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What is a supercapacitor?

A supercapacitor (ultracapacitor), is a capacitor which has a high capacity as its capacitance value is much higher than other capacitors, but lower voltage limits, which bridges the gap between rechargeable batteries and electrolytic capacitors. Super capacitors typically store 10 - 100 times more energy per unit volume or mass than electrolytic capacitors, can deliver and accept charge more quickly than batteries, and endures much more charge and discharge cycles than normal rechargeable batteries.  

How do Supercapacitors Work?

A capacitor stores energy by means of a static charge not using an electrochemical reaction. By applying a voltage differential across the positive and negative plates the capacitor is charged. The supercapacitor -rated in farads- is thousands of times higher than electrolytic capacitor. They use supercapacitors  for energy storage which requires regular charge and discharge cycles at higher current and during a short duration. The specific energy of the supercapacitor is 10–50 times less than Li-ion. The discharge curve is a disadvantage of supercapacitors. Unlike electrochemical batteries which deliver a steady voltage in the usable power band, the supercapacitor’s voltage decreases on a linear scale, which reduces the usable power spectrum. Supercapacitor  

Comparing a Supercapacitor and a Battery

There are unique differences between the battery and the supercapacitor. The battery’s chemistry determines the operating voltage, electrochemical reactions control charge and discharge. The capacitor is not electrochemical and the maximum allowable voltage is determined by the dielectric material type which separates the plates. Since the supercapacitor is non-chemical, the voltage is free to keep rising till the dielectric fails. This usually occurs as a short circuit so it is advisable not to go above the specified voltage. The supercapacitor is not a battery replacement to store long-term energy. When the charge and discharge times are more than 60 seconds, a battery should be used; if less, then the optimum solution is a supercapacitor. Supercapacitors are perfect for quick charge and to fill a short-term power requirement; whereas batteries are better for providing long term energy. Combining the two in a hybrid battery can satisfy both requirements while reducing the battery stress, which leads to a longer service life.  

Applications of supercapacitors

They use supercapacitors  in different applications like automobiles, trains, cranes and elevators, for short-term energy storage, regenerative braking or burst-mode power delivery. These applications all require rapid charge/discharge cycles, instead of compact long term energy storage. Supercapacitors are most effective in bridging power gaps which last from a few seconds to a few minutes and can be quickly recharged. A flywheel can offer similar qualities, and in some applications the supercapacitor can compete against the flywheel, as in the Long Island Rail Road (LIRR) trial in New York.  
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