Selecting coin-cell battery holders

Some key criteria should be considered before including one of these devices in a design

BY THOMAS BLAHA
Memory Protection DevicesFarmingdale, NY
http://www.batteryholders.com

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.

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 high-quality LCP plastic that offers exceptional dielectric strength at high temperatures, and can withstand 300C 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 25C 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.

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, applicable government or industry regulatory compliance requirements such as RoHS and lead-free should also be taken into consideration. ■