Ultracapacitors, also known as electrochemical double-layer capacitors (EDLCs), are electrochemical capacitors that possess an unusually high power and energy density when compared with traditional capacitors—typically several orders of magnitude greater than a high-capacity electrolytic capacitor. Most ultracapacitors are rated in Farads, and individual cells can typically be found in the 100F to 500F range, though they can extend higher or lower. Depending on the application needed, ultracapacitors may be used as battery replacements or enable smaller, more economical battery selection. Ultracapacitors have low equivalent series resistance (ESR), allowing them to deliver and absorb high currents. Their mechanical (rather than chemical) charge-carrier mechanisms enable their long, predictable life with a smooth performance change over time.

Ultracapacitors are not a new technology. The ultracapacitor effect was first noticed in 1957 by General Electric engineers experimenting with devices using porous carbon electrodes. It was believed that the energy was stored in the carbon pores and exhibited "exceptionally high capacitance," although the cause was unknown at that time.

General Electric did not immediately follow up on this work, and the modern version of the devices was eventually developed by researchers at Standard Oil of Ohio in 1966, after they accidentally rediscovered the effect while working on experimental fuel cell designs. Their cell design used two layers of activated charcoal separated by a thin porous insulator, and this mechanical design remains the basis for most ultracapacitors today. They first appeared as a low-energy, long-life power backup for consumer electronic devices like VCRs.

The past decade has seen substantial advances in ultracapacitor material science, construction, and manufacturing techniques, which has made them highly desirable products, especially for mission-critical applications. In recent years, these devices have found their way into consumer electronics, industrial products, and automotive applications. Today, the best ultracapacitors are extremely high-power devices with power densities of up to 20kW/kg. Compact in size (small-cell ultracapacitors are often no bigger than the size of a postage stamp), they can store much more energy than conventional capacitors and can release that energy quickly or slowly. They have long life and are designed to last the lifetime of the end product.