Rob Phillips, Managing Director of Accutronics, a leading independent battery design company servicing the medical sector, provides a brief overview of standards and design issues relevant to designing in batteries (and associated battery chargers) for medical devices.
It’s usually impractical for companies other than the largest to employ dedicated battery specialists internally. For many businesses, major battery (and associated charger) designs occur only every several years. Yet battery and the associated charger technology is a complex field that requires investment in continual reskilling in technology, as well as an understanding of multiple standards and legislation. Keeping internal battery specialists on-hand is often uneconomical and also, while Medtech designers generally know medical device standards, in my experience they don’t have the same knowledge of battery or charger standards.
However, whether you engage with a third party battery and charger specialist or not, project managers responsible for specifying the batteries in a medical device (and being able to charge them) will benefit from understanding the basics of the standards involved, and what some of the more common (and expensive) design issues are, so they can be avoided.
Let's examine a few top line considerations:
Standards – the fundamentals
All electrical and electronic devices (including batteries and chargers) require certification that they have been tested against a recognised standard for safety when the product is subjected to abnormal or abusive operation conditions. Certification to these safety standards is not a guarantee that the product will continue to function after being subjected to these conditions, but that the product has been proven to not cause injury or damage to personnel or property.
Certification requires that the principal safety components (cells, fuses, enclosure materials etc.) have already been certified to their individual applicable standards. If not, then certification will be considerably more difficult, take longer to complete and incur considerable additional qualification testing.
The type of standard required depends on a variety of factors:
- Type of product
- Intended use of the product
- Market that the product is to be sold into (despite harmonised standards, many countries still only recognise their local standards)
- Timing of the product placement onto the market (standards are revised after a number of years to reflect product and market changes)
A fundamental point is that the battery is tested to an appropriate standard for its relationship to the medical device, whereas the medical device is tested to an appropriate standard for its relationship to a patient. The battery is a “component” of the medical device.
For the same reason, a stand-alone battery charger is tested to an appropriate standard for the safe operation by all personnel as a management tool for the battery, and not tested as a management device of the patient.
Standards – Some specifics
Ultimately, an exhaustive discussion of standards related to qualifying batteries in medical devices could fill several very large tomes. For the layman, however, it’s enough to know roughly which standards might affect you. For anything more involved than this, you’ll probably need to seek advice.
The principle battery safety standard is IEC62133 “Secondary cells and batteries containing alkaline or other non-acid electrolytes – Safety requirements for portable sealed secondary cells”. Certain regions such as North America, principally recognise a similar standard; UL2054 “Household and Commercial Batteries”. Taiwan is introducing its own standard (CNS 15364) in 2014.
Although batteries are not classified as medical devices, it’s not unusual to adopt and test for specific elements contained of the Medical Device standard; such as drop-testing or EMC (Electromagnetic compatibility).
Lithium, Lithium-Ion, and Lithium-Polymer batteries are classified in “Class 9 – Miscellaneous dangerous goods.” They have the potential to generate a significant amount of heat or catch fire if damaged, improperly packaged or cared for, than do other battery chemistries. In order to be transported, they must be certified to separate transportation safety test standards, and against the requirements of Section 38.3 of the United Nations document ST-SG-AC10-11, “Recommendations on the Transport of Dangerous Goods – Manual of Tests and Criteria”.
Battery chargers are tested primarily to IEC60950-1 “Information technology equipment – Safety” (and equivalent UL60950-1). As with batteries there are a few geographic differences: again, Taiwan is introducing its own IT safety standard, CNS 14336-1 in 2014.
The Medical Electrical Equipment standard IEC60601 / UL60601, (also EN60601 in the UK), is a suite of separate tests, each with their own set of documents targeting a specific aspect of a medical device, such as direct patient intervention, something a battery or charger on their own, will never encounter.
We work with each customer to establish the most appropriate certification body, applicable standard(s) and additional testing suitable for the intended use and the intended geographic market(s). Customers sometimes ask for unnecessarily high certification standards (which carry a highly elevated test cost premium) and it has not been unknown for obsolete standards to be requested when that standard has been absorbed and refined into a much newer revision.
We recommend that customers contact us early within the development with their requirements for each product, so that we can establish what is needed, at that time, for that product, for their chosen marketplace and application.
Underspecified / over specified
One of the other most common problems battery consultancies see when designing in to medical devices is over specifying and under specifying.
The early warning signs that you, as a designer of the medical device, may have over specified your battery will be either commercial or physical. Either you will end up surprised by the cost of the battery required to meet your specified device runtime, or just how large or heavy the battery needs to be to meet the brief. Generally the negative effects are down to cost. The latest lightweight battery technology, with highly accurate fuel gauging and advanced protection systems, all have a cost attached to them.
We were once required to deliver a battery to a customer that was able to operate from -30°C to +70°C. Although this gave them a superior product to their competition, it drove their costs up significantly. Competitors could offer a battery at a fraction of the price that worked across 80% of the range, and which was ‘good enough’. If you create a product that can do everything for everybody, you may sell nothing to anyone, because you will be overpriced.
In battery design under specifying, on the other hand, tends to manifest itself in short runtimes or short battery lifetimes. Device developers will produce an initial ‘power budget’ outlining how much power each part of the system, (screen, processor, etc.) will use. Problems most commonly arise when additional components or software are added to the device design that increase the necessary power budget, or when the marketing department insists on a smaller battery, (to fit a certain form factor or increase mobility).
Under specifying on battery lifetime is worth a special mention: You need to consider what the total battery life and chargeability of your device is going to be, not just how much it charges initially, but how consistently it will charge. Even medical customers focus unduly on ‘out of the box’ performance and high initial capacities, as if they were buying a mobile phone rather than a high-end medical device. No-one wants to replace the batteries on a medical device that cost tens of thousands of pounds to purchase. But high initial capacity and long life rarely go hand-in-hand. Sometimes it pays to consider lowering initial performance in return for long-term battery lifetime benefits.
Other common issues
It’s also important for designers not to back themselves into a corner when it comes to cell selection. This is another reasonably common issue.
Before 1995 there were a small number of nickel cadmium and nickel metal hydride cell types, manufactured to IEC standard dimensions. Life was easier back then. Now there are literally hundreds of cell sizes and shapes, and you’re by no means guaranteed that a particular cell will be available in the same dimensions in another five years’ time.
In costly medical devices with operational life cycles of many years it’s important to select cells available from multiple manufacturers, and allow space inside cases for alternative cells if required.
Similarly, it can cause problems down the line if you do not opt for a chemistry-independent charging system. These so-called ‘smart’ charging systems allow batteries to request their own specific charging profile. This is useful if higher-capacity batteries or batteries with different charging profiles become available later.
Lastly, it’s frequently worth trying to keep your battery rating below 100Wh, where possible. Lithium-Ion batteries rated at over 100Wh are subject to quite particular transport restrictions. If you can’t keep battery energy beneath this threshold in your design, consider the use of multiple batteries where each battery is rated at under 100Wh.
A surprising number of companies underestimate the extent of the ramifications of battery choices for rest of their device design. Batteries are often seen as one of the ‘simpler’ parts of a design, and by many as a ‘commodity’ part. But getting it wrong, or failing to meet key standards, can lead companies to having to rework their entire product design.
For all these reasons designers need to ensure they work with battery experts that take a holistic view of product design, and can provide expert advice with regards to both the relevant standards and the wider effect battery considerations will have on your product design as a whole. Getting the specification nailed down accurately at the start of the project is key, as is ensuring that you’ve guarded your supply chain and support provision for the lifetime of your medical device.