In the 2013 film Gravity, there is a scene in which Sandra Bullock has to reduce the power budget of her escape vehicle to allow it to reach the next space station and salvation. Tom Hanks faces a similar power problem in Apollo 13. Both scenarios illustrate the need to match the power budget of a device to the capacity of its battery. Medical device designers face a similar quandary.
Many medical devices use AC electrical power from the grid and a battery as back up. This creates a clear contrast with the power budget required for a typical handheld consumer device, such as a smartphone or tablet. This means that, in many cases, the battery only really comes into play as a backup, or when the device is being moved around a hospital.
However, there are exceptions in the medical device environment. For instance, an aspirator on an ambulance would typically run primarily from battery power; it would have an AC power option as well as charging from the grid.
Creating a power budget
In order to create a power budget, the medical device designer has to decide on the current requirements of the device, including needs, such as screen size, the compressor (if needed), built-in pumps or fans, and any onboard computing needs. This will allow the designer to determine how much power will be required, which, when combined with the runtime, will create the power budget.
The best way to manage a device's power budget is to turn off things it doesn't need – just like Tom Hanks and Sandra Bullock did in their films. For instance, one could reduce the processor speed on the device or dim the screen.
Then, when developing the battery power strategy, the first question to be answered is how many batteries are required to provide the requisite power. For example, where the power budget is 50 watt-hours (Wh), a single Lithium-ion battery, with less than 100 Wh capacity would be more than adequate.
This sounds straightforward, but problems arise when further functionality is added in the relationship between the device, its power requirement, and the battery. For example, if there is a need to hot swap the battery during use, such as on a perfusion system that is keeping a patient's hearts or lungs functioning for instance, the design has to have at least two batteries in the system. This remains true even if only one could meet the power budget in a non-hot-swap environment.
In the perfusion system example, the design would have one battery operating most of the time and the other providing power during the hot swap. The first battery, which stays in the device during the swap, is called the bridging battery. The switch back from the bridging battery to the removable one, after it is charged, would normally be automatic.
Hot swapping allows a device to run for a long time without recourse to AC power from the grid. The only restriction is the number of charged backup batteries available and the lifespan of the bridging battery, which will itself eventually run down. A practical example of a medical system that utilises this bridging technique is a ventilator used for patient transport.
The crux is that the bridging battery has to be able to run the entire device on its own. Hot swapping won't work in a system with multiple batteries sharing the current equally across all of them.
Changing needs of medical devices
Technology is constantly changing, becoming smaller and more efficient. As a result, Original Equipment Manufacturers (OEMs) will often need to reduce the footprint of a battery to allow for a reduction in the size of the device itself. These size reduction demands are driven by increased requirements for portability, emulation of consumer technology, or the demands of the medical professionals using the devices.
To comply, a smaller battery can always be used, but an OEM has to accept that this will result in reduced runtime, reduced cycle life, or poor low temperature performance.
The universal drive is to design a device that is smaller and lighter, but the battery is often the last thought, which can create power budget problems. Bear in mind that there is no Moore's Law for batteries; Lithium-ion technology is the most energy-dense, commercially viable way of storing power and it can't be made smaller without storing less power. Designs are limited by the chemical couples of the metals that are available.
It's very difficult to provide an OEM with every performance trait they would like in an ideal world. As a result the negotiation about what elements of the original specification are most important, and which ones can be de-prioritised, is crucial.
Power budget metrics
Calculating the power budget requires considerations such as the required input operating voltage, how much power the device consumes (for calculating required current), the projected runtime, how long the battery can be allowed to re-charge, likely operating temperatures, the duty cycle, and the overall lifespan.
Normally in medical applications, good operation in high temperatures is a more important issue than low temperature operation. The battery sits inside the device where the temperature can be up to 120 °F (49 °C) continuously. In contrast, charging needs are very much a moveable feast; there are applications in which a battery has to charge in an hour and others where a 24 hour charge time is more than acceptable.
Battery lifespan can also be a radically different requirement depending on the medical application. In some instances, changing the battery every year is fine – from both a practical and financial perspective. However, if a battery designed for a frequently used application is only capable of 300 cycles, which is to say 300 full charges and 300 full discharges, it would be unlikely to last for even one year. Hospitals expect the battery to last for two to three years.
In a medical environment, more than any other, it is crucial that the batteries meet stringent safety standards. For instance, the testing of batteries to IEC62133 is mandatory for medical devices certified to IEC/EN60601-1 3rd edition.
In the U.S. the battery may also need to be tested to UL2054 and in Europe it needs to be CE marked to confirm compliance with all applicable EU legislation. Transportation testing of all Lithium ion batteries is mandatory and batteries with an energy exceeding 100 Wh have their transportation far more heavily regulated.
However, it is often sensible to subdivide a battery if it needs to provide more than 100 Wh capacity (Figure 1). This has the added bonus of improving reliability and allowing for hot swap functionality, as well as removing it from dangerous goods classification. Finally, in the U.S., the medical device manufacturer will need to be FDA audited and the battery partner should be able to provide all the relevant testing certificates to allow that audit to be completed satisfactorily.
In this context, designers can take a pre-tested and certificated range of batteries, such as Accutronics' Entellion range, and design them into a device. This saves a great deal of research and evaluation time, allowing for quicker time to market.
Case study: Reducing battery size
In one recent application, Accutronics was asked by an existing customer to reduce the thickness of its product by 50 percent to stay ahead of its competitors. This was achieved by moving from traditional Lithium-ion prismatic cells, which have heavy aluminium cans, to using Lithium polymer technology, like that used in tablets. This results in a larger footprint but thinner batteries. Moving from a liquid electrolyte to a gel was one factor that allowed for such a reduction to be possible.
This emulation of tablet technology might sound simple, but one has to consider that a tablet will normally have one cell, while this device needed nine cells to provide its power budget, and the battery had to be removable. Furthermore, embedding a battery to save space isn't an option when it has to be removed for replacement during the device's lifespan.
Meeting difficult medical device power budgets
The complete design process might seem arduous, but it's essential if the power budget of a new medical device is to be met. It's certainly better than the designer's equivalent of the desperate power budget quandary Tom Hanks and Sandra Bullock found themselves in at the end of Apollo 13 and Gravity.