Battery Power on the Move

28 February 2007 (Last Updated February 28th, 2007 18:30)

Portable devices without the right type of battery could have serious health consequences, in the event of failure. Huw Kidwell asks Dr Robin Tichy about the suitability of lithium ion battery power for the medical community.

Battery Power on the Move

The use of portable medical devices in and outside the hospital environment is on the rise. There has been an increase in the sophistication of the devices used by paramedics in the field (external defibrillators, suction pumps and ultrasound
equipment) and all of these have large power demands, which can only be satisfied by the use of advanced power cells.

In a similar situation, clinicians and technicians in hospitals are increasingly using portable medical devices to attend the patient in the ward or even in theatre. These devices also have substantial power requirements. The demands on battery pack
power are therefore greater than ever before, as the two most important factors in the healthcare arena are reliability and safe operation.

Dr Robin Tichy, product marketing engineer at Micro Power Electronics Inc, knows this better than most. The company currently supplies over 70% of the defibrillator market. She explains what is required from a power cell for use with medical
devices.

HK: How can manufacturers tell what kind of battery is needed for a particular device?

RT: Defibrillators are a good illustrative example of what is required from power cells. Defibrillators come in two different types – there is the frequently reusable kind in a hospital environment, which is used by experienced personnel, and
there is also the type for emergency use in an airport or gym.

"There are new higher power, faster recharging Li-Ion cell systems, that are approaching the kind of energy delivery rates of capacitors."

The latter is just for single use by an inexperienced operator and has to have a long shelf life, of at least five years. These are both good examples of battery-powered devices, because the environments are diverse, even though the function is the
same.

The outside defibrillator would use a non-rechargeable battery called a lithium primary battery, which has important characteristics such as a long shelf life, very high energy density and tolerance of a large range of temperatures. However, it is
essentially for one-time use.

The hospital defibrillator would use a rechargeable lithium ion battery, which needs to be recharged after every use and also doesn’t have such a good shelf life. But this type of device would have a definite maintenance schedule to check it
periodically.

HK: Defibrillators generally use a capacitor to concentrate charge prior to delivery – what kind of cells does this require?

RT: The battery pack uses cells fairly similar to those used in laptops, but there are more of them to give a bigger battery pack to charge the capacitor. The voltages are also similar. In terms of advancement of lithium ions there are new higher
power, faster recharging lithium ion cell systems, which are approaching the kind of energy delivery rates of capacitors.

These are currently receiving a lot of press attention, with the two most commonly cited from E One Moli Energy in Canada and A123 Systems of Boston. The A123 Systems cell is in production (1Q/2006) and is to be used in De Walt power tools.

Power tools require high current drain – for example, 80 amps for 30 seconds. Micropower designs and manufactures battery packs for orthopaedic power tools, and these are very similar to the ones used in the commercial product, with similar
power requirements. The biggest challenge for these is the fact that the medical device has to be autoclaved and the lithium ion cells in the battery packs do not respond well to autoclave temperatures; this requires a lot of special design to be
incorporated into the battery pack.

HK: How do you choose which battery type is most suitable for a device?

RT: For medical devices, a manufacturer needs to define a usage profile, which will provide all of the relevant parameters in terms of the amount of run time necessary, the operating voltage, the temperature range under which it is required to operate
and a host of other factors. Based on this, a battery pack can then be designed.

"The trend for battery packs is now towards smaller and lighter with a higher charge density."

We select power cells off the shelf which most suit the need of the device. Then, based on these, a battery pack is designed for the specific requirements of the medical device.

Requirements will include reliability, safety, protection circuitry and thermal sensors, which are incorporated to protect the cells and the medical device by preventing over charging. Another problem for lithium ion cells that has to be solved is the
issue of over and under voltage, which could cause heat dissipation problems. Latest designs also incorporate a "fuel gauge" to indicate how much charge is left for a given application.

The trend is now towards increasingly advanced fuel gauging, which has become a mandatory requirement for medical devices involved with life support, and you need an accurate countdown from 30 minutes to the end of the power.

HK: In the design and fabrication of a battery pack, are there any safety design and manufacturing quality factors that must be considered?

RT: The cells need to be of high quality and the same chemistry. Lithium ion chemistry cells have an advantage over nickel metal hydride chemistry in that they can provide three times the voltage and so you only need a third of the number of cells.
This therefore makes the battery pack lighter and smaller.

The trend for battery packs is increasingly towards smaller and lighter with a higher charge density. The advantage of designing battery packs for medical devices is that we can take advantage of the economies of scale with the laptop and mobile phone
industry, because the same power cells can be bought at a much better price off the shelf than going out and ordering custom power cells for a particular application."

The battery market is constantly innovating in order to be able to offer more efficient, safer and lighter cells for the power-hungry devices of the consumer. The medical device sector has now begun to tap into the same technology and now there are a
multitude of portable and cordless devices, which are beginning to become the norm for clinicians in hospitals and general practice.

Now doctors and paramedics will be able to treat patients effectively in all manner of emergency situations with sophisticated equipment, thus increasing survival rates and reducing ‘Golden Hour’ mortalities.