Ultracapacitors turn Malibus into Mercedes

Mike Everett
Vice President Technology Systems
Maxwell Technologies
San Diego, Calif.

Every year, automotive designers increase the average electrical load of new models by that of a 100 to 150-W bulb.

Demand for automotive electrical power has been climbing several percent per year, from l kW in the 1990s to over 2 kW today. Future demand is expected to hit 3 kW. But average power load is only part of the equation.

Peak power demand is growing at a rate of 100 to 150 W per year and can be as high as 6 to 18 kW for several seconds at a time. For perspective, it takes about 2 kW to power a power-steering unit. For this reason, the typical 14-V bus requires current from 500 to 1.5 kA for several seconds.

Such demands mean cable harnesses must carry a lot of amperage, and relay and switch ratings must be upsized. It is not clear at what point the industry will switch to 42 V, but there's no end in sight to demand for higher-powered electrical functions.

It used to be that the battery (aided by the alternator) could handle most vehicle electrical loads. But that was before electric-assist braking, electric-assist power steering, and a laundry list of features, once the exclusive domain of luxury models, found their way into mid-priced models. Switching to bigger, heavier, environmentally unfriendly batteries to handle the growing electrical load was an undesirable option. A far better choice was a device called an ultracapacitor.

Ultracapacitors, also known as electrochemical double-layer capacitors, first served as low-power, low-energy, long-life backup in VCRs and alarm clocks. More recently, capacitor technology has advanced to where the devices are acceptable in many critical applications.

These high-power energy storage devices offer extremely long cycle life, wide operating temperature ranges, low weight, flexible packaging, zero maintenance, and environmental friendliness. They best suit short-term power requirements ranging from a few seconds to a few minutes.

Theoretically, ultracapacitors are capable of storing energy forever. No electrochemical reactions take place in an ultracapacitor, so there's no electrochemistry to become depleted.

Ultracapacitor longevity is only limited by the degree of robustness in packaging, materials, and the assembly process.

To an extent, ultracapacitors resemble typical capacitive devices. They are filled with an electrolyte that provides a ready source of negatively and positively charged ions.

Upon charging, opposing collection plates assume the charge associated with the polarity of the charge source. The separation of the charges on the plates causes the ions to migrate to the appropriately charged collector. Migrating ions can find a home at many locations on the surface area of the collector. Tiny distances separate this large population of ions from the oppositely charged locations on the collector, creating a static electricitylike potential.

Usually, the collectors are charged carbon electrode elements that attract the ions within the electrolyte. A separator between the two collectors lets ions pass through while it precludes electrical contact, thereby preserving the charge separation until the device is discharged.

Once discharged, upon polarization of the collectors, the ions will line up again, ready for another round of electrical discharge. It is the very small charge separation and the high levels of charged pairs that put the "ultra" in ultracapacitor. The devices are capable of charge/discharge cycles which number in the 10⁶range and will outlast the vehicle itself.

Batteries are proven energy-storage devices, but while both batteries and ultracapacitors store energy, there are significant differences. Batteries store large amounts of energy in a relatively small volume; ultracapacitors store far less energy but are capable of producing more power. Ultracapacitors are efficient at high current; batteries are not. Batteries deteriorate under high cycling regimes; ultracapacitors can be recycled hundreds of thousands of times.

Initially, cost and availability were the primary reasons that ultracapacitors were not used in automobiles. However, with the current levels of production in Asia, Europe, and the United States, that is no longer the case.

While not directly responsible for rising power requirements, another factor in the growth of electrical functions is power stabilization. Take exterior lighting as an example. Replacing incandescent brake bulbs, headlamps, and other lighting needs with LEDs can reduce the lighting power budget to one-fourth of what it is today. But with LED lighting there is a greater need for electronic ballasting to regulate current. The ballasting reduces electronic loads, but at the expense of more electronics.

Electronic modules may not need much power but they are sensitive to fluctuations in voltage supply. Thus, introduction of more electronics introduces a whole new set of problems. Control electronics may have trouble operating if short-term demand for power causes voltage sags on the power net. This is a major issue because today's cars have multiple control modules. Safety functions may fail; electronic fuel controls could stop, stalling the engine; and lights and sound systems could fluctuate, causing distraction or worse.

Existing battery systems sufficient to support such loads would be prohibitively large. Combine this with batteryperformance drawbacks of poor low and high-temperature performance, poor cycle life and efficiency, maintenance and disposal issues, and it's clear another solution is needed. Ultracapacitor technology offers a made-to-order solution to stability requirements.

Ultracapacitors provide high peak power with rapid charge and recharge times. And putting ultracapacitors into series packs lets distributed power deploy throughout the power net. The distributed-power concept keeps wiring and cabling sizes down because power is delivered at point of use. This architectural shift makes modern systems more robust, while reducing the cost, weight, and complexity of the power net.

Centralized power-control demands a separate wire from the central control box (which contains fuses, relays, and switches) to the device being powered. Each wire must carry the full peak current of the load and is sometimes routed over long, tortuous routes. But in a distributed architecture, power is generated in one location and distributed via a limitednumber of common power buses. Control signals instructing the smart actuators and servos are also distributed over a limited number of common communications buses. The control signals along with power are sent to a local distribution node that incorporates intelligent electronic controls and, if necessary, energy storage.

Where local power is used intermittently, the local power node's energy storage can provide that power while the power bus need only supply average power. This use of a local power buffer reduces the size, cost, and weight of the main power bus wire. And limiting the number of common power buses reduces the total number of wires.

Power stabilization is one of the best applications for ultracapacitors. Shortterm power demands that cause voltage dipping can be buffered with a 14-V power module designed with enough energy storage to accommodate peak power demands. This offers many advantages. It eliminates the need for a second battery (reducing weight), lasts the life of the car, and performs reliably at 40°C. Acquisition cost in high volume is about the same as a second battery with cabling, and life-cycle cost is lower.

Electric-assist power steering (EPS) and electric-assist braking need distributed modules. Safety-critical electrified systems demand consistent availability of high-pulse power. A large battery several meters from the function is not as efficient in supplying peak power as a local distributed module. For example, a 1.2-kW peak power demand to an EPS for 2 sec (2.4 kW-sec) can be satisfied with a local pack of six ultracapacitor D-cells with a pack rating of 58°F at 15 V, storing 5.6 kJ (5.6 kW-sec) of energy. Plus, electromechanical braking reduces weight and cost while improving performance.

In the new hybrid-vehicle architectures, another trend is to reduce the load on the internal combustion engine (ICE) to improve fuel economy. Traditionally, the ICE carried the power steering, air conditioning, and start-stop (power train) loads. Now that those loads are supported electrically, smaller, more efficient engines can handle the task. Consequently, the ICE operates at lower stress levels, thereby extending its life.

The start-stop cycle of the power train and regenerative-braking-energy recapture are excellent means of boosting fuel efficiency. In the basic architecture of hybrid vehicles, an ultracapacitor module supplies the starting energy to an electric motor. At a stop, the ICE is turned off. To start, energy from the ultracapacitor bank feeds the electric-drive motors located in the drivetrain.

In either the series or the parallel-hybrid configurations, a belt-driven starter generator captures the braking energy, channeling it to the ultracapacitor pack and also starting the ICE. The reason ultracapacitors are better than batteries for this application is their high capability. They take up a lot of regenerative braking energy quickly and deliver it just as quickly to start the ICE or provide supplementary or solitary-burst power to move the car from a standstill. The combination of start-stop management and regenerativebraking-energy recapture produces fuel savings between 4 and 10%.

This is a perfect application for ultracapacitors. Not only does fuel economy improve during the off time of the ICE, but it is greatly enhanced by the boost power of the electric motors during the time of highest fuel consumption in nonhybrid vehicles — when moving the car off the line.

Also, the car generates the bulk of exhaust emissions during the initial acceleration phase of the velocity profile. This is where the engine load is the greatest and the most motive power is required. The use of electric motors to get the vehicle moving eliminates all the pollutants normally expelled in the exhaust. And starting the ICE after the car is moving also reduces the amount of pollutants generated over the entire driving cycle.

All in all, ultracapacitor technology has improved to the extent that performance and safety are no longer concerns. Lowering the cost of this technology to penetrate automotive markets is the main focus of nearly all ultracapacitor manufacturers.-Today, ultracapacitors are available at prices no one thought possible five years ago, and nothing on the horizon hints at a dimming of their prospects.

The illustration shows the types of advanced vehicular functions and their approximate power levels. Base electrical loads represent a continuous burden on the vehicle power system and form the basis for growth in installed load. High-power electrically driven functions are generally discontinuous and "on demand" with relatively short duty cycles, usually less than 10 sec and often only 1 or 2 sec. Electric-assist steering or electrohydraulic brakes, for example, have usage patterns of only seconds of activity followed by relatively long periods of quiescence.

In either of these configurations a beltdriven integrated starter generator captures the braking energy and channels it to the ultracapacitor pack. It also starts the ICE.