2011

Design Considerations for Battery Connectors

As medical devices become more portable and miniaturized, their power connections must evolve to meet these demands.

The future of batteries, like that of so many medical devices they power, is clearly toward smaller, lighter, and more powerful models. Developing concurrently with these new battery designs are battery connectors. As battery connection devices evolve, they have to meet a number of challenges, including: rapid growth of designs; rugged construction; high-density packaging; manufacturing costs; and environmental concerns.

Surface-Mount Button Cell Retainer for demanding applications." class="caption" align="right">Perhaps no device has undergone such continual evolution and development in the last 50 years as the portable primary power source: the battery. Portability, miniaturization, and computerization in medical devices have driven a good deal of this change. Common applications now include power source and memory backup for otoscope, ophthalmoscope, blood pressure machines, vital signs monitors, thermometers, defibrillators, blood glucose monitoring systems, dermascope, ultrasound equipments, audio/video systems, pulse oximeters, strength and fitness measuring instruments, data loggers, and other sophisticated applications.

Driven by these many applications, changes are being seen in a variety of battery characteristics. Lithium-ion batteries are commonly used in portable medical devices because of their high energy-to-weight ratios, lack of memory effect, and slow self-discharge when not in use.

Regardless of change, the goals in battery development over the years have remained the same: longer life, smaller and lighter packages, and more power, delivered economically and safely. New high rate cells that fulfill the most reliable high-power output with low space requirement and with high pulse current drain have been achieved. Low self-discharge is another performance goal, leading to longer shelf life. Today, a five-year shelf life is common and extends to 10 years in certain demanding applications. The coin cells used for memory backup in some medical equipment are likely to outlast the life of the equipment.

There is a direct connection between the battery and the device it powers. The bridge between them is the connection device, and this has had to evolve as well. Early battery connectors were crude compared with today’s devices. As battery connection devices have evolved, they have had to meet a number of challenges. The lithium batteries can be dangerous if not used properly. Improper use can cause Li-ion batteries to explode; therefore the holders and retainers must be designed to prevent such an occurrence.

Rapid growth of designs. There are virtually as many battery holders, retainers, contacts, clips, straps, and snaps as there are battery configurations. Most configurations today are designed so that the batteries can be quickly and easily installed and/or replaced. They come in horizontal and vertical mount configurations, with single or multiple contacts for series and parallel applications. They are used on PCBs or in self-contained battery compartments, including such features as polarity protection and battery retention latches.

Rugged construction. With the trend toward smaller and lighter designs, lighter contact forces must still provide reliable connections. They have to be rugged to hold the battery securely while withstanding shock and vibration, but at a fraction of the size and weight originally used.

In demanding applications where cost and space are concerns, contacts and clips are used. Retainer clips are available to lock the batteries in place while offering the lowest possible profile and preventing loosening or shifting of the battery.

Design considerations. Lower voltages and less amperage affect engineering on both sides of the connection, so contacts need to be made of the proper materials for given applications. Where such alloys as phosphor bronze and beryllium copper were once reserved for high-performance interconnects, they are now used in battery contacts.

High-density packaging. With space at a premium in many medical consumer electronics, not only the batteries, but also the holders and retainers are following a trend toward miniaturization. As a result, low-profile holders and compact battery contacts are necessary. They should be balanced and lightweight for reliable tape packaging and machine pick-up and placement.

Manufacturing costs. Today, the cost of materials, including specialized plating, has made tooling an integral part of any application. Connectors have to remain economical while providing reliable performance. Many manufacturers must use high-speed presses and multiple cavity molds to reduce cost.

Environmental concerns. In the past there were few if any environmental concerns regarding battery disposal. Recognizing the devastating effect that lead, cadmium, and lithium have on the environment, the government now regulates the disposal of batteries. Regulation, in turn, has encouraged the development of a battery holder that allows the battery to be removed quickly and safely. The utility of an inexpensive but durable battery connection solution is seen in today’s electronics. When PCBs are disposed of, the batteries on the boards may be disposed of separately, so removing a battery is as easy as putting it in. Battery holders and retainers must be environmentally friendly and RoHS-compliant.

This technology was done by Keystone Electronics Corp., Astoria, NY. For more information, visit http://info.hotims.com/34453-192.

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