Supercapacitors (double-layer or ultracapacitors) are devices that store extremely large amounts of charge (from 0.022 F to 55 F) much more than a normal capacitor. Their level of energy storage approaches around 1/10 that of a low-density battery. But, unlike a battery, the power output can be 10 times greater (a handy feature in high-current pulse applications).
A supercapacitor comprises of two non-reactive plates suspended within an electrolyte. A voltage applied to the positive plate attracts the negative ions in the electrolyte, whilst the voltage on the negative plate attracts the positive ions. This effectively creates two layers of capacitive storage, one where the charges are separated at the positive plate and another where the charges are separated at the negative plate. Conductive rubber membranes contain the electrode and the electrolyte material and make contact with a cell. Several cells are stacked in series to achieve the desired voltage ratings. Typical voltage ratings are 3.5 and 5.5 in keeping with their typical role as backup capacitors for 3.5 V or 5 V devices.
A supercapacitor can be charged to any voltage within its voltage rating extremely quickly and it can be stored totally discharged; on the other hand, many batteries are damaged by quick charging. The state of charge of a supercapacitor is simply a function of its voltage, whilst the state of charge of a battery is complex and often unreliable. Even though a battery will store more energy than a supercapacitor, a supercapacitor is able to deliver frequent high-power pulses without any harmful effect, whereas many batteries experience reduced life under similar conditions.
Supercapacitors can be utilized as an intermediate power source or a bridge between batteries and conventional capacitors. Several applications take advantage of the use of supercapacitors: from those that need short power pulses to those requiring low-power, long-duration support of critical memory systems for instance, they are capable of maintaining contents of low CMOS memory for a number of months.
Supercapacitors are outstanding solutions in a number of systems when used alone or combined with other energy sources, examples include: quick-charge applications that can be charged in seconds and then discharge over a few minutes (toys and power tools), short-term support for interruptible power systems, where the supercapacitor provides the power for short outages or as a bridge to a generator or other continuous backup power supply. Supercapacitors can also provide load-leveling to energy-rich, power-poor energy source such as a solar array.
When placed strategically within battery-powered systems, supercapacitors can prevent peak power-induced battery stress by supplying the peak power demands for the battery. This often allows a smaller capacity battery to be used and it can even extend the overall life span of the battery.
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