Supercapacitors evolve to meet market needs

Applications in electronics are constantly growing in diversity. In recent years, there has been a growing demand for very compact, battery-powered portable devices. Fueling that growth has been a steady improvement in the capabilities and cost effectiveness of rechargeable lithium batteries. Even with the advancements in battery technology, the developments have limits and improvements have slowed.

Many battery-powered devices have power needs that vary greatly as their mode of operation changes. Some need high-current power pulses that may be difficult or impossible for batteries to supply without increasing the risk of performance failures, reduced operational life, or even premature battery failures. The problem is that batteries have high internal resistances (such as ESR) too large to continue to consistently deliver high-power pulses. Devices that would require high-power pulses include:

• GPS tracking systems.

• Bluetooth communications devices.

• LED safety flashers.

• Bar-code equipment.

• Remote-control systems.

• Automatic meter readers (AMRs).

• RFID equipment.

• Medical equipment.

• Alarm and security systems.

• Portable music players and other audio amplifiers.

For some, there is a need to transmit data for only a few milliseconds each minute. The pulse currents during transmission range from 20 to 50 mA. While batteries are high-energy devices, they have difficulties delivering such short high-pulse currents without the battery becoming damaged, especially at low temperatures.

It is well known that adding supercapacitors in parallel with the source battery can help supply power during these pulses. As supercapacitors have become more cost effective, they have found an increasing use in this role.

An early use of supercapacitors was in auto sound systems with heavy bass. A 12-V car battery cannot sufficiently produce the momentary power to satisfy the needs of these stadium-level sound systems without causing the vehicle battery to fail after only a few months of usage. Adding a supercapacitor of a few farads across the battery is sufficient to maintain the bone-jarring sound pressure levels and save the battery from a premature failure!

Supercapacitor app

Placing a supercapacitor, with its huge storage capabilities, in parallel with a battery can easily supply almost any required pulse currents. After delivering the pulse current, the supercapacitor is quickly recharged by the battery between the pulse cycles. This reduces the stress on the battery, extending the overall life of the battery by a factor of three or more, compared to not having the supercapacitor in the circuit. Figure 1 illustrates this principle.

Fig. 1. Battery-powered pulse circuit.

Supercapacitors, also called EDLCs (electrochemical double layer capacitors) or ultracapacitors, are electrochemical devices that have high capacitance and high energy density compared to other common capacitors, such as tantalum and aluminum electrolytic types. Compared to lithium batteries, supercapacitors have up to 10 times the power density.

Being high-energy devices, batteries have an advantage over supercapacitors. They can deliver more energy over a longer time in a more efficient package than supercapacitors. There are applications, such as windmills, where supercapacitors are a superior choice over batteries for other reason other than energy storage efficiency. Supercapacitors on the other hand are high-power devices able to deliver a large amount of energy in a very short time.

Supercapacitors have been commercially available since 1978 at first, these were low-voltage devices that had high ESR and were primarily designed for computer backup applications. Since then, they have gone through considerable technical and manufacturing changes, to the point that there are several types commercially available. While terminology may vary, most supercapacitors fit into one of these categories: (1) Pseudocapacitor, (2) Prismatic, or (3) Pulse or battery support.

All supercapacitors are basically constructed from two carbon-based electrodes (mostly activated carbon having very high surface area), an electrolyte (aqueous or organic) and a separator that allows the transfer of ions, while providing electronic insulation between the electrodes.

Most high-energy and high-power supercapacitors are produced in a way that is similar to that of aluminum electrolytic capacitors. Producing supercapacitors using established mounting configurations allows designers to select parts that have screw terminals, radial leaded through-hole parts, in straight lead and snap-in lead styles, as well as surface-mount styles. This offers tremendous flexibility for designers to apply supercapacitors in their designs.

While basic appearances may be similar, supercapacitors store energy in a different way than electrolytic capacitors. As voltage is applied to a supercapacitor, ions in the electrolyte solution diffuse across the separator into the pores of the electrode of opposite charge.

Charge accumulates at the interface between the electrodes and the electrolyte; forming two charged layers (double layer) with a separation distance of just a few angstroms ( 0.1 nm). This is the distance between the electrode surfaces to the center of the ion layer. Since the capacitance value is proportional to the surface area and is the reciprocal of the distance between the two layers, high capacitance values can be achieved in a very small space.

Not all supercapacitors are suitable for all applications. For example, the larger pseudo and prismatic supercapacitors cannot be used in pulse applications because these capacitors have:

• Capacitance values too high (>1 F).

• Rated voltage too low.

• Internal ESRs to large.

• Electrolyte not environmentally friendly (ACN).

• Physical dimensions form factor not correct.

• High cost.

Flat-style supercapacitors

The form factor of typical supercapacitors precludes their use in pocket-sized instruments. To overcome this shortfall, several capacitor manufacturers have developed flat-style supercapacitors, designed specifically for battery support applications.

Most are rectangular or square in shape, are quite thin, encapsulated in an aluminum foil packet or rigid metal shell and can be surface mounted or stood on end as any ot

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