Advanced battery-operated wireless devices require cost-effective power management systems that are miniaturized, robust and reliable.By Tom Blaha, Memory Protection Devices
Figure 1. Glider-style holders. These low profile, nickel-plated phosphor bronze retainers secure a wide range of coin cells from ML-414 (4 mm) to larger CR-2430 (24 mm).
The vast majority of wireless devices are battery-powered. Although there is growing interest in super capacitors and energy harvesting devices, batteries remain a more proven, reliable and cost-effective alternative for the vast majority of wireless applications. Among battery-driven power management solutions, there are numerous alternatives, as competing battery chemistries differ significantly with regard to cost, performance, safety and ruggedness.
Requirements Dictate SolutionsWhen configuring a power management solution for a wireless device, your first step is to carefully evaluate application-specific performance requirements, addressing certain fundamental questions that can dictate the type of battery chemistry to be considered. Critical design considerations include size and weight requirements, energy density, capacity and high current pulse requirements (if applicable), access to a reliable AC power source (which is essential if a rechargeable system is being considered), environmental factors such as temperature range, moisture/humidity, as well as possible safety issues that could arise if the battery is subjected to extreme shock, vibration, puncture or breakage. If the device must be transported by air and the battery is considered hazardous goods, then it is important to know what shipping restrictions may apply with regard to IATA and/or other national or international regulatory agencies.
Figure 2. Assorted battery holders and contact solutions for a variety of applications.
By contrast, if the wireless device is intended for long-term use in remote locations with no direct access to AC power, then it is very likely that a primary (non-rechargeable) battery will be required. Remote wireless applications typically utilize some form of lithium battery, as lithium-based chemistries offer the highest potential, energy density and capacity of any competing battery technology.
Designing a battery holder for a remote wireless application often gets complex, including design restrictions related to size, weight, power and environmental performance considerations, as well as cost factors.
For example, if the wireless device is intended for use in the arctic, then the entire power management system must be designed to be highly robust, able to endure extreme temperature cycles, humidity, expansion/contraction and freeze/thaw cycles. Often, products designed for outdoor use require enhanced corrosion-resistance, so gold-plated battery contacts should be utilized instead of less costly tin/silver- or nickel-plated contacts, which may suffice in less harsh environments.
Common Sense Battery Holder SolutionsWhen designing a power management system for a wireless device, it is import to remember that like most products, battery holders are designed to often-contradictory requirements. When battery replacement is anticipated, it should be easy to install and remove a battery, but it should be held securely. At the same time, battery holders should be small, lightweight, have a minimal footprint, but should survive drop and vibration tests while holding massive batteries.
Product designers must do their best to juggle all of these competing requirements, but compromises are almost inevitable. For example, an application that utilizes coin-size batteries may call for a very secure battery holder, with a tool required to remove the coin cell. A lower cost alternative could call for a looser fitting coin cell, with finger notches underneath, making it easier to pry it out. A third solution could call for a latching mechanism that enhances performance, but the trade-off is added size, weight, and cost.
One simple way to make your product design more robust is to make the enclosure and the battery holder work as a team. For example, if your design allows for a battery access door, consider having ribs or similar features incorporated into the injection molding dies, which turns the inside surface of the door into a battery-holding device, and possibly permits the use of an easy insertion-removal holder without compromising battery retention. This type of solution can be extremely effective for multiple-battery holders (like 4xAAA or AA holders), where the batteries can have a tendency to buckle if not secure. Whenever possible, a support buffer should be added around coin cell holders as well. This may call for the addition of certain plastic features, which could minimally add to cost, but dramatically increases the robustness of the design.
Depending on the configuration, it may be possible to dispense with the battery holder altogether by using loose contacts and springs to fasten the battery. When considering these inexpensive components as an alternative, make sure that the spacing between contacts is appropriate. Also keep in mind that most batteries tend to be on the high side of the ANSI tolerance, so factor in the need to accommodate adequate spacing between the nominal and the upper tolerance limit to help ensure easy battery insertion and removal regardless of the brand.
In situations where it is difficult or impossible for the enclosure to provide additional support to the battery, make sure that the battery holder specified is designed for high retention and shock resistance. This is often the case with PCs or similar consumer electronics that surround the main circuit board with a metal enclosure. Spending a few extra pennies on a more robust battery holder can prevent the far greater expenses associated with thousands of dissatisfied customers, warranty repairs, service calls, and product returns.
A new and novel alternative for this circumstance is the MPD GLiDER line, which consists of a two-piece tray-and-cage design. Glider-style holders combine ease of insertion with excellent retention, and allow coin cell batteries to be stacked to save precious PCB real estate.
Design for manufacturability is also an important consideration, especially if high volume manufacturing techniques are being employed. In such cases, battery holders are typically supplied on standardized tape and reel packaging for pick-and-place assembly. In addition, if the product is being marketed globally, applicable government or industry regulatory compliance requirements such as RoHS lead-free should be followed.
If the circuit's assembly process calls for SMT components, then you might want to specify battery holders with BGA solder balls for permanent attachment, or with a BGA socket interface if battery removal is required. When specifying a battery holder for SMT assembly, it's also critical to make sure that the device is compatible with the soldering process. 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 supports the use of PBT/Nylon plastic insulator material. PBT/Nylon plastic that features a dielectric strength of 560 volts/mil at 25ºC for 5 seconds, resistance to chemicals and solvents, excellent strength and toughness, a wide temperature range with excellent thermal cycling performance characteristics, as well as an insulator resistance of 5000 MΩ min.
With thousands of knock-off products continually flooding the market, it is also becoming increasingly necessary to critically review and evaluate potential suppliers to ensure that the battery holder you purchase performs as promised. An integral part of this due diligence process should involve general requirements for all potential suppliers to provide comprehensive product test data to ensure that superior quality raw materials are being used, and that the battery holder meets or exceeds ANSI/EIA-5405000 standards.
In short, simply apply some basic common sense by performing the proper due diligence when evaluating battery holder manufacturers, then carefully scrutinize all economy-minded design ideas to ensure that corners are never cut and quality is never compromised. Choosing the solution that offers the best overall performance at the best possible price increases the likelihood that the battery holder you specify will be ideally matched to your intended application. Your sound decision-making will be rewarded with years of trouble-free battery performance.
Tom Blaha is president of Memory Protection Devices, Inc., www.memoryprotectiondevices.com, 631-249-0001.