A Designer’s Guide to Coin Cell Battery Holder Selection
While breakthrough technologies continue to grab headlines, components like the humble coin cell battery holder often get overlooked, despite playing a major role in support of their glamorous cousins. However, as electronic devices become increasingly feature-rich, power-hungry, and miniaturized, coin cell holders can become an unanticipated failure mode, as harsh manufacturing and operating environments take their toll.
Design engineers face growing challenges developing power management solutions that are capable of handling real-world environmental issues related to vibration, heat, shock, humidity and corrosion. These environmental conditions can jeopardize electrical connectivity, leading to product failure, service calls, and possible breakage caused when untrained end users attempt to replace a battery.
Facing ongoing challenges related to contradictory design requirements, optimal solutions often require a careful balancing act to ensure that cost targets can be achieved without compromising quality or customer satisfaction. By focusing on these product criteria, design engineers can achieve product reliability at the best possible cost.
Coin Cell Retention and Removal:
A well-designed coin cell holder must be resistant to shock and vibration while remaining flexible enough to allow for easy battery replacements, a requirement that can vary depending on whether the typical end user is a child, elderly or disabled. Unfortunately these two criteria are often in conflict, as better retention equals tougher removal. Look for holders that have special features that are designed to ease removal, or better yet, get samples and try them out for yourself.
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:
This is an especially critical requirement for applications where frequent battery replacement may be required over the expected lifespan of the product. 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 the holder has been tested for.
Also, 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 is 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. Corrosion problems can be further increased by the presence of electrochemically dissimilar metals, resulting in galvanic corrosion, which can be minimized through the use of insulators or gold-plating.
Highly conductive metals, such as gold, can have an unanticipated additional benefit. These materials can also have lower friction, resulting in decreased insertion forces. While housing and contact geometries are the primary contributors to insertion forces, all other things being equal, gold and similar materials will reduce the force over 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. Remember, too loose a connection is unacceptable, as it compromises electrical performance.
This is less of an issue if cells are installed at the factory and you have control over the battery that is used, but if the end user can purchase a replacement you should make sure that the full range of battery sizes is supported.
Solderability Requirements:
Soldering processes affect the choice of coin cell holder. For example, a coin cell holder requiring SMT soldering should be made of high quality LCP plastic that offers exceptional dielectric strength at high temperatures, and is capable of withstanding 300ºC lead-free reflow process temperatures. By contrast, wave soldering processes require less rugged materials, which allowing the use of PBT/Nylon plastic insulator material. This material offers a dielectric strength of 560 volts/mil at 25ºC for 5 seconds, as well as resistance to chemicals and solvents, excellent toughness and strength, a broad service temperature range with excellent thermal cycling performance, and insulator resistance of 5000 M Ω min.
An incorrect material selection for the manufacturing process can lead to a high level of line rejects.
Manufacturing Integration:
When designing products for high volume manufacturing, coin cell holders should be supplied on standardized tape and reel packaging for pick and place assembly. In addition, you need to factor in applicable government or industry regulatory compliance requirements such as RoHS lead-free.
