using a supercapacitor to manage your power

8/9/2019 Using a Supercapacitor to Manage Your Power

1/9

14 Dec 2009 | WorldWide

Using a supercapacitor to manage your power

Guest article by Pierre Mars, VP Applications Engineering,

CAP-XX (Australia) Pty Ltd.

This article will explain some of the key properties of

supercapacitors that design engineers creating energy

harvesting circuits need to be aware of, and explore how to

use supercapacitors in these circuits.

Why Super?

Why do supercapacitors have such high capacitance with such small volumes? Supercapacitor

electrodes are nano porous carbon coated on a current collector substrate such as aluminium. The

porous carbon has a surface area in the order of 2,500m2/gm. This gives a massive charge

storage area. Supercapacitors do not have a dielectric - they are electrical double layer capacitors.

Ions dissolved in an electrolyte provide charge transport. These ions nestle against the surface of

the porous carbon, so the charge separation distance is effectively the molecular width of the ions.

Since capacitance is proportional to charge storage area/charge separation distance, and thestorage area is 1000s of square meters, and separation distance is in nanometers supercapacitors

have enormous energy density. As an example, the CAP-XX HS230, which is 39mm x 17mm x

3.8mm, is 1.2F and has very low ESR, or Equivalent Series Resistance of 50mOhms, and is rated

to 5.5V. This results in an energy density of x 1.2F x 5.5V2/(39x17x3.8x10-6L) = 7.2KJ/L or

2Wh/L.

Since supercapacitors do not have a dielectric, their maximum operating voltage is limited by the

voltage at which the electrolyte starts undergoing electrochemical reactions. There are two types

of supercapacitor chemistries: those with organic electrolytes and those with aqueous electrolytes.

The maximum voltage for supercapacitor cells with aqueous electrolytes is the breakdown voltage

of water, ~1.1V, so these supercapacitors typically have a maximum of 0.9V/cell. Organic

electrolyte supercapacitors are rated in the range 2.3V - 2.7V cell, depending on the electrolyteused and the maximum rated operating temperature. Several supercapacitor cells are connected

in series to attain the working voltage needed. Although organic electrolyte supercapacitors have

superior energy density because they have a higher rated voltage, they are more difficult to

produce since they must be filled in a completely dry environment and hermetically sealed against

moisture. Aqueous electrolyte supercapacitors, having a water based electrolyte, have no such

problems. Supercapacitors have a porous separator to prevent the positive and negative

electrodes from shorting against each other, but allowing ions to pass through it for charge

transport.

Page 1 of 9Using a supercapacitor to manage your power - Energy Harvesting Journal

8/25/2010http://www.energyharvestingjournal.com/articles/using-a-supercapacitor-to-manage-your-...


2/9

Supercapacitors have been around for years, but were initially very low current devices such as

the Panasonic gold cap, with high ESR, suitable for RTC and memory backup. The breakthrough in

the last 10 years or so has to be reduce the ESR so supercapacitors can deliver high power.

When current is drawn from a supercapacitor, there is an instantaneous voltage drop = ILOAD

x

ESR. Hence ESR limits the amount of current that can be usefully drawn from the supercapacitor.

Again, consider the CAP- XX HS230 as an example: if a 2A load is drawn from this supercapacitor,

the instantaneous voltage drop will only be 2A x 50m = 100mV. The maximum power transfer

occurs when the load resistance = source resistance = ESR, so for an HS203 this = V2/(4 x ESR)

= 5.52/200m = 151.25W, so power density = 151.25W/0.0025L = 60KW/L.

An Ideal Power Buffer

The supercapacitor's high energy storage and high power delivery make it ideal to buffer a high

power load from a low power energy harvesting source, as shown in Fig 1.

Fig 1: Block diagram of energy harvesting power architecture with a supercapacitor

The source sees the average load, which with appropriate interface electronics, will be a low power

constant load set at the maximum power point. The load sees a low impedance source that can

deliver the power needed for the duration of the high power event. Consider a sensor thattransmits data at 100mW for 1 second once an hour. If an HS230 is charged to 3.3V just prior to

the transmission, then during the transmission it will only discharge to 3.27V. The average power

is 0.1W/3600 = 28W. If the circuit is 60% efficient, then the source only needs to deliver - 1.8V in order for U1, Q1 to operate and charge the

supercapacitor, otherwise a boost converter is needed.

Conclusions

Supercapacitors offer an important benefit for energy harvesting applications - the ability to buffer

a high power load from a low power source in a small form factor, but they do not behave like


8/25/2010http://www.energyharvestingjournal.com/articles/using-a-supercapacitor-to-manage-your-...


9/9

example circuit that can be used as a reference design and modified for other applications.

About the Author

Pierre Mars is the VP of Applications Engineering for CAP-XX Ltd. He jointly holds three patents on

supercapacitor applications. Mr. Mars has a BE electrical (1st class hons) and an MEng Sc from the

University of NSW, Australia, in addition to an MBA from INSEAD, France. He is also a member ofthe IEEE. Based in Sydney, Australia; the company can be reached at [email protected]. Design

tools, application notes and other details are available at http://www.cap-xx.com.

Top image: example of a supercapacitor from Cap XX

For more read: Energy Harvesting and Storage for Electronic Devices 2009-2019


using a supercapacitor to manage your power

Documents