Ultracapacitor Technology Powers Electronic Circuits

Oct 1, 2003 12:00 PM
By Youngho Kim, Director of Product Development, NESSCAP Co. Ltd., Korea

As engineering innovations continue to advance ultracapacitors, their enhanced performance capabilities are expected to hasten the convergence of batteries and ultracapacitors—strengthening the combination of both specific energy storage and pulse power design in future applications.

As the market strives for lighter, more compact wireless and portable devices with more ingenious features crammed into an ever-tighter space, a related quest ensues for the next power supply innovation — a powerful, compact, long-lasting, economical and safe battery. Although progressing toward this end, current battery technology often compromises the desired space and weight specifications without properly satisfying peak power requirements.

Ultracapacitors, also known as supercapacitors, offer an alternative source that promises to circumvent the battery scramble and extract greater efficiency from existing power sources. Because of high price and manufacturability issues, this electric double layer capacitor (EDLC), also known as a pseudo capacitor, isn't popular among engineers. However, it offers boundless growth potential because it responds to key market and societal needs: It's environmentally friendly, helps conserve energy, and enhances the performance and portability of consumer devices. Ultracapacitors also are free from characteristic battery problems, such as limited cycle life, cold intolerance and critical charging rates.

Why Ultracapacitors?

Ultracapacitors are being developed as an alternative to pulse batteries. To be an attractive alternative, ultracapacitors must have at least one order of magnitude higher power and a much longer shelf and cycle life than batteries. Ultracapacitors have much lower energy density than batteries, and their low-energy density is, in most cases, the factor that determines the feasibility of their use in a particular high-power application.

Available for decades, a conventional electrolytic capacitor is an energy-storage device that can be compared to a container that gradually fills with electrical energy and then delivers it when needed in a sudden burst. Offered just recently, an ultracapacitor is a high-energy version of a conventional capacitor, holding hundreds of times more energy per unit volume or mass than the latter by using state-of-the-art materials and high-tech microscopic manufacturing processes. When fully charged, these robust devices deliver instant power in an affordable, compact package.

Long considered an enigma because of price, the advent of inexpensive, compact ultracapacitors, characterized by an exceptionally high surface area, excellent conductivity, and superior chemical and physical stability, herald a new era of practical usage.

Ultracapacitors as Circuit Elements

The equivalent circuit used for conventional capacitors can also be applied to ultracapacitors. The circuit schematic in Fig. 2 represents the first-order model for an ultracapacitor. It's comprised of four ideal circuit elements: a capacitance C, a series resistor Rs, a parallel resistor Rp, and a series inductor L. Rs is called the equivalent series resistance (ESR) and contributes to energy loss during capacitor charging and discharging. Rp simulates energy loss due to capacitor self-discharge, and is often referred to as the leakage current resistance. Inductor L results primarily from the physical construction of the capacitor and is usually small. However, in many applications, it can't be neglected — particularly those operating at high frequencies or subjected to hard switching.

Resistor Rp is always much higher than Rs in practical capacitors. Thus, it often can be neglected, particularly in high-power applications. In that case, the impedance of the Fig. 2 circuit model is:

Z = R + i (2pfL-1/2pfC), where L is the inductance in [Henrys]. The impedance is purely resistive when 2pfL-1/2pfC = 0, or f = 1/2 p(LC)^½. This particular frequency is referred to as the resonance frequency of the capacitor. Thus, the impedance of circuit is simply the resistance at self-resonance. However, ultracapacitors exhibit non-ideal behavior, which result primarily from the porous material used to form the electrodes that cause the resistance and capacitance to be distributed such that the electrical response mimics transmission line behavior. Fig. 3 shows a more realistic circuit representing the real ultracapacitor's electrical response.

DC Behavior of Ultracapacitors

Ultracapacitors used in electric drivelines to load-level the battery experience large-steady (transient) dc, much like the battery, rather than small amplitude ac signals. The dc charge or discharge time (t_disch) of the capacitor is related to the fundamental characteristic frequency (f_AC in Hz) of the ac voltage on the capacitor by t_disch » 1/4f_AC. Hence, for several backup time applications, the ac signals are lower than 10Hz.

In testing ultracapacitors, it's convenient to model them as a simple series RC circuit when inductive effects are unimportant. In this case, Q = CV, E = 1/2 CV² and V_o - V = iR + (Qo - Q / C), where Q is charge on the capacitor, V is voltage on capacitor, E is energy stored in the capacitor, and V_o and Q_o are voltage and charge at t = 0, respectively.

Stacking Ultracapacitors

Many system applications require that capacitors be connected together, in series and/or parallel combinations, to form a “bank” with a specific voltage and capacitance rating. Because sustained overvoltage can cause an ultracapacitor to fail, the voltage across each cell in series stack must not exceed the maximum continuous working voltage rating of individual cells in the stack. The designer must either reduce the “rate of charge” being delivered to a cell, or completely stop charging a cell whose voltage approaches its surge voltage rating.

The easiest way to reduce the current that's charging an ultracapacitor cell is to divert some of it around the cell. One such method employs a passive bypass component. The other, more complicated procedure uses an active bypass circuit. After the stack has been held at voltage for a period of time, voltage distribution then becomes a function of internal parallel resistance. The cells with higher leakage current should have lower cell voltages, and vice versa in a series stack of ultracapacitors.

One technique to compensate for variations in parallel resistance is to place a bypass resistor in parallel with each cell, sized to dominate the total cell leakage current. This effectively reduces the variation of equivalent parallel resistance between the cells. The active balancing circuit has an active switching device, like a bipolar transistor or a MOSFET, connected in series with each bypass element ladder. The switches are controlled by voltage-detection circuits that only turn a switch “on” when the voltage across that particular cell approaches a value just slightly below the continuous working voltage rating of the cell. This is called the bypass threshold voltage. Fig. 4 depicts a typical block diagram of an active charging-current diversion circuit.

An ultracapacitor's voltage profile has a capacitive and resistive component. This can be represented by dV = i (R+dt/C), where dV is allowable change in voltage in Volt, i is the current in amperes, R is ESR in Ohms, dt is charge or discharge time in seconds, and C is capacitance in Farads.

The number of ultra-capacitor cells required can be determined by the system variables, such as allowable change in voltage (max. and min. voltage), current (or power) and required duration time.