Ultracapacitors are passive electronic components that, unlike batteries, store energy by physically separating positive and negative charges. They offer high power densities and provide significant energy storage capacities. Capacitance, a measure of energy storage ability, is typically expressed as C = K A/D, where A is the area of the electrodes, D is their separation, and K is a function of the dielectric between the electrodes. The formula yields a result in farads (F), but a farad is so large that the most commonly used values are expressed in microfarads (µf = 10-6F) or picofarads (pf = 10-12F). Applications for ultracapacitors include fuel cells for hybrid vehicles and robotic motion control systems.
Performance specifications for ultracapacitors include capacitance range and capacitance tolerance, a percentage of total capacitance. Other considerations include working DC voltage, rated current, leakage current, specific power, specific energy, and equivalent series resistance. Working DC voltage (WVDC) is the maximum voltage that can be applied continuously at any temperature between a lower category temperature and the rated temperature. Rated current is the maximum current that can be applied continuously across these same temperature intervals. Leakage current is the amount of current flowing from one conductor to an adjacent conductor through an insulating layer. Specific power measures the ability to deliver energy quickly. Specific energy measures the amount of energy that can be stored per unit of weight. Equivalent series resistance (ESR), a measure of total loss range, represents the extent to which an ultracapacitor acts like a resistor when charging or discharging.
Ultracapacitors can use axial, radial, flying, tab, screw, gull wing, or J-leads. Some devices bolt into place while others require or include mounting brackets. Pole-mounted capacitors are also available. Surface mount technology (SMT) adds ultracapacitors to a printed circuit board (PCB) by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board. Through hole technology (THT) mounts components on a PCB by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board.
Ultracapacitors are packaged in tape reels, trays or rails, shipping tubes or stick magazines, and in bulk packs. Tape reel assemblies include a carrier tape with embossed cavities for storing individual components. A cover tape seals the carrier tape in place and the composite tape is wound on a reel that can be loaded into industry-standard, pick-and-place board assembly equipment. Ultracapacitors with leads on four sides are often packed in trays or rails that are made of carbon-powder or fiber materials and molded into rectangular outlines that contain matrices of uniformly spaced pockets. Shipping tubes or stick magazines are containers made of rigid polyvinylchloride (PVC) and extruded in industry-standard sizes. Bulk packs are used to distribute components as individual parts.