Rechargeable Batteries for Medical Devices

Portable medical devices have benefitted hugely from various advanced technologies which were initially developed for consumer electronic devices such as notebook computers and smart phones, when coupled with these enabling technologies they are being developed faster, made easier to use, and now perform more reliably than ever before.

One continuing complaint from consumers is ‘battery life’. If the battery in your notebook computer runs out while you are writing a report it is annoying, if the battery fuel gauge on your smartphone jumps instantly from 25% to 0% and then shuts off while you are on the train home then it can be frustrating – but in either case no real harm is done. However if a doctor in a hospital is transporting a patient connected to a battery powered portable ventilator or, similarly a paramedic is working on a patient in the street using a battery powered aspirator, then the batteries used to power the portable medical equipment must be completely reliable or patient care could be at risk.  These batteries must work each and every time and never let the medical professional, or their patients down when they need them most.

 

 

Regulatory Framework

Until recently the regulatory framework surrounding the use of batteries in medical devices was poor. There was little or no regulation regarding testing requirements for batteries and it was down to the OEM (original equipment manufacturer) and the battery integrator to work out an agreed quality, safety and performance standard for the battery.

Today, the medical device standard IEC60601-1 (3^rd edition) requires that most battery powered medical devices have their batteries (and the cells within them) tested to IEC62133, which has the catchy title of “Secondary cells and batteries containing alkaline or other non-acid electrolytes - Safety requirements for portable sealed secondary cells, and for batteries made from them, for use in portable applications”. In addition, Lithium ion batteries (and the cells within them) must be tested to ensure they are safe for transport, and of course, the relevant electromagnetic compatibility standards must be met to allow for CE marking.

Of course, the long arm approach of purchasing a product that has met the relevant regulatory standards does not alone ensure a reliable and safe product.  The device OEM has the responsibility of selecting and partnering with a battery integrator who has a proven track record in the development and manufacture of batteries for medical devices – ones that are also certified to ISO13485 demonstrate they have the correct quality management procedures in place and will furthermore strive for the continuous improvements required for the medical device industry.

 

The need for a professional battery Integrator

A good battery integrator will advise the OEM on the correct battery chemistry to use.  Most OEM customers would simply specify ‘Lithium Ion’ because of its high gravimetric and volumetric energy density, but few realise the term is actually a catch-all for a wide number of different cells types that encompass different cell chemistries, cell manufacturing techniques and physical form factors. Selecting the wrong cell type can be very costly indeed and can result in poor performance, customer dissatisfaction or even product recall.

 

The Smart Battery

Understanding how much runtime remains is vital for users of portable medical devices. If a medical professional cannot rely on the battery fuel gauge then they lose confidence in their ability to use the device away from the AC supply and suffer from runtime anxiety – the worry that they cannot complete their task before the battery energy is depleted.

A good battery integrator will gain a deep understanding of how the medical device will be used and design an electronic fuel gauge system into the battery which constantly tracks the charge, rest and discharge activity. These electronic systems factor-in the prevailing environmental conditions and even the age of the battery to provide a runtime prediction which can be as accurate as ±1%. This runtime prediction is automatically communicated to the device which then displays it in a format the user can easily understand – the battery will even communicate warnings when runtime becomes critical so the user can take action and charge the battery or replace it with another.

As safety is paramount, a battery integrator will include active protection circuitry into the design. These protection circuits monitor the voltage of each cell in the battery and prevent its charge or discharge if one or more cells is in an over-charge or over-discharge condition. The battery is also protected against over temperature conditions or if discharge currents exceed predetermined levels. Industry best practise dictates that secondary over-charge protection is included when designing Lithium Ion batteries to prevent them from becoming unsafe should both the charger and active protection fail. Secondary over-charge protection activates a non-resettable fuse, permanently preventing further operation.

 

Conclusion

The market for portable medical devices is growing rapidly and the increased regulation surrounding the batteries that power them can only help to ensure the devices are safer and more reliable than in the past.

Battery integrators continue to play an important part in providing OEMs with the best battery technology available which allows medical professionals to take their equipment and skills closer to their patients where they can make a real difference - after all, the outcome for the patient is what matters most.

Neil Oliver
Technical Marketing Manager