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Designing-In Batteries for Medical Devices

Fri, 03/14/2014 - 1:44pm
Rob Phillips, CEO, Accutronics

When developing an electronic medical device, there are many factors that need to be considered. Power is just one of the critical areas that must be correct and conform to a variety of considerations. This article looks at battery standards and the design issues most frequently encountered when designing-in batteries.

An example of a custom designed battery and charger, kept separately from the main deviceBefore even starting to talk about battery standards and design pitfalls, it’s worth mentioning that device design companies usually face an up-front choice as to whether to appoint an in-house battery department or turn to independent experts in the battery field.

Although fundamental battery chemistries evolve fairly gradually, certain technologies that sit alongside batteries – such as charger technology, as well as standards – change pretty rapidly. “Battery experts” require
constant reskilling in technology and relevant legislation.

As such, it’s usually only large companies, or companies with an extremely rapid turnover of new device designs, that can afford to employ in-house battery experts. Businesses frequently choose to rely on the expertise of external battery design companies who have a more up-to-date working knowledge of the battery and charger standards and design issues.

Medical Device Safety Standards
Testing a new medical device (including batteries and chargers) in abnormal or abusive operation circumstances is a principal safety requirement. However, complying with safety regulations is not a guarantee that the product will continue to operate under such abnormal conditions, but, instead, simply that it will not cause injury or damage to personnel or property.

It is also required that the fundamental safety components, such as cells or fuses, have been certified to their individual applicable standards. Otherwise, certification is likely to take longer, frequently resulting in a need for additional qualification.

The type of product, the intended use of the product, the market that the product is sold into, and the timing of the product placement are the key factors that determine which standards need to be considered when designing for medical devices.

Despite geographically unified standards, many countries still recognize only their own local standards. This is something worth consulting if planning to introduce a product to a foreign market.

Battery Safety Standards
Both high performance lithium ion (Li-ion) or nickel metal hydride cell (NiMH) batteries present potential safety challenges. As a result, there is an extensive debate on standards with regard to qualifying batteries in medical devices. It is therefore of the utmost importance to stay informed and choose the ones that are most relevant to designing-in a battery for a medical device.

IEC 62133 is the principle battery safety standard with regard to “Secondary cells and batteries containing alkaline or other non-acid electrolytes – Safety requirements for portable sealed secondary cells”. The North American equivalent is UL 2054 “Household and Commercial Batteries,” whereas Taiwan is introducing its own standard (CNS 15364) in 2014.

The general requirements of IEC 60601/UL 60601 (also EN 60601 in the UK) apply to all specific categories of medical electrical equipment. This conformity standard is a combination of four basic tests, each of which relates to a particular aspect of a medical device.

In turn, battery chargers are tested primarily to IEC 60950-1 “Information technology equipment – Safety” (and equivalent UL 60950-1) with Taiwan again being a geographical area with its own IT safety standard – CNS 14336-1, to be introduced in 2014.

Batteries and Transport
Lithium, lithium-ion, and lithium-polymer batteries are often classified as dangerous products compared to other battery chemistries. They have the potential to generate a significant amount of heat or catch fire if damaged, improperly packaged or cared for, and their transport requirements are specified in Section 38.3 of the United Nations document ST-SG-AC10-11, “Recommendations on the Transport of Dangerous Goods – Manual of Tests and Criteria.” Additionally, lithium-ion batteries rated at over 100Wh are subject to strict transport restrictions; therefore, in order to circumvent these regulations, consider the use of multiple batteries, where each battery is rated at under 100Wh.

An example of a concept for a smart battery, such as might be used in a range of medical devices

The Dangers of Underspecifying
An underspecified battery is usually evidenced in either short runtimes or short lifetimes. Short runtimes are usually generated by late additions to a device’s required “power budget”; for example, by the adding-in of additional components once the battery has been specced.

It is therefore important to consider what the total battery life and chargeability of the device is going to be; not only how much it charges initially, but how consistently it will charge. The choice that often needs to be made is between the battery’s high initial capacity and long life, in order to avoid the need to replace a new battery sooner than expected.

Under specifying on battery lifetime is worth a special mention; what the required total battery life and chargeability of the device is needs to be considered. High initial capacity and long battery life rarely go hand-in-hand, so it can pay to consider lowering initial performance in return for long-term battery lifetime benefits. A good battery specialist will already have in-depth understanding of the market sector and be able to contribute to achieving the performance and commercial objectives.

The Dangers of Overspecifying
Overspecified batteries, in turn, tend to cause either commercial or physical issues; either the cost or the size of the battery may be unacceptable.

The medical device industry often faces a choice between expensive, but lightweight batteries, or less expensive, but rather heavier ones. The lightest battery technology, with accurate fuel gauging and advanced protection systems, is a costly matter. And it should not be forgotten that, sometimes, “heavier” is good enough for the task at-hand.

For example, a client once asked for a battery that operated between -30 and 70°C. Such a broad range of operating temperature increased the battery cost significantly. While it presented, technically, the best battery on the market – and one that could cope with every situation thrown at it – the client’s competition was busy making a battery that could cover 80% of the range, but at a fraction of the cost. This was “good enough.”

Such overspeccing is something seen often. Without a doubt, sacrificing some specific unnecessary requirements can lower the cost of a battery design greatly. Determine what is essential, and what is simply perfectionism.

Be Careful with Cell Selection
Today’s wide selection of cell types can be both a blessing and a curse. The availability of hundreds of cell sizes and shapes ensures a better flexibility in design, but at the same time, as the cell market fragments, there’s no guarantee that a particular cell will be available in the same dimensions in a few years’ time.

Ideally, when designing-in a battery, it’s best to select cells available from multiple manufacturers, and allow space inside cases for alternative cells if required in the future.

Consider a Chemistry-Independent Charging System
It’s always possible that higher-capacity batteries or batteries with different charging profiles might become available later in the lifecycle of a product. As such, it often makes sense to adopt a “smart” charging system; these are able to adapt their charging methods in line with whatever a new battery requests of them.

Chemistry-independent “smart” charging systems allow greater flexibility in terms of the batteries available to a device should there be a need to change battery suppliers, or should more advanced battery designs become available.

Conclusion
The number of factors to consider when designing-in batteries are numerous, and it’s impossible to go into enough detail to cover them all in one article. However, rather than embarking on the continual back-and-forth of speccing and respeccing that occurs all too often with battery specification, a designer might be surprised how smooth and simple the process can feel once a third-party expert is included.   MDT

For more information, visit www.accutronics.co.uk.

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