How Super Capacitors Different Than Others

7/22/2019 How Super Capacitors Different Than Others

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Super Capacitors - Different Then Others (on photo Super Capacitor 2.7V, Capacitance 0.22~20F, E.S.R. 40~2000ohm)

Super Capacitors - Different Than Others (part 2) | EEP

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Continued from first part:Super Capacitors Different Then Others (part 1)

Content

1. Equivalent circuit

2. How to measure the capacitance?

Charge Method

Discharge Method

Measure Capacitance

Capacitor types

Advantages o f the supercapacitors

Equivalent circuit

Super capacitors can be illustrated similarly to conventional film, ceramic or aluminum electrolytic capacitors.
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Figure 3 - First order model of a super capacitor

Figure 4 - Model of a su per capacitor

This equivalent circuitis only a simplified or

first order model of a super capacitor. In

actuality super capacitors exhibit a non ideal

behavior due to the porous materials used to

make the electrodes. This causes super

capacitors to exhibit behavior more closely to

transmission lines than capacitors.

Below is a more accurate illustration of the

equivalent circuit for a super capacitor:

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to

Content

How to measure the capacitance?

There are a couple of ways used to measure the capacitance of super capacitors:

1. Charge method

2. Charging and discharging method.

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Charge Method

Measurementis performed using a charge methodusing the following formula:

C = t / R

t = 0.632 x Vo where Vois the applied vo ltage.
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Figure 5 - Charge and discharge methods

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Discharge Method

This method is similar to the charging

method except the capacitanceiscalculated during the discharge cycle

instead of the charging cycle.

Discharge time for constant current

discharge:

t= Cx( V0 V1) / I

Discharge time for constant resistance discharge:

t= CR ln ( V1/ V0)

Where:

t discharge time,

V0 initial vo ltage

V1 ending voltage

I current

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Measure Capacitance

Super capacitors have such large capacitance values that standard measuring equipment cannot be used to

measure the capacity of these capacitors.

Capacitance is measured per the following method:

1. Charge capacitor for30 minutesat rated voltage.

2. Discharge capacitor through a constant current load.

3. Discharge rate to be 1mA/F.

4. Measure voltage drop between V1to V2.

5. Measure time for capacitor to discharge from V1to V

2.

6. Calculate the capacitance using the following equation:

C = I * ( T 2 T1)

V1 V2

Where:

V1 = 0.7 Vr, V2 = 0.3 Vr(Vr rated voltage of capacitor)

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Capacitor types

We group capacitors into three family types and the most basic is the electrostatic capacitor, with a dry separator.
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Capacitor types

This capacitor has a very low capacitance and is used to filter signals and tune radio frequencies.

The

size

ranges from a few pico-farad (pf)tolow microfarad (uF).

The next member is the electrolytic capacitor, which is used for:

1. Power filtering,

2. Buffering and

3. Coupling.

Rated in microfarads (F), this capacitor has several thousand times the storage capacityof the electrostatic

capacitor and uses a moist separator.

How a Capacitor Works by Dr. Oliver Winn

Cant see this video? Click hereto watch it on Youtube.

The third type is the supercapacitor, rated in farads, which is aga in thousands of times higher than the electrolytic

capacitor. The supercapacitor is ideal for energy storage that undergoes frequent charge and discharge cycles at

high current and short duration.

Faradis a unit of capacitance named a fter the English physicist Michael Faraday. One farad stores one coulomb of

electrical charge when applying one volt. One microfaradis one million times smaller than a farad, and one pico-

farad is again one million times smaller than the microfarad.

Engineers at General Electricfirst experimented with the e lectric doublelayer capacitor, which led to the

development of an early type of supercapacitor in 1957. There were no known commercial applications then.

In 1966, Standard Oil rediscovered the effect of the double-layer capacitor by accident while working onexperimental fuel cell designs. The company did not commercialize the invention but licensed it to NEC, which in

1978 marketed the technology as supercapacitor for computer memory backup.

It was not until the 1990s that advances in materialsandmanufacturing methodsled to improved performance and

lower cost.
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The modern supercapacitor is not a battery per se but crosses the boundary into battery technology by using

special electrodes and electrolyte. Several types of electrodes have been tried and we focus on the doublelayer

capacitor (DLC) concept. It is carbon-based, has an organic electrolyte that is easy to manufacture and is the most

common system in use today.

All capacitors have voltage limits. While the electrostatic capacitor can be made to withstand high vo lts, the

supercapacitor is confined to 2.52.7V. Voltages of 2.8V and higher are possible but they would reduce the service

life.

To achieve higher voltages, several supercapacitors are connected in series.

This has disadvantages.

Serial connection reduces the total capacitance, and strings of more than three capacitors require voltage balancing

to prevent any cell from going into over-voltage. This is similar to the protection circuit in lithium-ion batteries.

The specific energy of the supercapacitor is low and ranges from 1 to 30Wh/kg. Although high compared to a

regular capacitor, 30Wh/kg is one- fifth that of a consumer Li- ion battery. The discharge curve is another

disadvantage. Whereas the electrochemical battery delivers a steady voltage in the usable power band, the voltage

of the supercapacitor decreases on a linear scale from full to zero voltage.

This reduces the usable power spectrum and much of the stored energy is left behind.

Consider the following example.

Take a 6V power source that is allowed to discharge to 4.5V be fore the equipment cuts o ff. With the linear discharge,

the supercapacitor reaches this voltage threshold within the first quarter of the cycle and the remaining three-quarters

of the energy reserve become unusable.

A DC-to-DC convertercould utilize some of the residual energy, but this would add to the cost and introduce a 10 to

15 percent energy loss. A battery with a flat discharge curve, on the other hand, would deliver 90 to 95 percent of its

energy reserve before reaching the voltage threshold.

Table 1below compares the supercapacitor with a typical Li-ion:

Funct ion Supercapacitor Lithium- ion (general)

Charge time 110 seconds 1060 minutes

Cycle life 1 million or 30,000h 500 and higher

Cell voltage 2.3 to 2.75V 3.6 to 3.7V

Specif ic energy (Wh/kg) 5 (typical) 100200

Specific power (W/kg) Up to 10,000 1,000 to 3,000

Cost per Wh $20 (typical) $0.50-$1.00 (large system)

Service life (in vehicle) 10 to 15 years 5 to 10 years

Charge t emperature 40 to 65C (40 to 149F) 0 to 45C (32to 113F)

Discharge temperature 40 to 65C (40 to 149F) 20 to 60C (4 to 140F)

Rather than operating as a stand-alone energy storage device, supercapacitors work well as low-maintenance

memory backupto bridge short power interruptions. Supercapacitors have also made critical inroads into electric

powertrains.

The virtue of ultra-rapid chargingand delivery of high current on demand makes the supercapacitor an idealcandidate as a peak- load enhancer for hybrid vehicles, as well as fuel cell applications.

The charge time of a supercapacitor is about 10 seconds.

The charge characteristic is similar to an electrochemical battery and the charge current is, to a large extent, limited


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by the charger. The initial charge can be made very fast, and the topping charge will take extra time.

Provision must be made to limit the initial current inrushwhen charging an empty supercapacitor.

The supercapacitor cannot go into overcharge and does not require full-charge detection; the current simply stops

flowing when the capacitor is full. The supercapacitor can be charged and discharged virtually an unlimited number

of times. Unlike the electrochemical battery, which has a defined cycle life, there is little wear and tear by cycling a

supercapacitor.

Nor does age affect the device, as it would a battery.

Under normal conditions, a supercapacitor fades from the original 100 percent capacity to 80 percent in 10 years.

App lying higher voltages than specified shortens the life. The supercapacitor functions well a t hot and cold

temperatures.

The self-discharge of a supercapacitoris substantially higher than that of an electrostatic capacitor and somewhat

higher than the electrochemical battery. The organic e lectrolyte contributes to this.

The stored energy of a supercapacitor decreases from 100 to 50 percent in 30 to 40 days.

A nickel-based battery self-discharges 10 to 15 percent per month. Li- ion discharges only five percent per month.

Supercapacitors are expensive in terms of cost per watt. Some design engineers argue that the money for thesupercapacitor would better be spent on a larger battery.

We need to realize that the supercapacitor and chemical battery are not in competition; rather they are different

products serving unique applications.

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Advantages of the supercapacitors

1. Cell voltage determined by the circuit application, not limited by the ce ll chemistry.

2. Very high cell voltages possible (but there is a trade-off with capacity)

3. High power available.

4. High power density.

5. Simple charging methods. No special charging or voltage detection circuits required.

6. Very fast charge and discharge. Can be charged and discharged in seconds. Much faster than batteries.

7. No chemical actions.

8. Can not be overcharged.

9. Long cycle life of more than 500,000 cycles at 100% DOD.

10. Long calendar life 10 to 20 years

11. Virtually unlimited cycle life not subject to the wear and aging experienced by the e lectrochemical battery.

12. Low impedance enhances pulse current handling by paralleling with an electrochemical battery.

13. Rapid charging low- impedance supercapacitors charge in seconds.

14. Simple charge methods voltage- limiting circuit compensates for selfdischarge; no full-charge detection

circuit needed.

15. Cost-effective energy storage lower energy density is compensated by a very high cycle count.

16. Almost zero maintenance and long life, with little degradation over hundreds of thousands of cycles.

While most commercially available rechargeable batteries can be charged 200 to 1000 times, ultracapacitors

can be charged and discharged hundreds of thousands of times with no damage.However, in reality, they can

be charged and d ischarged virtually unlimited number of times, and will last for the entire lifetime of mostdevices and applications they are used in, thus making them environmentally friendly.

Battery lifetime can be optimised by only charging under favorable conditions, at an ideal rate and, for some

chemistries, as infrequently as possible.

Ultracapacitors can help in conjunction with batteries by acting as a charge conditioner, storing energy from

other sources for load balancing purposes and then using any excess energy to charge the batteries at a
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suitable time.

17. Increased safety since they can handle short circuit and reverse po larity. Also, there is no fire and explosion

hazard.

18. Improved environmental safety since there is no corrosive electrolyte and toxicity of materials used is low.

Rechargeable batteries on the other hand wear out typically over a few years, and their highly reactive

chemical e lectrolytes present a disposal and safety hazard.

19. Rugged since they have Epoxy Resin Sealed Case which is non corrosive.

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How Super Capacitors Different Than Others