Discover how the supercapacitor can enhance the battery

The supercapacitor, also known as ultracapacitor or double-layer capacitor, differs from a regular capacitor in that it has very high capacitance. A capacitor stores energy by means of a static charge as opposed to an electrochemical reaction. Applying a voltage differential on the positive and negative plates charges the capacitor. This is similar to the buildup of electrical charge when walking on a carpet. Touching an object releases the energy through the finger.
There are three types of capacitors and the most basic is the electrostatic capacitor with a dry separator. This original capacitor has very low capacitance and is used to filter signals and tune radio frequencies. The size ranges from a few pico-farads (pf) to low microfarad (μF).
The electrolytic capacitor provides more farads and these larger units are used for filtering, buffering and signal coupling. Rated in microfarads (μF), this capacitor has several thousand times the storage capacity of the electrostatic capacitor and uses a moist separator. The third type is the supercapacitor, rated in farads, which is thousands of times higher than the electrolytic capacitor. The supercapacitor is used for energy storage undergoing frequent charge and discharge cycles at high current and short duration.
Farad is a unit of capacitance named after the English physicist Michael Faraday. One farad stores one coulomb of electrical charge when applying one volt. One microfarad is one million times smaller than a farad, and one pico-farad is again one million times smaller than the microfarad.
Engineers at General Electric first experimented with an early version of supercapacitor in 1957, but there were no known commercial applications. In 1966, Standard Oil rediscovered the effect of the double-layer capacitor, which makes the supercapacitor work, by accident while working on experimental 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 materials and manufacturing methods led to improved performance and lower cost.
The supercapacitor has evolved and crosses into battery technology by using special electrodes and electrolyte. While the basic Electrochemical Double Layer Capacitor (EDLC) depends on electrostatic action, the Asymmetric Electrochemical Double Layer Capacitor (AEDLC) uses battery-like electrodes to gain higher energy density, but this is affected by a shorter cycle life and other burdens shared with the battery. Graphene electrodes promise improvements with supercapacitors and batteries but such development is 15 years away.
Several types of electrodes have been tried and most are based on the electrochemical double-layer capacitor concept, which 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 volts, the supercapacitor is confined to 2.5–2.7V. Voltages of 2.8V and higher are possible but they reduce the service life. To get higher voltages, several supercapacitors are connected in series. Serial connection reduces the total capacitance and increases the internal resistance. Strings of more than three capacitors require voltage balancing to prevent any cell from going into over-voltage. Lithium-ion batteries share a similar protection circuit.
The specific energy of the supercapacitor ranges from 1 to 30Wh/kg, 10 to 50 times less than Li-ion. 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, reducing the usable power spectrum.
Take a 6V power source that is allowed to discharge to 4.5V before the equipment cuts off. 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. An optional DC-DC converter helps the recover the energy that dwells in the low voltage band but this adds costs and introduces energy loss. A battery with a flat discharge curve, in comparison, delivers 90 to 95 percent of its energy reserve before reaching the voltage threshold.
Figures 1 and 2 demonstrate voltage and current characteristics on charge and discharge. On charge, the voltage increases linearly and the current drops by default when the capacitor is full without the need of a full-charge detection circuit. On discharge, the voltage drops linearly and to maintain a steady wattage level as the voltage drops, the DC-DC converter begins drawing more current. The end of discharge is reached when the load requirements can no longer be met.

charge-web2 discharge-web2

The charge time of a supercapacitor is 1–10 seconds. The charge characteristic is similar to an electrochemical battery and the charge current is, to a large extent, limited by the charger’s current handling capability. The initial charge can be made very fast, and the topping charge will take extra time. Provision must be made to limit the inrush current when charging an empty supercapacitor as it will suck up all it can. The supercapacitor cannot go into overcharge and does not require full-charge detection; the current simply stops flowing when full.
Table 3 compares the supercapacitor with a typical Li-ion.

Function

Supercapacitor

Lithium-ion (general)

Charge time

Cycle life

Cell voltage

Specific energy (Wh/kg)

Specific power (W/kg)

Cost per Wh

Service life (industrial)

Charge temperature

Discharge temperature

1–10 seconds

1 million or 30,000h

2.3 to 2.75V

5 (typical)

Up to 10,000

$20 (typical)

10 to 15 years

–40 to 65°C (–40 to 149°F)

–40 to 65°C (–40 to 149°F)

10–60 minutes

500 and higher

3.6 to 3.7V

100–200

1,000 to 3,000

$0.50-$1.00 (large system)

5 to 10 years

0 to 45°C (32°to 113°F)

–20 to 60°C (–4 to 140°F)

The charge time of a supercapacitor is 1–10 seconds. The charge characteristic is similar to an electrochemical battery and the charge current is, to a large extent, limited by the charger’s current handling capability. The initial charge can be made very fast, and the topping charge will take extra time. Provision must be made to limit the inrush current when charging an empty supercapacitor as it will suck up all it can. The supercapacitor cannot go into overcharge and does not require full-charge detection; the current simply stops flowing when 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. Age is also kinder to the supercapacitor than a battery. Under normal conditions,
a supercapacitor fades from the original 100 percent capacity to 80 percent in 10 years. Applying higher voltages than specified shortens the life. The supercapacitor is forgiving in hot and cold temperatures, an advantage that cannot be met equally well by batteries.
The self-discharge of a supercapacitor is substantially higher than that of an electrostatic capacitor and somewhat higher than the electrochemical battery; the organic electrolyte contributes to this. The supercapacitor discharges from 100 to 50 percent in 30 to 40 days. Lead and lithium-based batteries, in comparison, self-discharge five percent per month.

Applications

Supercapacitors are deployed to deliver short-term power; batteries are chosen to provide long-term energy. Combining the two into a hybrid battery satisfies both needs and reduces battery stress that reflects in longer service life. Such batteries are being made available today in the lead acid family.
Supercapacitors are most effective to bridge power gaps lasting from a few seconds to minutes; they also have the capability to recharge quickly. A flywheel offers similar qualities and an application where the supercapacitor competes against the flywheel is the Long Island Rail Road (LIRR) trial. LIRR is one of the busiest in North America.
The 2MW supercapacitor bank and a 2.5MW flywheel system are being tested to prevent voltage sag and reduce peak power during acceleration at the LIRR.  Both systems must provide continuous power of 2MW and 2.5MW respectively for 30 seconds and fully recharge in the same time. The systems must have low maintenance and should last for 20 years. The goal is to achieve a regulation that is within 10 percent of the nominal voltage. (Authorities believe that flywheels are more rugged and energy efficient for this application than batteries. Time will tell.)
Japan also deploys large supercapacitors. The 4MW systems installed in commercial buildings reduce grid consumption at peak demand times and ease loading. Other applications are starting backup generators during power outages and providing power until the switch-over is stabilized.
Supercapacitors have also made critical inroads into electric powertrains. The virtue of ultra-rapid charging during regenerative braking and delivery of high current on demand makes the supercapacitor ideal as a peak-load enhancer for hybrid vehicles, as well as fuel cell applications. The broad temperature range and long life offers an advantage over the battery.
Supercapacitors have low specific energy and are expensive in terms of cost per watt. Some design engineers argue that the money for the supercapacitor would better be spent on a larger battery. Table 4 summarizes the advantages and limitations of the supercapacitor.

Advantages

Virtually unlimited cycle life; can be cycled millions of time

High specific power; low resistance enables high load currents

Charges in seconds; no end-of-charge termination required

Simple charging; draws only what it needs; not subject to overcharge

Safe; forgiving if abused

Excellent low-temperature charge and discharge performance

Limitations

Low specific energy; holds a fraction of a regular battery

Linear discharge voltage prevents using the full energy spectrum

High self-discharge; higher than most batteries

Low cell voltage; requires serial connections with voltage balancing

High cost per watt