Medical devices have become increasingly reliant on batteries to ensure proper operation during power disruptions--especially if the device is PC based, which requires enough energy to operate the system until safe shutdown is achieved. And while most critical devices have some sort of internal battery for backup, these batteries are often neglected and can be unreliable when needed. Recent tragic events, such as the tsunami in Japan and tornadoes in the Midwest, underscore this need for a more reliable form of energy storage. Offering an alternative technology, the present generation of ultra capacitors is proving to offer a reliable solution to these energy-storage drawbacks.

All batteries operate on the same basic principle: They use a reversible chemical reaction to store energy, and therefore have a limited lifetime that typically lasts two to three years. In addition, batteries will be destroyed if left in a zero-voltage state for any length of time. While battery manufacturers have been improving the energy density and cycle life of batteries, batteries will need to be replaced at some point, regardless of the design. And that's where the problem occurs.

Enter the capacitor. Capacitors have actually been in existence longer than batteries and offer a significant benefit over batteries in the way that they store energy. Instead of relying on a reversible chemical reaction, capacitors store energy electrostatically--accumulating opposite charges on a conductor plate separated by an insulator. Because of this energy-storage method, there is no limit to how many times capacitors can be cycled. They are also not as affected by temperature as batteries.

The main drawback to capacitors has traditionally been their size relative to the amount of energy they can hold (volumetric efficiency). Recent developments in design, however, have improved energy densities by many orders of magnitude. As a result, a new class of capacitors called ultra capacitors has emerged that are carbon based and environmentally friendly.

In contrast to traditional batteries, ultra capacitors do not present a maintenance or replacement issue; basically, once they are installed, you can forget about them for the life of the product. Furthermore, the typical cycle life of an ultra capacitor exceeds 500,000 cycles compared with about 2000 cycles for the best lithium-ion units, which are typically the go-to batteries of choice.

Another problem that all batteries have in common is that there is no truly accurate way to determine the remaining charge on batteries. We have all seen the icon on our computer indicate one hour of power left, only to have the unit shut down in 20 minutes! The remaining energy in ultra capacitors, on the other hand, can be precisely determined simply by measuring the terminal voltage, thereby ensuring reliable operation.

In that same vein, charging is a critical part in maintaining capacity for both batteries and ultra capacitors. Batteries do suffer from limited charge acceptance, however, and typically require from two to six hours of charging. Ultra capacitors, in contrast, have no limit as to how quickly they can be charged other than available power limitations. Typically, ultra capacitors are charged in minutes rather than hours.

But no technology is perfect. Current ultra capacitors are about two to four times larger than lithium-ion batteries with the same capacity. However, this will change. Capacitors' stored energy is proportional to the square of the terminal voltage, which means that, unlike batteries, their capacity is simply proportional to voltage. Consequently, improvements in maximum operating voltage will produce big dividends in capacity gains. Ultra capacitor manufacturers have also been developing higher-voltage units, which, at some point in the future, will rival or exceed the energy density of lithium-ion batteries. In addition, ultra capacitors are slightly more expensive than lithium-ion batteries at present, but cost will be competitive with batteries as the ultra capacitor market grows.

When ultra capacitors finally reach energy parity with lithium-ion batteries, they will be the ultimate energy-storage device, eliminating all of the shortcomings of batteries and providing a long-term, bullet-proof solution for portable and backup medical applications.

The opinions expressed in this article solely represent the views of the author and do not necessarily represent the opinions of MPMN, Qmed, or their parent company UBM.