More pop for power packs

Leland Teschler
Executive Editor

Most engineers are probably familiar with simple electrochemical capacitors. Called by various trade names that include Supercapacitors and Ultracapacitors, these devices have for years served as backup power on consumer electronics containing clocks and memory. An electrochemical capacitor (ECC) took over when it was time to replace the main battery or unplug from ac mains for awhile. These special charge storage devices could provide enough juice to keep basic circuits alive during brief interruptions of power.

Now ECCs are taking on a new role. Recent developments have let them serve as adjuncts to batteries during normal operation. The big benefit: A battery of a given capacity can power certain kinds of loads far longer with an ECC helping out than if working alone. When space is at a premium, a battery/ECC combo can be a means of providing decent operating life while staying on a tight packaging budget. Moreover, ECCs use relatively inexpensive materials. Many consist of little more than carbon, polymers, and electrolytic salts.

The electronics that benefit most from inclusion of ECCs are those with pulsed loads. A typical example is that of a cell phone employing digital transmission protocols. In the case where a battery alone powers such phones, the battery voltage drops slightly during transmission as the demand for current rises. This slight voltage drop poses no problem when the battery is charged to an appreciable percent of its capacity.

But difficulties arise as the battery nears depletion. Battery voltage begins to drop under load to a larger degree. It eventually drops below the point where phone circuits function. Result: The handset cuts out. What is interesting is that the battery still has some capacity left, even after its voltage dips below the point where circuitry can draw current.
ECCs make it possible to extract this last bit of battery capacity before the phone goes dead. The idea is to put an ECC in parallel with the battery. Pulsed loads then initially draw current from the ECC rather than from the battery. This comes in handy as the battery nears the last of its capacity because it can eliminate the voltage sag that would ordinarily take place during the last part of the battery -discharge curve.

The net effect of adding an ECC in this case is to increase the effective battery life without increasing the battery capacity. The benefits are obvious for ever-smaller handheld devices. But ECCs are also finding a home in bigger appliances. ECCs operating alone, for example, are candidates for powering an electric toothbrush. The benefit is that the ECC can charge up and be ready to go in a few seconds, unlike conventional rechargeables. And ECCs can hold enough juice to power a toothbrush long enough for even the most discerning oral hygiene freak.

Packaging is another area getting attention. ECC makers have gone beyond typical tubular capacitor housings. New form factors for OEM use lend themselves to automatic insertion in cramped quarters.

Superlarge ECCs are coming out of research labs as well. Experts say they are likely to help power the next generation of hybrid vehicle and will debut on a limited scale in service trucks later this year.
Inside an ECC

An ECC doesn't work like a conventional capacitor. The reason why becomes clear from an analysis of its internal makeup.

Consider that ordinary capacitors generally consist of two metal plates separated by a dielectric. Charge in the form of electrons and electron holes build up on the plates until the capacitor voltage reaches that of the supply voltage.

In contrast, there are no plates in an ECC. The structure is analogous to a bowl full of shredded wheat in milk, with a piece of saran wrap dividing the bowl in half. Both the anode and cathode in an ECC consist of superporous carbon. These electrodes sit in a common bath of electrolyte. A thin porous separator (usually paper or a plastic) divides the anode from the cathode.

Charge storage in an ECC takes place at the interface of the electrodes and the electrolyte. At this interface there are actually two different storage mechanisms at work in any given ECC, one called double-layer capacitance, the other called pseudocapacitance. One or the other predominates depending on the materials used in the ECC.
In double-layer capacitance, application of a voltage causes cations and anions to accumulate at the surface of the charged electrodes. The double-layer moniker comes from the capacitorlike separation of the ionic charges on the electrode surface. Thus capacitance takes place at the electrode surface and is proportional to the electrode surface area. The separation between charges is relatively small, perhaps a factor of 10 smaller than that in an ordinary electrolytic capacitor. So an ECC of a given size can have a much higher capacitance than a conventional dielectric capacitor.

In contrast, charge carriers in pseudocapacitance are electrons and electron holes, not ions. The charge storage is also at the interface between the electrode and electrolyte, but arises out of redox reactions when voltage is applied.

One thing to note about ECCs is that most of their volume consists of electrode material. Separators occupy relatively little space. This gives ECCs a much larger energy density than ordinary electrolytic capacitors, where separator material takes up as much as 40% of the capacitor volume.

A simple equivalent circuit model of an ECC contains a series resistance and a resistance in parallel with a series capacitor. A more accurate equivalent circuit model of the ECC accounts for the porous nature of the ECC electrodes by the use of a transmission line containing numerous Rs and Cs in parallel.

The affect of the distributed resistance and capacitance is to limit the rate at which ECCs can charge up and discharge. But recently developed ECCs optimized for pulse performance limit series resistances through various means. This improvement in dynamic response minimizes the time needed to charge them up. But their response remains in the 10-Hz range or below. They are unlikely to compete in the near future with conventional capacitors for 60-Hz filtering applications or other general-purpose tasks.

Smart sizing

The process of sizing a modern-day ECC involves deciding how much energy the device should provide for expected current pulses. 'An ECC typically can be designed to deliver 50 to 90% of the pulse energy,' says John R. Miller, founder of JME Inc. in Shaker Heights, Ohio. Miller's firm consults with military and commercial clients using ECCs in critical applications.

'A rule of thumb is that a quarter of the volume available for a power source should be allotted to the capacitor, provided the ratio of peak-to-average power is four-to-one or greater,' says Miller. 'In the case of a GSM (Global System for Mobile Communications) cell phone, for example, you might see a 10-to-1 ratio between the pulse and background power. So the rule holds. If the peak is only two times the average, you are better off putting more battery energy in the available space rather than more pulse power.'

Web sites maintained by ECC capacitor vendors often include online application engines that provide a first approximation for sizing calculations. 'In pulsed applications, you basically examine the energy under one pulse and decide how much of it you want to come from the ECC. That's where sizing calculations start, but there are also nonideal qualities of the ECC that need to be considered as well,' explains Miller.

Just don't expect ECCs for portable electronics to completely take over from batteries in any but a few special niches. 'ECCs are 1003 cheaper than any aluminum electrolytic cap and 1,0003 cheaper than a film capacitor,' says Miller. 'But the technology is expensive if compared to a battery in terms of the energy stored.'

Garbage trucks may be the next hot ECC app

Portable electronics aren't the only products to benefit from advances in ECCs. It looks as though these novel capacitors may find a place in the next generation of hybrid vehicles. The first beneficiaries may be service vehicles for National parks and airports that run repeatedly over short routes. The Air Force is also looking into ECCs for electric tugs pulling planes between hangers. The common thread is that such vehicles never stray too far from a charging station. But once plugged in, they can rapidly hit their full capacity and be ready to go.

'Hybrids could be the premier application for big ECCs,' explains industry veteran John R. Miller of JME Inc. Miller chaired the Advanced Capacitor World Summit held last month in Washington, D.C., where progress was reported for a variety of cutting-edge ECC niches. 'You want to store charge every time you slow down a hybrid. ECCs are a good way of doing that with regenerative braking.'

Honda has a demo unit that incorporates ECCs, as does Bowling Green State University with a hybrid bus. But one of the most intriguing uses for the technology is in garbage trucks.

'The typical garbage truck has over 800 start/stop cycles daily, and garbage hauler Waste Management Inc. says its biggest maintenance item is brakes,' explains Miller. 'It makes sense to capture the energy in a capacitor rather than wear the brakes. The DOE is funding a program in this area with Oshkosh Truck Corp. and should have a demo going in two years.'

Boeing's Combat Survivor Evader Locator (CSEL) communication system carries an electrochemical capacitor from Maxwell Technologies for backup power in the event the 21-day battery needs to be replaced during a mission. This locator radio is designed as an aid for downed pilots. It uses multiple satellite links and contains a state of-the-art military GPS receiver.

OEM packaging for ECCs is becoming more widespread as manufacturers strive to make products that can be easily inserted by machine. Units from Cap-XX USA, for example, carry solderable tab terminations and are only a few millimeters thick at most. Typical applications are in PDAs, two-way radios, digital cameras, and hand-held data terminals. Cap-XX also claims its devices have the highest power density of any ECC.

ECCs make it possible to use battery capacity that would otherwise be unavailable once battery-cell voltage drops below a certain level. The ECC is typically wired in parallel with the battery. When the load demands a sudden high current peak, it sees a low resistance in the capacitor and a much higher resistance in the battery. Consequently, the capacitor supplies the current peak. The battery restores charge to the ECC when the load demands a lower current. An example of where ECCs come in handy is in GSM cell phones. The GSM signal requires a current pulse of as much as 2 A for 500 µsec at a 4-msec repetition rate.

In the simplified electrical model of an ECC, a separate capacitance represents each electrode. Charge transfers through resistances representing the carbon electrodes, the electrolyte, and the interface between the two. A resistance in parallel with the electrode capacitances represents leakage paths. A more detailed electrical model recognizes the high porosity of the electrodes through use of a transmission line network with pore resistance elements (R_P) as well as distributed capacitance and leakage resistance R_F.

BestCaps from AVX Corp. are an example of new electrochemical capacitors optimized for pulse performance. They use a patented, highly conductive polymeric electrolyte that provides a series resistance of 200 mOhms or less. BestCaps address a problem seen in most ECCs, says AVX, that lose more than 90% of their capacitance when asked to supply pulses in the millisecond range.

Electrochemical capacitors can provide high capacity in a relatively small package. An example is this 10F device from Maxwell Technologies in San Diego. ECCs of this size sometimes serve as the primary source of power for small hand appliances. For example, an electric toothbrush powered by an ECC could charge up after only a few seconds in its holder. Other potential applications might include dental drills or even dialysis pump motors.