The button cell was developed to miniaturize
battery packs and solve stacking problems. Today, this architecture
is limited to a small niche market. Non-rechargeable versions
of the button cell continue to be popular and can be found
in watches, hearing aids and memory backup.
The main applications of the rechargeable
button cell are (or were) older cordless telephones, biomedical
devices and industrial instruments. Although small in design
and inexpensive to manufacture, the main drawback is swelling
if charged too rapidly. Button cells have no safety vent and
can only be charged at a 10 to 16 hour charge rate.
New designs claim rapid charge capability.
Figure 3-2: The button cell.
The button cell offers small size
and ease of stacking but does not allow fast charging. Coin
cells, which are similar in appearance, are normally lithium-based
and are non-rechargeable. Photograph courtesy of Sanyo
Corporation; design courtesy of Panasonic OEM Battery Sales
Group, March 2001.
The prismatic cell was developed in response
to consumer demand for thinner pack sizes. Introduced in the
early 1990’s, the prismatic cell makes almost maximum use
of space when stacking. Narrow and elegant battery styles
are possible that suit today’s slim-style geometry. Prismatic
cells are used predominantly for mobile phone applications.
Figure 3-3 shows the prismatic cell.
Prismatic cells are most common in the lithium
battery family. The Li-ion polymer is exclusively prismatic.
No universally accepted cell size exists for Li-ion
polymer batteries. One leading manufacturer may bring out
one or more sizes that fit a certain portable device, such
as a mobile phone. While these cells are produced at high
volume, other cell manufacturers follow suit and offer an
identical cell at a competitive price. Prismatic cells that
have gained acceptance are the 340648 and the 340848. Measured
in millimeters, ‘34’ denotes the width, ‘06’ or ‘08’ the thickness
and ‘48’ the length of the cell.
Figure 3-3: Cross-section of a
prismatic cell.
The prismatic cell improves space
utilization and allows more flexibility in pack design. This
cell construction is less cost effective than the cylindrical
equivalent and provides a slightly lower energy density. Design
courtesy of Polystor Corporation, March 2001.
Some prismatic cells are similar in size
but are off by just a small fraction. Such is the case with
the Panasonic cell that measures 34 mm by 50 mm
and is 6.5 mm thick. If a few cubic millimeters can be
added for a given application, the manufacturer will do so
for the sake of higher capacities.
The disadvantage of the prismatic cell is
slightly lower energy densities compared to the cylindrical
equivalent. In addition, the prismatic cell is more expensive
to manufacture and does not provide the same mechanical stability
enjoyed by the cylindrical cell. To prevent bulging when pressure
builds up, heavier gauge metal is used for the container.
The manufacturer allows some degree of bulging when designing
the battery pack.
The prismatic cell is offered in limited
sizes and chemistries and runs from about 400mAh to 2000mAh
and higher. Because of the very large quantities required
for mobile phones, special prismatic cells are built to fit
certain models. Most prismatic cells do not have a venting
system. In case of pressure build-up, the cell starts to bulge.
When correctly used and properly charged, no swelling should occur.
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