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.
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 (tdisch) of the capacitor is related to the fundamental characteristic frequency (fAC in Hz) of the ac voltage on the capacitor by tdisch » 1/4fAC. 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 Vo - 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 Vo and Qo are voltage and charge at t = 0, respectively.
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.
Ultracapacitors benefit many applications, from those involving short power pulses, to those requiring low-power support of critical memory systems. Whether used alone or with other power sources, ultracapacitors provide an excellent solution in several system configurations and high-power applications, such as cellular electronics, power conditioning, uninterruptible power supplies (UPS), industrial lasers, medical equipment, and power electronics in conventional, electric and hybrid vehicles.
In electrical vehicle applications, ultracapacitors permit faster acceleration, increased range, and extend battery life by freeing it from stressful high-power tasks. In addition, ultracapacitor technology now can do load-leveling to extend the life of EV batteries and provide the high power essential for EV acceleration. For example, a vehicle might use this power burst to accelerate and climb a steep hill. Ultracapacitors also can absorb regenerative braking energy and thus limit the otherwise very high charging current to the battery.
UPS: Ultracapacitors provide short-term support for UPS. With less energy storage capability than a battery, an ultracapacitor isn't a viable substitute in UPS as a long-term power source. However, for short-term support, its instant power and rapid response capability allows it to act as a bridge during power outages until an alternative source kicks in, such as a generator or other backup power supply. In addition, ultracapacitors can serve as a load-leveling function by absorbing power surges and spikes and then releasing clean quality power essential for precision high-tech equipment.
Peak pulse power: Ultracapacitors are optimal for applications requiring a high energy burst not achievable with a battery alone, such as with consumer electronics and wireless devices. For example, unlike analog equipment that draws steady current, digital wireless communication devices often burden the battery with short, heavy current spikes during transmission mode. An added ultracapacitor can provide the intermittent pulse power while the battery supplies the steady current. End users benefit from an extended battery life and lengthened time between charges. Other electronics with ultracapacitor applications include VCRs, CD players, electronic toys, security systems, computers, scanners, smoke detectors, microwaves and coffee makers.
Quick-charge applications: Ultracapacitors charge in seconds whereas batteries often require hours, thus potentially benefiting quick-charge applications. Wireless power tools employing an ultracapacitor can be charged just before use. Moving toys, such as miniature racing cars, also benefit from the fast-charge properties of the ultracapacitor.
Memory backup applications: Widely used in consumer electronics, small ultracapacitors defend against data loss during short-period power outages or, in the case of portable devices, during battery replacement. For this, an ultracapacitor is superior to a battery because it's less expensive and requires no replacement during the device's lifetime.
All-weather quick start: Today's car batteries meet peak power requirements during engine startup, even in the coldest weather conditions. Ultracapacitors, unaffected by weather, can supply the seconds-long peak power, thus permitting the battery to be downsized and its useful life extended. The current catalytic converter in vehicles sends untreated exhaust gas into the environment for a few minutes until it warms and begins functioning. Ultracapacitors quickly preheat the catalytic converter and enable it to function immediately.
Applications to decrease battery size: Often, power loss in batteries can be traced to voltage “spikes” that occur when a device is activated or draws a higher-than-average current. Adding an ultracapacitor to handle voltage spikes allows either a prolonged life or smaller battery — whatever is application optimal.
Although some ultracapacitor applications, such as memory backup, are already in widespread use, many of the applications just discussed are still in the initial phase of adoption. Higher-voltage ultracapacitor technology looms on the horizon, and the implications are enormously far-reaching.