SAN DIEGO -- Farads can now fit in the palm of your hand. That's the message of Maxwell Technologies Energy Products Inc., a new venture here that is commercializing capacitor R&D funded by the Department of Defense. The company is using a proprietary activated-carbon fabric technology to produce capacitors ranging from a 15-farad can with a volume of only 2 milliliters, up to an astounding 2,700-farad cell that's smaller than a quart of milk and weighs only 800 grams.

The new devices have such high capacitance compared with a vailable commercial alternatives that they are forcing basic rethinking in the electronics, automotive and other industries, said Jim Nickerson, vice president of sales and marketing at Maxwell. "In markets ranging all the way from hybrid gasoline/electric vehicles to palmtop communicators," he said, "the new capacitors make it possible to temporarily store energy in ways that no one had thought much about previously."

The capacitors are likely to find their earliest application in the automotive industry, where Honda and other Japanese manufacturers are using the parts to store power to supply peak loads on the electric motors of hybrid vehicles. Meeting peak demands from a bank of capacitors, rather than from the car's battery, greatly reduces the maximum current drawn from the battery, which helps increase the battery's life and can make it considerably less expensive.

The same principle is being used for uninterruptible power supplies in communications applications. A rack-sized bank of the new capaci tors can keep the vital 48-V supply in a control cabinet operating for minutes-sufficient time for a standby generator to be started.

But the most interesting applications may turn out to be in microelectronics. The same techniques used to build the 2,700-farad capacitors can be employed to produce a 15-farad can about the size of a crystal, or surface-mounted capacitors in the 1-farad range. Such components would open new alternatives to designers of battery-based handheld systems.

"Think of this as a way to replace expensive batteries with cheap ones," Nickerson said. "Look at any application where the demand is peaky: a CPU that only runs fast part of the time, or a cellular handset that only transmits once in a while. You can use a small battery to trickle-charge the capacitor, and then draw the peak current from the cap rather than the battery."

In Japan, several manufacturers are developing electric dual-layer capacitors as a substitute for rechargeable batteries, with Elna Co. Ltd. (Fujisawa, Jap an) taking an early lead. The company, a capacitor and printed-circuit-board manufacturer located outside of Tokyo, introduced a 100-farad electric dual-layer capacitor last April, while competitors generally carry about 30-farad capacitors.

"Elna is, we believe, the only manufacturer in the world that produces 100-farad-class capacitors in volume," a company spokesman said. The 100-farad capacitor has a cylindrical form with a 35-mm diameter, stands 50 mm high and weighs 60 grams. It can power at 1 mA for 50 hours or at 1 A for four minutes.

Taking advantage of the long life and contamination-free materials of the electric dual-layer capacitor, Elna intends to promote it as a substitute for rechargeable batteries.

"In the near future, 2,000-farad to 4,000-farad capacitors will be in strong demand. Elna is now developing 1,000-farad to 2,000-farad capacitors as the first step," the spokesman said.

At Maxwell, the principle employed by the company for its new capacitors has already been used by the Defense Department in an emergency locator beacon for military aircraft. When activated, the beacon only transmits a burst every few minutes, so the designers used a Maxwell capacitor bank to power the transmitter. A small battery-much smaller than those in conventional designs-charges the caps during the quiescent interval.

The capacitors, which Maxwell is calling UltraCapacitors, are built from a combination of novel materials. Vice president Richard Smith said the electrodes are formed by embedding activated carbon-which has an enormous surface area-in an aluminum-impregnated matrix. The result, in effect, is a metal-impregnated carbon cloth.

The primary advantage of this material is that a gram of it has a surface area of 2,000 square meters. Another important advantage is that the equivalent series resistance is reduced by the aluminum.

The fabric plates are coated with a non-aqueous organic electrolyte. They are then put together in such a way that the effective standoff distance between the plat es is on the order of Angstroms.

The result is enormous capacitance in a small package, with minimal leakage and very small equivalent series resistance. For example, the company's PC7223 cell-which measures only 164 mm by 62 mm2, or about 0.6 liters-stores up to 7,200 joules of energy. The cell is rated at 2,700 farads at 2.3 volts.

The other electrical characteristics of the component are as impressive as its capacitance. Equivalent series resistance is under 1 mý at 25ýC. But leakage current is only 7 mA. "That gives a charged capacitor a shelf-life ranging from long weeks to short months," Smith said.

The capacitors are not polarized and require no particular forming regimen. Their rated life at 2 V is about 20 years, dropping to 5 years at 2.3 V.

Maxwell Technologies Energy Products, which is a subsidiary of Maxwell Technologies, a DOD research contractor, is building the capacitors in a prototype lab. The company is bringing up a pilot line, which will lead to a full-production facility with cap acity of 10,000 units per month. "For the auto industry, we have to step through every phase of our manufacturing plans," Smith said. "We can show them how we intend to get to a million units a month."

-Additional reporting by Yoshiko Hara.

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