Selecting coin-cell battery holders

While breakthrough technologies
continue to grab headlines,
components like the
coin-cell battery holder often get overlooked,
despite their major role in support
of their glamorous cousins. However,
the holder can fail due to issues
like vibration, heat, shock, humidity,
and corrosion, and design engineers
face growing challenges developing
power management solutions to handle
these real-world conditions. By focusing
on the following criteria, engineers
can achieve product reliability at
the best possible cost.

Coin-cell retention, removal

A well-designed coin-cell holder must
resist shock and vibration while remaining
flexible enough to allow easy
battery replacements. Unfortunately,
these criteria are often in conflict, as
better retention equals tougher removal.
Look for holders with features
designed to ease removal.
It is particularly important to test
the holder in situ. It is often a lot easier
to remove a battery when the coin-cell
holder is in your hands than when it is
soldered to a pc board and surrounded
by other components and a housing.

Durability

Durability is especially critical for applications
where frequent battery replacement
may be needed over the
product’s expected lifespan. These
problems can be exacerbated if the battery
holder has high grip due to increased
wear and tear during battery
insertion and removal. If your application
will require relatively frequent cell
changes, find out the cycle count that

the holder has been tested for.
All battery holders should have
polarity protection, so they will not
make contact if the battery is inserted
improperly. This becomes more important
when a large number of battery
replacements are anticipated.

Conductivity and corrosion
resistance

Products exposed to excessive heat
and humidity, caustic chemicals, or
airborne pollutants can often have
problems associated with corrosion
build-up, which can negatively impact
electrical performance. To minimize
these effects, locate holders that are
constructed from corrosion-resistant
materials. The presence of electrochemically
dissimilar metals can further
increase corrosion problems resulting
in galvanic corrosion. These
effects can be minimized through the
use of insulators or gold-plating.
Highly conductive metals, such as
gold, can have an unanticipated additional
benefit. They can have lower
friction, resulting in decreased insertion
forces. While housing and contact
geometries are the primary contributors
to insertion forces, when all
other things are equal, gold and similar
materials will reduce the force more
than tin and copper.

Required tolerances

Competitive batteries can differ substantially
in terms of dimensional
specifications. For example, according
to industry standards, a CR2032 coin
cell can vary in height by ±0.3 mm, or
10% of its total height. Therefore, it is
critical that a coin-cell holder be adaptable
to normal height variances without
accepting incompatible batteries.
It is unacceptable to have a connection
that is too loose, as it compromises
electrical
performance.
This is less of an issue if the
cells are factory installed and designers
have control over the battery that
is used, but if the end user can purchase
a replacement, the full range of
battery sizes should be supported.

Manufacturing integration

When designing products for highvolume
manufacturing, coin-cell
holders should be supplied on standardized
tape-and-reel packaging for
pick-and-place assembly. In addition,
applicable government or industry
regulatory compliance requirements
such as RoHS and lead-free should also
be taken into consideration.

Solderability requirements

Soldering processes also affect the
choice of coin-cell holder. For example,
a coin-cell holder requiring SMT
soldering should be made of highquality
LCP plastic that offers exceptional
dielectric strength at high temperatures,
and can withstand 300°C
lead-free reflow-process temperatures.
By contrast, wave-soldering processes
require less-rugged materials, which
allow the use of PBT/Nylon plastic insulator
material. This material offers a
dielectric strength of 560 V/mil at
25°C for 5 s, as well as resistance to
chemicals and solvents, a broad service
temperature range with excellent
thermal cycling performance, and insulator
resistance of 5,000 MΩ min.
An incorrect material selection for
the manufacturing process can lead to
many line rejects. Comprehensive
product test data should be requested
to ensure that the coin-cell holder
meets or exceeds ANSI/EIA-5405000
standards and that the quality of all
raw materials used is superior.