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A Short Primer on Ultracapacitors

By • May 18th, 2009 • Category: Battery Technology for Hybrid and Electric Cars

Ultracapacitors are super high capacity versions of electric capacitor components that store energy in an electric field. These are devices that have raised the “capacity” of a capacitor so high that ultracapacitors or “supercapacitors” are now thought of as energy storage devices that can replace batteries in some applications. They are available in various cylindrical, square, and flat shapes and sizes. With increasing production volumes and economies of scale, their prices can be competitive with batteries.

Ultracapacitor Characteristics

Maxwell Technologies ultracapacitor cells and modulesUltracapacitors, sometime called Ucaps, are high-power/low-energy devices while batteries tend to be high-energy/low-power devices. The lower equivalent series resistance of an ultracapacitor gives it higher roundtrip (in and out) energy storage efficiency, especially with lower losses at high currents. Ultracapacitor packs are ideal for storing high power braking regeneration energy and supplying quick acceleration energy. The original Honda FCX hybrid fuel cell car used ultracapacitors for the energy storage, as do some of the new hybrid electric transit buses.

Being a capacitor, the state of charge (SOC) energy is precisely determined by the square of the voltage (E=½CV²), where C is the capacitance. The SOC discharge profile is significantly different from a battery, but offers some unique characteristics and advantages. Low temperature performance down to a -40°C (-40°F) shows little degradation. Above 89°C (192°F) internal temperature the electrolyte will vaporize and cause a flash explosion. In general the cycle life is limited by the temperature history of the electrolyte, which slowly decomposes. Over a million complete 100% charge/discharge cycles can be expected.

Ultracapacitor Construction

Capacitors are electric components that store energy in an electric field between two electrically conducting plates or “electrodes”. Ultracapacitors or “supercapacitors” have electrodes are made from a high porosity carbon much like the carbon in air and water filters. This high porosity carbon (in some new devices carbon nanotubes) is somewhat like a microscopic sponge that has extremely high surface areas that increase the “capacity” of a capacitor to such a high density that ultracapacitors are now thought of as energy storage devices that can replace some battery applications.

Ultracapacitors are known as double layer carbon devices because the actual construction is like a layer cake with carbon electrodes on the top and bottom. In between the electrodes is an electrolyte to facilitate the movement of charged microparticles. A special separator sheet lies in the middle of the electrolyte to isolate the voltage between the top and bottom electrodes. Ultracapacitors are rolled up like a “jelly roll” to make cylindrical devices and stacked in flat sheets to make prismatic devices. The cylindrical devices dissipate most of the charging/discharging heat through the end electrode connections while the prismatic devices dissipate most of the heat through the flat sides.

Unlike some modern batteries of similar construction no chemical reaction takes place to store or release energy. One of the electrodes in a battery is made from a different material and different electrolytes are chosen to act as catalysts for the chemical reaction.

Ultracapacitor and Batteries

The combination of ultracapacitors with batteries offers potential advantages in power, energy, temperature range, and life. However, the different SOC voltage profiles make such combinations challenging especially in high power vehicle applications. There have been over 50 technical papers published on combining batteries and ultracapacitors.

I maintain that, for many applications, a simple parallel ultracapacitor pack combination with a battery pack could double the life of the battery pack and pay for the extra complexity.

At least one manufacturer is developing a type of hybrid battery/ultracapacitor cell for use in vehicles. An actual vehicle test using an energy storage pack built with hybrid battery/ultracapacitor cells more than doubled the pack life. The AFS Trinity hybrid SUV offered a combination of battery and ultracapacitor packs. Small scale testing in a camera circuit demonstrated the combination advantages and I suspect that there are some small cordless tool applications that have already implemented this design.

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is an industry veteran with 30 years of experience in general business, marketing, project and product management, and engineering research and development. Mr. Bartley provided executive management support including technical and business oversight to heavy-duty hybrid-electric prototype projects as they evolved into production. He developed cost models for energy storage and fuel savings, and power models for ultracapacitor packs. Mr. Bartley is well known throughout the industry of heavy-duty hybrid-electric buses and trucks, having delivered many papers and presentations since 2003. Mr. Bartley maintains a blog at Follow
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