Capattery® Series

Introduction

The Capattery is a new high-reliability double layer capacitor. It is an ideal standby power source in memory back-up and bridge power applications. It has more than twenty times the capacitance density of conventional capacitors, essentially unlimited cycle life, and stable operating performance throughout the -55° to +85°C temperature range. The high capacitance of the Capattery results from an electrostatic charge stored at the interface between activated carbon and an aqueous electrolyte in the so-called electric double layer. Long life is possible because the energy storage is physical rather than chemical in nature and because the component uses an innovative welded package. These features also provide stable performance during extremely high shock loading.

The Capattery improves upon previous double layer capacitors because of its patented Permselective valve, which allows the escape of CO2 generated by all double layer capacitors, while it maintains its sealed construction. Being a capacitor, the Capattery requires a very simple charging circuit. It is leakproof, exhibits no memory effect, and can deliver currents from microamps to amps. Unlike most batteries, state of charge can be directly measured.

We developed the Capattery to satisfy high reliability requirements. Capable of withstanding exceedingly high shock loads, it is an ideal maintenance-free back-up power source for applications like satellite electronic systems, advanced military equipment and complicated computer networks.

Material Data Safety Sheet

 

Features

High Capacitance Density
Typical capacitance densities exceed 30 farads per gram of activated carbon. Thus, an eleven volt 0.47F Capattery is less than one cubic inch in volume.

High Reliability
The Capattery is sealed in an innovative tantalum package using welded construction. This ensures long, stable operation. Capatteries are not damaged and do not explode if short-circuited. Their charge state is easily ascertained by measuring the voltage.

Welded Construction
The Capattery package, anode, and cathode plates are formed from capacitor grade tantalum, with a matched coefficient of thermal expansion glass-to-metal anode feedthrough. All materials used in cell construction and insulation have proven stability in contact with the sulfuric acid electrolyte. Automated TIG welded assembly and exclusive Evans design features prevent the escape of electrolyte.

Wide Operating Temperature Range
Unlike many batteries, the Capattery can be safely and reliably operated throughout the -55° to +85°C temperature range.

Long Cycle Life
Since energy is stored physically rather than chemically in the Capattery, extremely long cycle life results. Capatteries do not dry up like electrolytic capacitors, exhibit no discharge memory effect like NiCad batteries and can be charged and discharged indefinitely. Due to the Capattery's relatively high ESR, use in AC circuits is not recommended.

Maintenance-free
Unlike NiCad bameries, Capatteries require only a simple charging circuit. And they do not have to be routinely replaced like batteries.

   As shown, a multi-cell double layer capacitor resides between and is contacted between the anode and cathode plates. Several cells connected in series are needed to achieve the desired operating voltage. A patented Permselective valve allows the release of CO2 while retaining electrolyte vapor. The part will pass Helium Leak Test.

Open Failure Mode
Excess voltage or temperature when applied over long time periods to the Capattery will lead to an open cixcuit failure. The Capattery is de signed to prevent electrolyte leakage during these conditions.

   Each cell has two identical activated carbon-sulfuric acid electrodes separated by a porous membrane. The outer faces of each electrode are made electrically conductive to allow series stacking.

 

High Shock Survivability
Because energy is stored at the molecular-scale electric double layer formed on high surface area carbon, stable operation is maintained during high G loading.
Intermittent opens, shorts or voltage ringing commonly seen with batteries and conventional capacitors - do not occur with the Capattery under these severe conditions.

Ease of Application
Incorrect installation, causing reverse voltage, will not damage the Capattery because of its symmetric construction.

Safety
The safety of the Capattery is indisputable. Its design contains features that prevent catastrophic failure due to excessive thermal or electrical loads. It contains no heavy metals or highly toxic materials. When made to fail in testing, the Capattery has done so in a benign manner.

More detailed technical information is available on request.


Dimensions and Standard Ratings


Custom Capabilities

We can design parts and packaging to meet your specific application.


Application Notes

The typical back-up circuits for the Capattery are shown in Fig. 1. The current limiting resistor, "R", is specified by the maximum current that can be provided by the power supply. The diode D isolates the power supply from the Capattery during power interruptions.

The back-up time capability of the Capattery is related to the current draw of the load. Fig. 2 shows Voltage vs. Time for the one farad Capattery under four different load conditions: constant power, constant current, fixed resistance, and semiconductor devices. Each starts with the same 100µA current draw.


Figure 3 shows the relationship between back-up current and back-up time for CMOS RAM applications for the lF RS or RE series Capattery. For example, 1000 hours of back-up time is provided for a mcmory circuit that draws 1µA at 5 volts.

Selection of the optimal model Capattery for a given application depends on:

  • Operating voltage window
  • Load current
  • Time period required for power back-up
  • Volume available for energy storage

Capattery cells have been optimized in two versions -- minimum self-discharge rate (RS), for low-current applications, and minimum ESR, for high-current applications (RE).
Charge/self-discharge characteristics of the 5.5V, 1F RS055105 Capattery are shown in figures 4 and 5.

The Capattery, like all electrochemical devices exhibits performance changes at temperature extremes. These are shown in Fig. 6 and 7 and should be considered when specifying the Capattery model. The capacitance decrease at low temperatures usually present few problems because load currents typically decrease substantially under these conditions.


General Suggestions

When power source voltages greater than 11V are required, Capatteries of the same model can be series connected. However, zener diodes or voltage balancing resistors are recommended to ensure that maximum rated voltages are not exceeded. Parallel connections present no difficulties.
Due to the component weight, mounting attachments should be made directly to the package.
Although the Capattery is non-polar, optimum performance will be achieved by maintaining the case lead at negative polarity.

 

Specifications

Operating temperature range: -55° to +85°C

Capacitance tolerance: Plus 80%/minus 20%

Equivalent series resistance: Refer to table 1

Leakage current: Refer to table 1

 

Operating Principles

The high volurnetric efficiency of the Capattery is derived first from the use of high surface-area carbon electrode material. A lF, 5.5V Capattery has an effective "plate area" of over 2000m2. Second, the "plate separation" is extremely small -on the order of 10 angstroms. The naturally occurring diffuse double layer, created at the interface between the carbon and a liquid electro lyte when a voltage is applied, establishes this remarkably thin dielectric layer. Capacitance densities in excess of 30F/g of carbon are thus readily achieved. Helmholtz first investigated these highly reversible charge storage phenomena over 100 years ago.


Principle of the Electric Double-Layer


Capattery Selection

Capacitor back-up time from initial voltage Vc to final voltage Vf. The self-discharge current has been ignored in these model equations, which are applicable for initial loads greater than 50µA.

Notes:
1. Vf is the final load voltage.
2. Vc is the t=0 capacitor voltage.
3. R is the capacitor's ESR, which can be neglected for small current loads.
4. Vo=Vc for small current loads.
5. a and b are the semiconductor load constants. b=0.5V(-1) for typical CMOS devices.

As a guide for proper Capattery selection, a lumped-element double layer capacitor circuit model is used (Figure 8). Unlike an ideal capaci tor, the behavior of the Capattery depends upon a number of unique factors inherent in double layer capacitors. This behavior can be accurately predicted mathematically using the formulas given in Table 2 for initial load currents greater than 50µA.

Consult the engineers at Evans for further details or assistance in Capattery selection.


Property Measurements

We use the following methods to determine Capattery property characteristics:

1. DC Leakage current. DC leakag is measured using thc following method:

Notes:
1. Rc = 10 ohm.
2. Eo = maximum rated voltage
3. VR = voltage drop by resistance Rc after 24 hours
4. Initial Vc is less than or equal to .05 volt

2. Capacitance. Capacitance is calculated using the following method:

Where:
Eo = maximum rated voltage
Rc = 100 ohm (0.47, 0.75,1.0F) = 51 ohm (1.5F)
T = Time constant, that is, time period from 0 to 0.632 Eo volt

3. Equivalent series resistance (ESR). The ESR is measured directly or determined from measurements obtained from a bridge. The magnitude of the ac voltage is limited to 05 volt rms maximum.

Where:
Cx: Capacitor
A: Ampere meter (ac)
V: Voltmeter(ac)
Vc: Capacitor voltage


Conclusion

A new and better double layer capacitor, the Evans Capattery is designed exclusively to power circuits reliably during power interruptions.
The Capattery replaces both batteries and less volumetrically efficient electrolytic capacitors.
Sealed in a welded tantalum package, it offers

  • excellent resistance to hostile environments,
  • unparalleled reliability,
  • extremely long life.

The Capattery is ideal for memory back-up and bridge-power applications during power out ages. It is leakproof, non-polar and maintenance free. It can supply currents from microamps to amps.
Capatteries can be series comlected to provide higher voltages. They have unexcelled shock re sistance, do not require complicated charging circuits, will not leak or explode, and exhibit no memory effect.

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