Perhaps you’ve had the following experience. You grab a long-ignored, battery-powered device off the shelf and try to turn it on. Nothing happens. “Dead batteries,” you think. You can’t remember the type of batteries this particular device uses, so you open it up and take a look. You get an ugly surprise. The battery compartment is filled with goo, or caked white and green crusty stuff, or a mix of the two. The batteries may be swollen to the point that you have trouble removing them. If you’re particularly unlucky, the leaking battery electrolyte has dissolved part or all of one or more battery contacts and the device is ruined. Bad, bad batteries.

Of course, the culprit is you. You didn’t properly maintain the device by instituting a program of regular maintenance and periodic battery checkups. How could you? The device was tucked away on a shelf, perhaps years ago, and you forgot all about it. After all, batteries aren’t the most important thing in your life. You’re a busy person.

This same scenario applies to batteries in all sorts of embedded equipment and servers. As little electrochemical electricity factories, batteries need to be maintained or they will eventually give you a nasty surprise. It’s that simple.

Over the last 10 years or so, ultra-capacitors have started to replace batteries in a variety of electronic applications where large amounts of energy storage are needed. One of the primary uses for ultra-capacitors is memory backup. A more conventional approach to memory backup employs ultra-capacitors to provide backup power to SRAM or DRAM subsystems in the event of a power outage. However, even large banks of ultra-capacitors cannot back up memory for years. A different sort of approach to preserving data in the event of a power mains failure is to draw energy from ultra-capacitors just long enough to move critical data from volatile SRAM and DRAM into non-volatile Flash memory. Then the Flash memory can retain the data for ten years or more with no power at all.

Ultra-capacitors get their high capacities from porous carbon electrodes that provide massive amounts of surface area in tiny spaces. As nanotech research delves into the mysteries of carbon-based nanostructures such as nanotubes, ultra-capacitor storage capacities improve. Coincidentally, the number of ultra-capacitor vendors has recently been increasing and therefore the effort required to evaluate the various offerings has also been increasing.

Characterizing an ultra-capacitor isn’t simple. For long-term use in critical embedded and server systems, you need to know how an ultra-capacitor’s electrical storage abilities change over time, temperature, and voltage. It turns out that the long-term characteristics of these low-voltage devices are extremely sensitive to temperature and to operating voltage. They’re also sensitive to the way they’re charged and discharged, so the design of charging and discharging circuitry is critical to the safe, long-term use of ultra-capacitors in multiple-device banks.

If you’re designing memory subsystems and wish to use ultra-capacitors for power backup, you have two choices. On the one hand, you can mount your own ultra-capacitor characterization program and develop your own charging and discharging circuitry. Alternatively, you might choose to use a pre-designed, pre-characterized power module based on ultra-capacitors that’s specifically designed for memory-backup applications such as the PowerGEM offered by AgigA Tech. Either way, ultra-capacitors offer a good alternative to backup batteries, one well worth investigating.

References:

Energy Storage Industry Needs Novel Circuits And Semiconductors, Bobby Maher, http://electronicdesign.com/Articles/Index.cfm?AD=1&ArticleID=21973&bypass=1

Ultracapacitors Challenge the Battery, John M Miller, http://www.kilofarad.org/files/Ultracap-%20World%20&%20I%20-%20June%202004.pdf