is important to match the available technologies in primary
(disposable) and secondary (rechargeable) batteries to the appropriate
applications. Key primary batteries today include alkaline and
lithium/manganese dioxide (Li/MnO
the emerging nickel-metal-hydride (NiMH), lead-acid (Pb-acid) and
nickel-cadmium (NiCd) cells. Several other battery technologies that are
currently in the research and development stages, including lithium
rechargeables (such as lithium-ion and polymer) and nickel-zinc.
Battery design-in considerations
These are the key criteria for selecting an effective power source,
whether a primary or secondary battery: * Capacity and energy density *
Low-temperature performance * Shelf life and charge retention *
Discharge profile * Power management * Dimensional considerations *
Availability * Costs–both initial and operating
Capacity and energy density Capacity and energy density represents the
battery's output energy per unit weight and volume. Table 1 compares the
rated performance of various primary and secondary batteries in the
cylindrical AA size. Among primary batteries, lithium manganese dioxide
(Li/MnO2 ) batteries have higher watt-hour capacities and deliver
longer service compared to alkaline-manganese dioxide (alkaline-MnO2 )
batteries. Among rechargeable batteries, nickel-metal-hydride (NiMH)
offers the highest capacity. As shown in Fig. 1, rechargeable batteries
have the ability to deliver most of their rated capacity when discharged
in high-rate applications. Primary batteries, on the other hand, can lose
much of their capacity at extremely high discharge rates. However, because
primary batteries have a higher rated capacity, they generally maintain
good performance at moderately high discharge rates.
Low-temperature characteristics Because of their higher rated capacity,
primary batteries can maintain a high performance level over a wide
temperature range, although this advantage is reduced at higher discharge
rates. Related to their high rate performance and lower internal
resistance, rechargeable batteries can deliver more of their room
temperature capacity at lower temperatures.
Shelf life and charge retention Primary batteries can retain their
capacity over longer periods of storage time than secondary batteries.
However, in applications where the batteries will be used frequently–such
as in portable phones and personal computers–shelf life is not as crucial.
Secondary batteries can be recharged to restore any capacity that may be
lost during storage.
Discharge profile. Secondary batteries maintain a flat discharge curve
(that is, they can be fully discharged within a relatively narrow voltage
band.) Primary batteries, on the other hand, show a more sloping profile.
Therefore, to maximize primary battery capacity discharge, it is important
to set a lower end voltage.
Power management. Voltage or power regulators, which control the voltage
or power demands on the battery, can be designed to use the battery's
energy more effectively. Figure 2 illustrates the advantages of a constant
current, and particularly, a constant power discharge increase for the
service life that a battery can deliver. The voltage profile of a cell
under the three modes of discharge is plotted, assuming that the power
delivered at the end of discharge for the three modes is the same. Under
these conditions, the longest discharge (service time) is obtained under
the constant power mode of discharge.
Dimensional considerations. As product size increasingly becomes an
important purchasing consideration for consumers, designers must also
consider battery size during the design process. In portable products, the
space typically allocated to batteries is at a premium and depends on the
needed battery voltage and the type of battery chosen. Except for lithium
batteries (which are available in 3-V systems) all of the commonly
available systems are 1.2 to 1.5 V. A 6-V application requires the series
connection of either two lithium batteries or four alkaline batteries. In
the case of rechargeable batteries, five batteries are needed. Figure 3
shows the minimum volume required for the power source by battery voltage
and type.
Availability. Regardless of whether the portable product uses a primary
or secondary source of power, eventually, the user will need to replace
the battery. Preferably, the replacement battery will be readily available
at a local retail store. Primary alkaline batteries are currently the most
accessible option for consumers worldwide. Battery manufacturers like
Duracell are working with designers to establish standard rechargeable
pack designs. These standard packs will respond to retailer needs for
limited stock-keeping units (SKUs). As retailers begin to stock a limited
number of standardized rechargeable packs, consumers will begin to realize
widespread availability of rechargeable battery packs.
Cost. Cost can be viewed from two different perspectives: initial cost
and operating cost. Table 2 estimates battery costs for six systems. The
cost shown for rechargeable batteries is based on a standard NP55 pack,
rather than on individual cells, since rechargeable batteries are
typically used in pack configurations. The initial cost of rechargeable
batteries at retail is more expensive than primary batteries. The
long-term operating cost, on the other hand, is significantly less for
rechargeable batteries because they can be recharged many times.
Making the appropriate selection Selecting the most appropriate battery
that meets the needs of new portable products and their users is a
challenge. Final selection could be influenced by factors like the
deployment of the equipment, operating environment, size and weight,
required service time, duty cycle, frequency of use, capital investment,
cost, and, most important, user preferences. Both primary and
rechargeable batteries offer particular advantages for powering portable
electronic products. In some cases, a “dual power option” is appropriate,
where manufacturers offer customers the choice of either a rechargeable or
a primary battery pack. As an example, Blaupunkt, Fujitsu, Goldstar, and
NovAtel all offer alkaline battery cartridges containing five AA alkaline
cells to power handheld cellular phones in addition to standard
rechargeable packs. The alkaline option, which delivers up to two hours of
continuous talk time, can be refilled easily once the batteries are
drained. In addition, spare alkaline batteries can easily be carried in a
travel bag or briefcase, where chargers might be less practical. As
rechargeable battery packs become available in standard sizes, the dual
power option will provide consumers with a viable combination of choice
and convenience.
CAPTIONS:
Fig. 1. Rechargeable batteries like nickel-cadmium and
nickel-metal-hydride maintain much of their rated capacity in
high-discharge-rate applications. Primary batteries like alkaline
manganese dioxide and lithium/manganese dioxide, on the other hand, can lose
much of their capacity at these rates.
Fig. 2. A constant current or constant power discharge scheme can result
in longer service life for the battery.
Fig. 3. Minimum volume required for the power source, shown for AA and
2/3A size cells, varies by battery voltage and type.
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