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Design Guide Cellergy Super Capacitors: For Improved Power Performance

Background

Film capacitors store charge by means of two layers of conductive film that are separated by a dielectric material. The charge accumulates on both conductive film layers, yet remains separated due to the dielectric between them. Electrolytic capacitors are composed of metal to which a thin layer of non-conductive metal oxide is added, which serves as the dielectric. These capacitors have an inherently larger capacitance than that of standard film capacitors. In both cases the capacitance is generated by an electronic charge, therefore the power capability of these types of capacitors is relatively high while the energy density is much lower.

The Electrochemical Double Layer Capacitor (EDLC) or Super Capacitor, is a form of hybrid between conventional capacitors and the battery. The electrochemical capacitor is based on the double layer phenomenon that occurs between a conductive solid and a solution inter-phase. The resulting capacitance, coined the "double layer capacitance", is a result of charge separation in the inter-phase. Electronic charge is accumulated on the solid electrode, while counter charge in the form of ionic charge is accumulated in the solution. The EDLC embodies high power and high energy density

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Electrochemical Capacitors

The operating principle of the super capacitor is similar to that of a battery: pairs of electrodes are separated by an ionic conductive, yet electrically insulating, separator (Fig. 2). When a Super Capacitor is charged, electronic charge accumulates on the electrodes (conductive carbon), and ions of opposite charge from the electrolyte approach the electronic charge. This phenomenon is coined "the double layer phenomenon".

The distance between the electronic and ionic charge is very small, roughly 1 nanometer, yet electronic tunneling does not occur. The ions and electrons shift locations between charging and discharging. In the charged state, a high concentration of ions is located along the electronically charged carbon surface (electrodes). As the electrons flow through an external discharge circuit, slower moving ions shift away from the double layer. During EDLC cycling, electrons and ions constantly move in the capacitor but no chemical reaction occurs. Therefore electrochemical capacitors can undergo millions of charge and discharge cycles.

This phenomenon, which occurs with carbon electrodes of a very high surface area and a three-dimensional structure, leads to incredibly high capacitance when compared to standard capacitors. One can envision the model of the EDLC as two capacitors formed by the solid (carbon) and liquid (electrolyte) inter-phase, separated by a conductive ionic membrane. An equivalent electronic model is two capacitors in a series connection (Fig. 3), where Cdl is the capacitance of each electrode; Rp is the parallel resistance to the electrode; and Rs is the resistance of the separator.

We conclude that the energy density of electrochemical capacitors is higher than that of electrolytic capacitors, and therefore they have applicability for systems with lower frequency requirements.

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Cellergy Technology

Through the use of a unique patented production and manufacturing process, Cellergy has developed a small footprint, low Equivalent Series Resistance (ESR), high frequency EDLC capable of storing relatively large amounts of energy. This development is based on an innovative printing technology allowing the production of EDLC’s in many different sizes, with varied dimensions and shapes. As a result, Cellergy produces one of the smallest, low ESR footprint EDLC's on the market today.

Since the patented printing technology is based on conventional printing techniques, the simple manufacturing process enables the manufacturing of large wafers of EDLC's. The basis of the technology is a printable aqueous electrode paste based on a high surface area carbon paste that is printed in an electrode matrix structure on an electronically conductive film. The electrodes are then encapsulated with a porous ionic conducting separator, after which another electrode matrix is printed on the separator. This bipolar printing process can be repeated as many times as required, enabling Cellergy to tailor the product to the specifications of the end user. The finished wafer is then cut into individual EDLC's that are packaged for the customer.

Cellergy's EDLC's boast low equivalent series resistance as well as a low leakage current due to the unique encapsulation technology and electrode composition. Cellergy's EDLC's require no cell balancing or de-rating. The combination of the separator and carbon paste allow very high power bursts within low milli-second pulse widths.

Cellergy’s technology is based on aqueous off-the-shelf components that are all environmentally friendly and non-toxic. Though the system is water based, the capacitor can work at temperatures of between -40°C - 70°C. This temperature range is achieved through the unique water based electrolyte that penetrates the high surface carbon. Since the chemistry of the system is based on water, the performance of Cellergy’s EDLC’s is not affected by humidity.

Application Notes for EDLC

Cellergy's Super Capacitors offer high power and high energy. These characteristics, coupled with a battery, offer the designer a unique opportunity to solve power related issues.

The following table lists the characteristicsof the EDLC (Table 1):

Characteristics  
Working Voltage 1.4V-18V
De-rating Not required
Capacitance 10-100's of mF
Foot print Selectable down to 12mm by 12.5 mm
Operating Temperatures -40°C to +70°C
SMT Under development.
ESR 10's-100's mO
Safety Environmentally friendly materials, No toxic fumes upon burning
Power 10's of amps, short pulse widths
Polarity No polarity
Number of cycles Not limited

Voltage Drop

Two main factors affect the voltage drop of all capacitors, including EDLC's. The first voltage drop is defined as the Ohmic voltage drop. The capacitor has an internal resistance defined as ESR (Equivalent Series Resistance). As current flows through the capacitor, a voltage drop occurs that obeys Ohms law. This voltage drop is instantaneous and will diminish once no further current is drawn. The second voltage drop, capacitance related voltage drop, is due to capacitor discharge.

The voltage of the capacitor is directly proportional to the charge accumulated in the capacitor. During current discharge, capacitance is consumed (current emitting from the capacitor) thus causing a linear voltage decrease in the capacitor. When the current stops, the voltage of the capacitor indicates the charge left in the capacitor. The combination of the Ohmic related voltage drop and the capacitance related voltage drop determine the actual working voltage window of an EDLC under drain conditions (Fig. 4).

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EDLC and Battery Coupling

Under drain conditions, a battery undergoes a voltage drop similar to the EDLC. Because of many physical and chemical constraints, the battery cannot always supply the power required while still retaining its open circuit voltage. The working voltage of the battery reflects the load on the battery, thus the larger the voltage drop of the battery, the larger the load on the battery.

Many difficulties are encountered by the designer planning the online power demand of a system, mainly because the power of batteries is so

limited. If the battery needs to supply high power at short pulse widths, the voltage drop may be too great to supply the power and voltage required by the end product (cutoff voltage). The large load on the battery may decrease the useful energy stored in it, harming the battery and shortening its work life. This problem may be resolved by connecting the battery in parallel to an EDLC (Fig. 5).

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Under conditions of high power and short duration current pulses, a voltage dampening effect is achieved. The voltage drop of the battery decreases, resulting in better energy management and superior energy density of the battery (Fig. 6).

The power supplied is produced by both the EDLC and the battery, with each supplying relative power inversely to its own ESR. The inefficiency of batteries at lower temperatures is well known. The capacitance of most batteries decreases with decreasing temperatures. The decrease is due to the slow kinetics of the chemical reaction in the battery, which increases the internal resistance of the battery. At low temperatures, the voltage drop of the battery increases and reduces the usefulness of the battery. This voltage drop can be reduced greatly through the coupling of the battery and the EDLC.

In conclusion, coupling the battery and EDLC results in superior power management for many short interval and high power applications.


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Distinct Applications for Cellergy’s Super Capacitors

  • Extends the battery’s lifetime - By connecting a primary battery in parallel to Cellergy’s Super Capacitor, the designer can reduce the voltage drop during a high current pulse.
  • Extends secondary battery operation - Reduces the voltage drop at low temperatures (-40°C).
  • CF, PCMCIA Cards - Cellergy's EDLC overcomes the current limitation encountered when connecting boards in an application that utilizes batteries.
  • Backup or current boosters for mechanical applications such as DC motors
  • Extends the battery lifetime of digital cameras
  • Rechargeable backup power source for microprocessors, static RAM's and DAT
  • AMR – Automatic Meter Readings
  • GPS-GSM Modules

Manual Soldering

The following instructions should be followed when considering the manual soldering of the Cellergy Super Capacitor:

  • When using a soldering iron, it should not touch the cell body
  • The temperature of the soldering iron should be less than 410°C or 435°C
  • Soldering time for terminals are less than 5 seconds or 3 seconds respectively

Handling Cautions

Please read the below instructions regarding the handling of the Cellergy Super Capacitors:

  • Do not apply more than the listed rated voltage
    • If you apply more, Cellergy electrolytes will be electrolyzed and the super capacitors ESR may increase.
  • Do not use Cellergy for ripple absorption
  • Operating temperature and life
    • Generally, Cellergy has a lower leakage current, longer back-up time and longer life in the lower temperature range i.e. at room temperature. At elevated temperatures, it will have a higher leakage current and a shorter life.
    • Please position the Cellergy super capacitor so that is not adjacent to heat emitting elements.
  • Short-circuiting Cellergy Super Capacitors
    • You can short-circuit between terminals of Cellergy Super Capacitors without a resistor. However, when you short-circuit frequently, please consult us.
  • Storage
      For long term storage, please store Cellergy Super Capacitors under the following conditions:
    • Temperature: 15 ~ 25 °C
    • Humidity: 45 ~ 75 %RH
    • No dust
  • Do not disassemble Cellergy Super Capacitors - They contain electrolyte.
  • The tips of Cellergy terminals are very sharp - Please handle with care.
  • Reflow process is not recommended for Cellergy Super Capacitors

Note: The Cellergy EDLC is a water based component. Extended use of the EDLC at elevated temperatures may cause evaporation of water leading to ESR increase.