Path: EDN Asia >> Design Centre >> Communications/Network >> EDLCs' maximum capacitance complement apps that require low maintenance
Communications/Network Share print

EDLCs' maximum capacitance complement apps that require low maintenance

02 Jul 2012  | Dan Strassberg

Share this page with your friends

Marketers describe ultracapacitors, EDLCs (electrochemical dual-layer capacitors), as true capacitors in an effort to distinguish them from rechargeable batteries. You can buy EDLCs in several surface-mountable package styles, as well as in cylindrical and rectangular packages with axial or radial leads, so it's easy to think that EDLCs' performance must resemble that of true capacitors (Figure A). However, many characteristics distinguish EDLCs from true capacitors—even from electrolytic capacitors, which, before the advent of EDLCs, provided the highest capacitance of any form of capacitor.

Controversy even surrounds the meaning of the letter E in EDLC. Some say that the E stands for electrochemical, pointing to EDLCs' use of a liquid electrolyte. Others say that the E stands for electric, pointing out that, even though the electrolyte becomes polarized when you apply a dc voltage to an EDLC's terminals, energy storage within the device—unlike that in an electrolytic capacitor—involves no chemical reactions.

Today, EDLCs with capacitance of tens of farads commonly appear in many applications. Devices with capacitance of more than 100F already exist, and EDLCs' maximum capacitance is likely to continue increasing as engineers devise new applications that require larger energy reservoirs, such as consumer, automotive, and "green" systems.

AVX's EDLCs

EDLCs may appear outwardly simple. Most have only two terminals, which may not even be polarized the way electrolytic capacitors' terminals are. However, you cannot successfully apply them without first familiarizing yourself with their many unusual characteristics. Even if you believe that you can skip the self-education step, you'd be wise to allot some time in your project schedule for a learning experience.

From freshman physics, you probably remember the formula for a capacitor's capacitance: C=?A/d, where C is the capacitance, ? is the dielectric constant of the insulating material between the capacitor's plates, A is the area of the plates, and d is the distance between them. EDLCs achieve their high capacitance values by reducing d by as many as three orders of magnitude and maximizing A through the use of rough-surfaced electrodes. The device manufacturer's challenge is to reliably achieve stable, high values of capacitance plus stable, low ESR (equivalent series resistance) to deliver high stability in other parameters and to produce parts in large unit volumes under tight cost constraints.

An unexpected phenomenon in EDLCs is a result of the fact that actions within the device do not take place instantaneously. For example, if you impress an ac voltage across its terminals and measure a device's capacitance, you may discover that the capacitance depends on the ac voltage's frequency. A 20F device operating at, say, 5Hz is likely to exhibit a capacitance of less than 10% of that value at 1kHz. Suppose you want to use the capacitive reactance, XC, to limit the ac current in a simple circuit to which you want to apply sinusoidal ac excitation. As you increase the excitation frequency, the current decreases because capacitive reactance is inversely proportional to capacitance. This effect may require you to redesign the circuit or choose a higher-value capacitor. You would also likely find that, over time at temperatures above the rated range, the capacitance decreases and the ESR increases.

Unlike a rechargeable battery, which has a typical cycle life of hundreds of charge/discharge cycles—after which its ability to store a charge drops significantly—an EDLC may suffer no permanent catastrophic damage after hundreds of thousands or even millions of charge/discharge cycles. Nevertheless, its capacitance after, say, 100,000 cycles may be only 90%, and its ESR may be as much as 125% of what it was after the first cycle. Moreover, these changes are likely to increase if you operate the device at elevated ambient temperatures beyond its ratings.

EDLCs have a generally well-deserved reputation for high leakage, or self-discharge current. In many cases, however, you can find ways around the shortcoming. For example, whenever the primary power source—usually, the ac line—is operating normally, uninterruptible power supplies that use EDLCs commonly keep the devices in a trickle-charge mode. This mode provides just enough current to satisfy the leakage requirement.

EDLCs can be expensive components, and most batteries store more energy than do EDLCs of the same size. Why, then, would you want to use EDLCs—especially in a wireless-sensor network? A key answer lies in their virtually unlimited cycle life. When you use them alone or in combination with primary—that is, nonrechargeable—or rechargeable batteries, EDLCs can eliminate or at least greatly reduce the need for maintenance of somewhat-inaccessible network nodes. Such maintenance represents a major operating cost. EDLCs are also an important part of those wireless-sensor networks that accept the penalty of higher node power to achieve faster response to emergency conditions in the system or the equipment they monitor.

With the aid of EDLCs, you can substantially increase the cycle life of rechargeable batteries. If you parallel an EDLC that guarantees low ESR with a battery that periodically delivers high-current pulses—a common operating condition in energy-harvesting applications—the battery's cycle life is likely to increase by a factor of 4-to-1. Thanks to their low ESR, EDLCs deliver most of the pulse current, thereby limiting the battery's contribution and limiting thermal cycling due to internal heating of the battery.

Typically, you can purchase prism-shaped EDLCs with low-profile designs and voltage ratings of 4.5 to 15V for approximately $4.50 to $7.50 each. At volumes of many millions of pieces, the unit prices are correspondingly lower.




Want to more of this to be delivered to you for FREE?

Subscribe to EDN Asia alerts and receive the latest design ideas and product news in your inbox.

Got to make sure you're not a robot. Please enter the code displayed on the right.

Time to activate your subscription - it's easy!

We have sent an activate request to your registerd e-email. Simply click on the link to activate your subscription.

We're doing this to protect your privacy and ensure you successfully receive your e-mail alerts.


Add New Comment
Visitor (To avoid code verification, simply login or register with us. It is fast and free!)
*Verify code:
Tech Impact

Regional Roundup
Control this smart glass with the blink of an eye
K-Glass 2 detects users' eye movements to point the cursor to recognise computer icons or objects in the Internet, and uses winks for commands. The researchers call this interface the "i-Mouse."

GlobalFoundries extends grants to Singapore students
ARM, Tencent Games team up to improve mobile gaming