Safety and reliability head the priority list when designing battery packs into medical devices.
The healthcare industry has been trying to cut the electric cord since the 1950s when Earl Bakken and Palmer Hermundslie invented the first transistorized cardiac pacemaker with an internal battery. Over the ensuing decades, manufacturers have continued to introduce devices—hearing aids, digital thermometers, defibrillators, nerve stimulators, suction pumps, ultrasound equipment, all manner of surgical instruments, and even a new implantable heart-assist pump—that use internal batteries as a power source.
Convenience, portability, and ease of use have fueled this market trend. But one of the biggest challenges remains designing well-engineered battery packs. While improvements in battery technology have created new opportunities for the introduction of smaller, lighter, smarter, and more portable medical devices, design engineers who work on battery packs must balance many operational and environmental needs to ensure that the power source is both reliable and safe.
Battery Power Requires Expertise
Over the last 20 years, the marketplace has increasingly favored the use of lithium-ion (LiIon) cells—for good reasons. Most LiIon cells have a charge voltage three times greater than a nickel-cadmium (NiCd) cell, which means that a single LiIon cell produces three times the voltage of a NiCd cell. When it comes to performance, a LiIon battery provides nearly 10 times the energy density of a comparable sealed lead acid (SLA) battery. This makes LiIon an attractive alternative to lead-acid and even NiCd batteries, especially when size and weight are key considerations.
Creating LiIon battery packs that are safe and reliable requires skill, specialized knowledge, and experience. LiIon battery cells can be extremely safe to use in a range of applications, but assembling multiple cells into a pack with sufficient power and run time to operate a device requires careful design to deliver optimal performance. All battery packs, but especially LiIon packs, need to be part of a properly designed system or they may rupture, ignite, or explode when exposed to high temperatures, drops, or other abuses.
Improperly assembled or counterfeit battery cells also can be an issue. For example, a counterfeit cell can be contaminated with small metallic particles that can cause a battery pack to short-circuit. Detecting counterfeit cells can be extremely difficult; therefore it’s best to secure battery cells from a reliable and traceable source. In this way, you can quickly track an issue if a problem is discovered after a product is released.
If a cell in a LiIon battery pack does short, it can cause adjacent cells to overheat and lead to a condition known as thermal runaway, in which the cell’s metallic lithium melts and catches fire, generating temperatures hot enough to melt aluminum (1220°F). Thermal runaway prompted battery recalls in 2006 and again in 2008, when consumers began reporting problems with the battery packs in their Apple, Dell, HP, Toshiba, and Lenovo laptops. A similar battery pack problem affected 5.9 million cell phones in 2006. The resulting recall cost the cell-phone maker $250 million in replacement battery packs. But that wasn’t the complete picture. After the firm added in free battery replacement, shipping, labor, sales loss, phone replacements, and other damages, the recall actually cost a whopping $429 million! In addition, the bad publicity pushed the company’s stock price to historic lows.
The chart reflects some serious battery recalls, which can have an enormous financial impact.
Safety is especially important in medical applications, because patients are often wearing (or implanted with) a device with a battery pack. Even though the medical-device community is subject to rigorous regulatory oversight, it has had issues with battery pack malfunctions and isolated recalls of implantable and external defibrillators, instant thermometers, insulin pumps, and surgical instruments. Improvements in technology and manufacturing processes have led to fewer incidents since 2006, but continued vigilance and process improvement are necessary to ensure that this trend continues.
Make it safe and reliable
All electrical devices are subject to safety and reliability requirements. But how and where the device will be used are perhaps even more important considerations when dealing with a battery-powered medical device. Will it be used in an oxygen-rich operating room, in a doctor’s office, or in an ambulance? How long will the device run? Are hot-swappable battery packs required? What type of battery-pack assembly is most cost-effective for the application?
When designing a battery pack, you should consider all these issues along with requirements for voltage, accessibility, run time, charge time, and form factor. Each requirement has to be weighed and considered to ensure that the battery system will deliver optimal performance under real-world conditions.
For example, a battery pack can act as an ignition source under specific conditions if it isn’t designed with non-sparking components. Similarly, there’s the potential for thermal runaways (at least with LiIon batteries) if protection circuits aren’t included to mitigate power surges. Good engineering is even more critical with some newer types of advanced batteries, such as those that use lithium iron phosphate (LiFePO4) as a cathode material.
Specific types of protection devices include integrated circuits (ICs) for controlling battery cell voltage. ICs prevent cells from over-charging or -discharging by controlling a cut-off switch and monitoring voltage across the switch’s MOSFET structures. Well-designed battery packs also include a resettable fuse (also known as a polymeric positive temperature coefficient device, or PPTC). Such safeguards are essential for preventing surges or sags in voltage that could damage the medical device, its battery pack, or both. Finally, an effective design also includes a thermistor that measures temperature and shuts down the pack if it increases beyond a predetermined maximum.
All these protection strategies can be tied to a “smart battery” solution, or a microchip that communicates with the battery pack and recharging source. The microchip can sense when a recharging is needed, and during recharging it can protect the battery pack against overcharging. This recharging management technology helps promote cell life and protects against adverse conditions like thermal runaway. Simple onboard indicators can also tell users whether a device is ready for use or if it requires recharging or maintenance.
Medical devices are subject to unique requirements, such as the need for sterilization after procedures, which can affect the pack’s design. To ensure that a battery-powered device will remain safe and reliable, its pack must be designed for repeated exposure to sterilization conditions. In the case of an autoclave, this usually means treatment for a minimum of 20 minutes at a temperature of 266°F. These kinds of high temperatures, moisture, and pressure conditions can stress the most robust piece of electronic equipment.
Use industry guidelines
Battery packs don’t have to be approved by regulatory agencies such as the FDA, but packs for medical devices should be independently tested and certified. Fortunately, there are many independent agency certification programs to ensure LiIon battery pack performance and safety. These programs are stringent and rigorous, and having their seal of approval provides users with added confidence that they are implementing the safest battery packs possible.
Let’s now detail some of the more critical industry standards, guidelines, and certifications for ensuring reliability. Underwriter Laboratories (UL) certifications such as UL 1642 and UL 2054 subject a device with LiIon batteries to a variety of tests to evaluate its electrical, environmental, mechanical, and user-replaceable performance. These two certifications apply to LiIon and nickel-based batteries rated at less than 10 Amp-hours, but UL 2054 is a much broader standard and offers an end-product-level certification or UL listing. The testing under this standard is divided into five main categories of evaluation: electrical, mechanical, battery enclosure, fire exposure, and environmental.
The International Electrotechnical Commission (IEC) oversees a series of standards as part of IEC 60601. Testing and certification for medical devices are focused on demonstrating risk management. It should be noted that this gives companies the option of following the IEC’s question-based approach or a registration audit approach developed by the International Organization for Standardization (ISO). UL 60601 is the harmonized standard for IEC 60601 and provides another pathway to certification.
The Cellular Telecommunications Industry Association (CTIA) has expanded the LiIon safety and performance testing program to include the IEEE 1625 and IEEE 1725 programs. The uniqueness of these two standards is that they look at the entire system (Host device, Battery, Charger, Cell and User) for safe design.
The IEEE 1625 standard was developed for mobile computing by manufacturers of rechargeable lithium-ion or lithium-polymer cells, battery packs, battery and power management semiconductors, and portable computing systems. It’s specific to batteries with LiIon cells in a multi-series configuration (for example, two or more batteries in series), which can be used in wireless network devices. IEEE 1625 offers design methodologies to facilitate developing reliable and safe battery packs that still provide desired features and functions. A designer uses these standards to determine critical operational parameters over time and across different environmental conditions.
Gaining overall compliance requires conformance with each and every subsection of the standard. A portable device can’t achieve compliance without consideration of all the related subsystems—including the user. To achieve compliance, designers of a subsystem like a battery pack must thoroughly review their designs individually and in conjunction with other subsystems to identify faults that could propagate hazards. Once the designers ascertain that the subsystems all conform to their particular standard requirements, they must conduct further analysis to assess the overall system compliance to ensure that the design doesn’t allow two faults of any type to propagate a hazard.
The IEEE 1725 standard establishes criteria for design analysis for qualification, quality, and reliability of rechargeable LiIon and lithium-polymer batteries for cell-phone applications. This standard covers the construction of the pack’s electrical and mechanical components, packaging of the technology, and mechanisms for controlling cell-level charge and discharge, among other considerations.
The purpose of this standard is to ensure reliable user experience and operation of batteries in single series or 1S application in devices using wireless networks. No single piece (cell, battery, charger, or phone) can be compliant alone—the combination of pieces must be compliant. Although these standards and compliance to the standards are currently being used in U.S. by the cell phone carriers (Verizon, AT&T, etc), they provide valuable guidelines for other battery applications.
The International Protection Rating (also known as the Ingress Protection Rating) defines how well an enclosure excludes solids and liquids as defined in IEC 60529. Battery packs for medical devices typically are designed for a rating of IP67. The first digit of this rating indicates that the battery pack enclosure is dust-tight. The second digit indicates that the enclosure will remain watertight to a depth of 3.28 ft. North American enclosure rating systems are defined in National Electrical Manufacturers Association (NEMA) 250, UL 50, UL 508, and Canadian Standards Association C22.2 No. 94. NEMA addresses a wider range of product features than does the IP rating. This means an IP67 rating is equivalent to NEMA 6, but NEMA 6 (which is more encompassing) may not be equivalent to IP67.
The ISO13485:2003 standard applies to medical devices and incorporates the FDA’s good manufacturing practices. Companies with this certification have implemented a quality management system that ISO has certified consistently meets customer and regulatory requirements for producing medical devices and delivering related services.
Even if you don’t have an in-house lab, you can ensure compliance with industry guidelines by consulting with an independent battery testing facility. Such a facility will meet CTIA battery certification and UL 1642 and UL 2054 certifications requirements and be authorized to carry out the performance tests noted previously.
Using this approach will enable you to validate that sourced batteries are legitimate and capable of meeting your marketplace’s requirements. You can also ensure that the battery pack being developed for your device meets all necessary performance, safety, and environmental requirements.
Battery power is growing
The popularity of battery-powered medical devices will increase as medical practitioners discover the advantages of going cordless. Added flexibility means they can complete procedures more quickly during routine visits while still providing the high-quality service patients want.
Device makers are presented with a unique market opportunity. But any cordless device, no matter how groundbreaking, will be only as effective as the battery-pack system that powers it. Close attention to system design and rigorous adherence to industry and government standards are essential at all phases of product development.
About the author
Riad Nakhleh is the director of product solutions for Palladium Energy and leads the development of innovative design engineering solutions for the company across a wide variety of verticals.