Chapter 3: The Battery Pack
In the 1700 and 1800s, cells were encased in glass jars.
Later, larger batteries were developed that used wooden containers.
The inside was treated with a sealant to prevent electrolyte
leakage. With the need for portability, the cylindrical cell
appeared. After World War II, these cells became the standard
format for smaller, rechargeable batteries.
Downsizing required smaller and more compact
cell design. The button cell, which gained popularity in the
1980s, was a first attempt to achieve a reasonably flat geometry,
or obtain higher voltages in a compact profile by stacking.
The early 1990s brought the prismatic cell, which was followed
by the modern pouch cell.
This chapter addresses the cell designs,
pack configurations and intrinsic safety devices. In keeping
with portability, this book addresses only the smaller cells
used for portable batteries.
The cylindrical cell continues to be the
most widely used packaging style. The advantages are ease
of manufacture and good mechanical stability. The cylinder
has the ability to withstand high internal pressures. While
charging, the cell pressure of a NiCd can reach 1379 kilopascals
(kPa) or 200 pounds per square inch (psi). A venting
system is added on one end of the cylinder. Venting occurs
if the cell pressure reaches between 150 and 200 psi.
Figure 3-1 illustrates the conventional cell of
a NiCd battery.
Figure 3-1: Cross-section of a
classic NiCd cell.
The negative and positive plates
are rolled together in a metal cylinder. The positive plate
is sintered and filled with nickel hydroxide. The negative
plate is coated with cadmium active material. A separator
moistened with electrolyte isolates the two plates. Design
courtesy of Panasonic OEM Battery Sales Group, March 2001.
The cylindrical cell is moderately priced
and offers high energy density. Typical applications are wireless
communication, mobile computing, biomedical instruments, power
tools and other uses that do not demand ultra-small size.
NiCd offers the largest selection of cylindrical
cells. A good variety is also available in the NiMH family,
especially in the smaller cell formats. In addition to cylindrical
formats, NiMH also comes in the prismatic cell packaging.
The Li-ion batteries are only available
in limited cells sizes, the most popular being the 18650.
‘Eighteen’ denotes the diameter in millimeters and ‘650’ describes
the length in millimeters. The 18650 cell has a capacity
of 1800 to 2000mAh. The larger 26650 cell has a
diameter of 26 mm and delivers 3200mAh. Because of the
flat geometry of the Li-ion polymer, this battery chemistry
is not available in a cylindrical format.
Most SLA batteries are built in a prismatic
format, thus creating a rectangle box that is commonly made
of plastic materials. There are SLA batteries, however, that
take advantage of the cylindrical design by using a winding
technique that is similar to the conventional cell. The cylindrical
Hawker Cyclone SLA is said to offer improved cell stability,
provide higher discharge currents and have better temperature
stability than the conventional prismatic design.
The drawback of the cylindrical cell is less
than maximum use of space. When stacking the cells, air cavities
are formed. Because of fixed cell size, the pack must be designed
around the available cell size.
Almost all cylindrical cells are equipped
with a venting mechanism to expel excess gases in an orderly
manner. Whereas nickel-based batteries feature a resealable
vent, many cylindrical Li-ion contain a membrane seal
that ruptures if the pressure exceeds 3448 kPa (500 psi).
There is usually some serious swelling of the cell before
the seal breaks. Venting only occurs under extreme conditions.
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