Battery manufacturers face the power budget quandary

In the film Gravity, there is a scene in which Sandra Bullock has to reduce the power budget of her escape vehicle to allow it reach the next space station; there is a similar scene in Apollo 13, starring Tom Hanks. Both films illustrate the need to match the power budget of a device to the capacity of its battery.

Many medical devices will use AC electrical power from the grid, using a battery as back-up. This creates a clear contrast with the power budget required for a typical hand held consumer device - such as a cell phone or tablet. This means that in many cases the battery only really comes in to play when power is removed or 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 but would have an AC power option, as well as charging from the grid. 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 needed, which, when combined with the run time, will create the power budget. Most designers working on medical devices are very conscious of this and are expert in reducing their power requirements. When this design process is complete, the battery manufacturer can create a strategy for providing the power that’s needed in the device. The first question to be answered is how many batteries are required to provide the requisite power. In an example where the power budget is 50Wh, a single Lithium-Ion battery, with less than 100Wh 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, on a perfusion system that is keeping a patient’s hearts or lungs functioning for instance, at least two batteries would be needed in the system. This remains true even if only one could meet the power budget in a non hot swap environment.

In this example, one battery would be operating most of the time and the other one providing power during the hot swap. The first battery, which stays in the device during the swap, would be referred to as the bridging battery. The switch back from the bridging battery to the removable one, after it is charged, would normally be automatic.

Changing needs

Hot swapping means that a device can run for a long time without recourse to AC power from the grid. The only restriction is the number of charged back-up batteries that can be accessed 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.

Technology is constantly changing, becoming more efficient and smaller. As a result, 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 can be driven by increased requirements for portability, emulation of consumer technology or simply the demands of the medical professionals using the devices.

To comply, a battery manufacturer can always provide a smaller battery, but OEMs have to accept that this will result in a reduced run time, 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. We have to 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 simply can’t be made smaller without storing less power; it is limited by the chemical couples of the metals available.

It’s very difficult to provide an OEM with every performance trait they would like in an ideal world. As a result the conversation about what elements of the original specification are most important, and which ones can be de-prioritised, is crucial.

Power budget metrics

A battery manufacturer needs to establish, by working with the OEM, a number of factors that combine to allow the power budget to be calculated. These include the required input operating voltage, how much power the device consumes (to calculate the required current), the projected run time, 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 50°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. Most OEMs working with hospitals report that the expected lifetime for a battery is two to three years.

In one recent application, Accutronics was approached by an existing customer that needed to reduce the thickness of its product by 50% to stay ahead of its competitors, which was achieved by moving from traditional Lithium-ion prismatic cells, which have heavy aluminium cans, to using Lithium polymer technology, as used in tablets; this resulted in a larger footprint but thinner batteries. Moving from a liquid electrolyte to a gel was one factor that allowed the requirement to be met, which wouldn’t otherwise have been 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 to be replaced during the device’s lifespan.

Certification requirements

In a medical environment, more than any other, it’s crucial that the power budget is provided for in a compliant way. For instance, the testing of batteries to IEC62133 is mandatory for products certified to EN60601. In the US the battery may also need to be UL 2054 certified and in Europe it needs to be CE marked. It will also need mandatory UN transportation testing and, if the battery is more than 100Wh, its transportation is far more heavily regulated.

However, it is often sensible to subdivide a battery if it needs to provide more than 100Wh capacity. 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 United States, 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, it makes more sense to 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. 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.