Once the staple of brute-force power supplies and large industrial and consumer power applications, ultracapacitors are now finding their way into products of all sizes, particularly portables. Also called supercapacitors, these components are notable for capacitance values ranging into the thousands of Farads and fast charge/discharge rates.
With the ability to store massive amounts of energy for long periods of time, ultracapacitors behave more like a battery than a standard capacitor. In fact, as things progress, they will replace rechargeable batteries in a plethora of products, from computers and digital cameras to cell phones and other handheld devices.
WHAT'S AN ULTRACAPACITOR?
Basically put, the ultracapacitor or supercapacitor is a very large, polarized electrolytic (electro-chemical) capacitor. In describing these components, though, “large” refers to capacity, not necessarily their physical size.
True, when it comes to generic electrolytic capacitors, the greater the capacitance and/or voltage values, the bigger the overall package. Electrolytic caps typically offer capacitance values in the micro-Farad range from about 0.1 µF and topping off at approximately 1 F with voltage ratings up to 1 kV dc. Usually, the higher the voltage rating, the lower the capacitance value, and the bigger the capacitor and the higher the capacitance value, the bigger the package even as operating voltage may decrease.
The same rules of thumb essentially apply to ultracapacitors. These components come with capacitance values from 1 F and up and operating voltages ranging from 1.5 to 160 V dc or greater. As both values increase, so does capacitor size.
Early-generation ultracapacitors with values in the tens of Farads were hefty clunkers, relegated to large power-supply applications. Smaller ultracapacitors with miniscule voltagehandling capabilities found employment as short-term power backups in consumer electronics.
As capacitor technology evolved, new developments haven’t only leveled the playing field, they’re also beginning to tip it favorably for the ultracap market. Despite the vast similarities between supercapacitors and their electrolytic counterparts, there is a significant variation in their electrical and physical proportions.
For example, a general-purpose 10-µF electrolytic with a 25-V dc rating may measure only slightly smaller or even the same as a 1- to 10-F, 2.7-V dc ultracap. And, with recent advancements, boosting the operating voltage to 25 V dc may translate into a size increase of less than double for the supercap, which may or may not be significant depending on the application.
ANATOMY OF AN ULTRACAPACITOR
Basically, one can view an ultracapacitor as a rechargeable battery. It stores a charge proportional to its capacitance and releases a charge when called upon to do so. What sets the ultracap apart from its electrolytic brethren is an electrical double-layer architecture, which enables higher capacities.
Standard capacitors sandwich a dielectric substrate between two electrodes attached to plates (Fig. 1). Depending on the type of capacitor, the dielectric can be aluminum oxide, tantalum tetroxide, titanium oxide barium, or polyester polypropylene, each of which determines capacitance and voltage capabilities (Fig. 2). The amount of dielectric and the distance between the plates also affects capacitance levels. However, the maximum allowable distance between the plates limits the amount of dielectric.
In this single-layer topology, increasing the amount of dielectric to boost capacitance is usually achievable in one of three ways: widen the package and increase plate size, lengthen the package and increase plate distance, or a combination of both. Any of the three solutions translates into a physically larger capacitor as a tradeoff for more micro-Farads.
Electric double-layer capacitors (EDLCs), as the name implies, solves this problem by adding a second dielectric layer within the same package that works in parallel across a separator with the first layer (Fig. 3). EDLCs also employ nonporous dielectrics such as activated carbon, carbon nanotubes, carbon aero gels, and select conductive polymers, which exhibit higher storage capabilities than standard electrolytic materials. This combination of the extra layer and more efficient dielectric material enables a capacitance boost in the neighborhood of four orders of magnitude.
There is a tradeoff in terms of voltage capability, traceable to the dielectric. In EDLCs, the dielectric is extremely thin, measured in nanometers, creating a large surface area that’s responsible for higher capacitance. However, these thin layers lose some of the desirable insulating properties of conventional dielectrics and therefore require lower operating voltages.
In relation to standard capacitors and batteries, EDLCs have several advantages that make them desirable alternatives. These benefits include a greater number of charge/discharge cycles than rechargeable batteries, efficiencies in the realm of 98%, a lower internal resistance, high output power, better thermal capabilities, and better safety margins than both batteries and standard capacitors.
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Unlike batteries of any type, EDLCs have no special disposal requirements, making them environmentally friendly throughout their life cycle. Formerly large and heavy, supercaps are now available in a wide range of sizes to suit just about any application and almost any budget.
SUPERCAPS FOR PORTABLES
As stated earlier, large capacitance values are no longer a barrier in terms of physical size. Supercapacitors with values of 5 F and greater are finding their way into many portable and handheld products. In some cases, these components can replace the batteries that power these products.
Under the PowerBurst name, the Tecate Group offers a wide range of ultracapacitors in a variety of configurations. For generalpurpose pulse power, hybrid battery, and portable product applications, the radiallead TPL and radial-snap-in TPLS series double-layer components offer capacitance values from 0.5 to 70 F and 100 to 400 F, respectively (Fig. 4). Both series specify a voltage rating of 2.7 V and an operating temperature range from –40°C to 65°C. Maximum heights for the TPL and TPLS series are 45 mm (100 F) and 60 mm (400 F), respectively.
Specifically targeting portable markets, CAPXX offers the GS/GW series single- and dualcell supercapacitors (Fig. 5). Providing an alternative to the power limitations of batteries, the components promise long-life performance up to 2.3 V in single-cell versions and 4.5 V in dual-cell configurations with two cells connected in series.
Both versions operate from –40°C to 75°C. GW series features include a 28.5- by 17-mm footprint, capacitance values up to 0.4 F at 4.5 V, and equivalent series resistances (ESRs) as low as 60 mΩ. Measuring 39 by 17 mm, the GS series offers values up to 0.7 F at 4.5 V and ESRs as low as 34 mΩ.
Also for space-critical designs but where higher temperatures are an issue, the HS and HW series from CAP-XX feature a thin profile and operate from –40°C to 85°C (Fig. 6). With an operating voltage from 4.5 to 5.5 V, the HW measures 28.5 by 17 mm and offers capacitance values up to 0.4 F at 5.5 V with ESRs as low as 100 m at 5.5 V.
Depending on the component, thicknesses range from 0.9 to 2.9 mm. Available in values up to 0.7 F, the HS series measures 39 by 17 mm with thicknesses ranging from 0.9 to 2.9 mm. ESRs are as low as 55 mΩ. Both series handle a pulse current of 20 A and specify an RMS current of 4 A.
Supplanting batteries for memory backup power, Kanthal Globar’s Maxcap double-layer capacitors specify a volumetric efficiency beyond 5.5 F/in.3 and unlimited service life, rapid charge/discharge capabilities, and very low leakage current, according to the company (Fig. 7). Kanthal Globar also claims the capacitors are safer than batteries and will not explode or experience damage under short-circuit conditions. They are non-polarized components and require no current limiting resistors or overvoltage protection, eliminating assembly errors and related costs.
Maxcaps are available in radial-lead (LP, LC, LK, LT, LF, LV, LX, and LJ series) and surface-mount (LM series) versions. With voltage ratings of 3.5 or 5.5 V, capacitance values range from 0.01 to 5 F and 0.47 to 1 and 5.6 F, depending on voltage rating. There is also a 5-F/11-V package on board. Operating temperature ranges from –40°C to 85°C or –25°C to 70°C depending on the series. Additionally, all Maxcaps are low-profile components, deployable in remote locations, and don’t require access ports.
SUPERCAPS FOR LARGER APPS
Although it seems like all electronic designs are shrinking with designers desperately fighting for every nanometer of space, there are many areas where miniaturization is both impossible and undesirable. These include automotive and transportation, renewable energy, military, and aerospace. In these sectors, the physically larger ultracapacitors are the norm.
Perhaps the most visible player in the large supercap sector, Maxwell Technologies broke ground and set standards with its BOOSTCAP products. Based on proprietary electrode technology, components are available in both single-cell and multicell modular configurations.
Modular BOOSTCAP configurations consist of the BPAK and BMOD series spanning 14 modules (Fig. 8). Depending on their applications, users can choose from capacitance/operating-voltage values of 20, 23, 52, or 58 F at 15 V dc; 110, 250, or 500 F at 16.2 V dc; 80, 110, or 165 F at 48.6 V dc; and 94 F at 75 V dc or 63 F at 125 V dc. These modules range in size from approximately 178 by 52 by 32 mm to larger than 515 by 263 by 211 mm. Target applications include industrial, automotive, and consumer.
Also under the BOOSTCAP umbrella, the company offers a range of large singlecell components that achieve very high capacitance levels, though at a lower voltage. The BCAP series offers five cells with capacitance values of 650, 1200, 1500, 2000, and 3000 F at an operating voltage of 2.7 V dc (Fig. 9). Primarily, these cells perform in tandem with batteries for applications requiring a constant low-power discharge and a pulse power for peak loads.
Targeting heavy-duty military applications, the 3STHQ3 and 3PTHQ3 capacitor banks from Evans Capacitor Co. integrate three of the company’s THQ3 hybrid capacitors into an anodized, epoxysealed aluminum case measuring 4.47 by 1.59 by 1.07 in. (Fig. 10). For higher operating voltages, the 3STHQ3 banks wire the capacitors in series to offer four options: 0.004 F/160 V dc, 0.0028 F at 200 V dc, 0.0019 F at 250 V dc, and 0.0011 F at 300 V dc.
For higher capacitance, the 3PTHQ3 banks configure the capacitors in parallel to provide options ranging from 0.45 F at 10 V dc to 0.01 F at 125 V dc. Both configurations operate in temperatures ranging from –55°C to 85°C and include all necessary balancing resistors and wiring.
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For high-current environments, German manufacturer Wima provides a comprehensive range of double-layer, cylindrical components with operating-current ratings up to 400 A and pulse-current withstanding capabilities to 1400 A. The SuperCap C and R lines consist of 2.7-V dc capacitors in values from 110 to 600 F with operating and pulse current ratings up to 100 and 800 A, respectively.
The largest and most muscular, Wima’s SuperCap MC specifies an operating voltage and current of 14 V dc and 400 A, respectively. Weighing 1.7 kg and measuring 325 by 60 mm, 90 mm in height, and 265 mm between poles (±), the component can withstand pulse currents up to 1.4 kA.
Other specifications include a capacitance of 110 F with a ±20% tolerance, internal resistance of 7 m, maximum stored energy of 10 kJ, an operating temperature from –30°C to 65°C, and an operating lifespan of 90,000 hours.
As noted earlier, the big push for ultracapacitors is to replace rechargeable batteries in a range of applications. This is a logical progression, particularly with the current interest in green technologies and the quest for cost-effective alternative power sources.
One of several initiatives under way entails the recent partnering of supercap maker CAP-XX and Perpetuum, noted for its energy-harvesting solutions, to create battery-free, wireless-sensor condition monitoring systems. A case study presented at the nanoPower forum back in June describes how Perpetuum’s PMG17 vibration energy-harvesting microgenerator paired with a CAP-XX supercapacitor will enable the design of battery-free condition monitoring systems. These systems collect and display data on machinery for improving asset management.
According to the companies, traditional condition monitoring systems require manual data collection or the use of battery- powered wireless sensors. Allegedly, batteries may survive only two to five years in the harsh environments associated with these systems. It stands to reason that in a plant with possibly thousands of batterypowered wireless sensor nodes, the cost of replacing and disposing of batteries can add up fast.
In operation, the PMG17 converts unused mechanical vibration into electrical energy, providing a steady power source between 0.5 and 50 mW. The CAP-XX supercapacitor stores this harvested energy and then delivers the peak power needed to transmit sensor condition data over wireless networks such as IEEE 802.15.4 and 802.11.
The PMG17 can provide the necessary power for intermittent radio sensor systems such as Wireless HART, SP-100, and Wi-Fi. However, its output impedance is too high to supply the 10 to 100 s of mW required by the sensor nodes. Resolving this, the high capacitance and low ESR of the supercapacitor provide approximately 1 s of peak power to transmit data.
“The micro-generator and supercapacitor combo eliminates battery reliability issues and time-consuming maintenance while enabling significant savings in operational costs and energy use,” says Stephen Roberts, Perpetuum’s technical manager.
“Wireless system manufacturers can now easily design battery-free systems using this fit-and-forget self-generating power source,” adds Pierre Mars, vice president of applications engineering at CAP-XX.
For details on the PMG17, see “Energy Harvester Perpetually Powers Wireless Sensors,” by Pierre Mars of CAP-XX at www.electronicdesign.com, Drill Deeper 20033. Additionally, visit www.perpetuum.co.uk/resource/PMG17%20-%20Technical%20Specification%20Rev%202%200.pdf. For more information on CAP-XX supercapacitors, visit www.cap-xx.com/products/products.htm.