Top 5 ways to select the right battery for your device
One of the key themes in the film surrounds the effectiveness of battery powered devices. Set during the backdrop of hurricane Katrina in 2005, the film depicts the struggle of Nolan Hayes (Walker) who brings his pregnant wife Abigail to a hospital in New Orleans for the delivery of their daughter. Tragically, Abigail passes away following a complication during child birth and the premature baby is placed on a ventilator to keep her breathing.
The hospital is evacuated a few hours later when the hurricane hits, taking out the mains power supply. Subsequent flooding of the hospital takes out the backup generators, leaving the ventilator running on nothing but the onboard battery. By sheer luck, Nolan manages to find a hand-crank generator and connects this to the battery, only to see the monitor show "battery remaining – 03:00 minutes"
The rest of the film charts the trials and tribulations of Nolan as he tries to go about getting a message to rescue services, all the time having to contend with an ever diminishing battery capacity as well as dwindling food supplies and thieves.
Although this situation appears at first to be completely farfetched, in reality it may not be that implausible. The last few years have certainly seen a rise in the use of portable medical equipment – specifically high powered devices such as acute ventilators and anaesthesia workstations where batteries are required to provide backup power in the event of a power outage.
Although increased portability gives doctors and paramedics the flexibility to transport patients, undertake more fieldwork and rely on auxiliary power when mains power fails, a lot is being demanded of the humble battery. Whilst it’s not a big deal if your mobile phone dies mid-conversation, it’s slightly more important that your life-support machine keeps going as any failure can, quite literally, mean the difference between life and death.
At Accutronics, we design and manufacture batteries and chargers for medical devices and have worked with many of the world’s leading medical device manufacturers to ensure their batteries are reliable, safe, cost effective and easy to use. So why did the battery in the film only have three minutes of charge and what could have been done to avoid this situation?
Here are our top five tips for getting the right battery for your medical device and to ensure it works when it is needed most:
1. Select the right chemistry
There are numerous battery technologies to choose from, each with their own specific performance traits. Established chemistry couples such as Nickel Cadmium have now mostly been excluded from the market due to environmental legislation and lead acid batteries are large and heavy making medical devices bulky and cumbersome.
Nickel Metal Hydride offers a cost effective solution with higher energy density but issues with heat evolution and a lack of technology investment make it a doubtful choice for the future.
It is Lithium ion which is proving itself to be the reliable chemistry of choice for new medical devices. ‘Lithium ion’ is the umbrella term for a battery technology that uses the intercalation of Lithium ions between a graphitic anode and a layered oxide cathode. The technology provides high energy density, excellent safety, low self-discharge and outstanding cycle life.
Through careful selection of cathode formulation and cell construction a wide range of Lithium ion cells have been developed that provide specific performance attributes, such as high discharge capability or high volumetric energy density.
2. Specify a ‘smart battery’
To gain maximum performance from batteries in medical devices they should be made part of the power management system where battery, charger and host device communicate with each other to maximise safety, efficiency and performance. These so called ’smart batteries’ only request charge when they need it, smart batteries charge more efficiently and use less power.
Smart batteries maximise the runtime per discharge cycle because they tell their host device when to shut down based on a highly accurate remaining capacity prediction. This method is superior to dumb systems that use a fixed voltage cut-off. Host medical systems that use smart battery technology can provide accurate, meaningful runtime information to users – of vital importance in a medical environment where power failure is not an option.
Smart batteries constantly track their own capacity whether they are being charged, discharged or stored. Their battery fuel gauges use correction factors to adjust for changes in temperature, charge rate and discharge rate together with further modifications as the battery ages. Properly designed and calibrated smart batteries can predict their capacity to within 1% which means that medical device manufacturers can provide users with a device runtime figure they can trust.
3. Size the battery correctly
It is important that the battery inside a medical device is correctly sized for the job it needs to do. Users that think their battery may not power the device for long enough may suffer from runtime anxiety and refuse to use the device on battery power or never move far away from an AC power outlet.
Medical device designers should think carefully about their device’s power budget when running on battery power and then factor in the effect of battery aging, ambient temperature before deciding on a battery energy rating. If space is at a premium and long runtime is required then hot-swappable batteries may be considered to lengthen the operation of a medical device.
4. Get Certified
Medical devices that are certified to IEC60601-1 must have their batteries certified to IEC62133 which is a safety standard for portable batteries. If the product is to be sold in the USA then the FDA also recommend batteries are certified to another safety standard – UL2054.
For batteries using Lithium ion cell technology then mandatory transportation testing must be performed to provide the battery is safe for transport. A good battery integrator will manage this certification process seamlessly, ensuring worldwide market compliance.
5. Ensure it is reliable and safe
Without the battery, any portable medical device becomes an expensive liability. It is the battery that is the heart of machine and it must work when needed and never pose a risk to its environment.
A good battery integrator will ensure the battery contains ‘nested’ layers of safety that include its cell selection, mechanical design, passive protection and active electronic protection elements. Battery integrators that are certified to ISO13485 have the expertise and processes in place to ensure risk assessments, design reviews and performance verification is conducted diligently and thoroughly.
Although this movie presents a highly unlikely scenario, it’s not impossible. And, of course, if all the tips above had been followed there would not have been a movie but it certainly provides us with things to think about to prevent a similar scenario. OEMs and medical practitioners would be wise to give a second thought to the backup power in their portable devices.
So how did the movie end? In the end, it wasn’t all doom and gloom. Despite the subpar medical equipment, baby Abigail, fittingly named after her mother, pulls through and wakes up just as the storm passes… a happy ending naturally.
About the author:
Neil Oliver is Technical Marketing Manager at Accutronics Ltd – www.accutronics.co.uk
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