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Ultracapacitors Are In Our Future

Just about everything we use today requires a battery. Think about it. (computers, mobile cell phones, hedge clippers, flashlights, hybrid electric cars, school buses, trucks, personal entertainment devices, and more).

The digital age has brought about an increase in functionality. Our reliance on the traditional battery has increased tremendously. Yet the battery has not progressed far beyond the basic design developed by Alessandro Volta in the 19th century. Until now.


MIT's Laboratory for Electromagnetic and Electronic Systems (LEES) recently came up with the most economically viable alternative to conventional batteries in more than 200 years.  The Ultracapacitor is both a battery and a capacitor.

Just think, laptops and cell phones could be charged in a minute. Unlike the traditional laptop battery, which starts to lose its ability to hold a charge after a year or two (several hundred charge/discharge cycles),  ultracapacitors have hundreds of thousands of charge/discharge cycles and could still be going strong long after the device is obsolete.

Introducing the Ultracapacitor - Both a Battery and a Capacitor


Just how does the Ultracapacitor work? Ultra capacitors & Super Capacitors store electricity by physically separating positive and negative charges— different from batteries which do so chemically. The charge they hold is like the static electricity that can build up on a balloon, but is much greater thanks to the extremely high surface area of their interior materials.

An ultra capacitor, also known as a double-layer capacitor, polarizes an electrolytic solution to store energy electro statically. Though it is an electrochemical device, no chemical reactions are involved in its energy storage mechanism. This mechanism is highly reversible, and allows the ultra capacitor to be charged and discharged hundreds of thousands of times.

Once the ultra capacitor is charged and energy stored, a load (the electric vehicle's motor) can use this energy. The amount of energy stored is very large compared to a standard capacitor because of the enormous surface area created by the porous carbon electrodes and the small charge separation created by the dielectric separator.



Design Engineers! 6 Reasons To Use Ultracapacitors 

All electronic devices we have used have had one energy supply.  Engineers designing the device designed it to have too much energy or too much power at any given time.  Today, we see the ultracapacitor where you can leverage both energy and power and have high performance that cost less. Here are 6 reasons to use Ultracapacitors:

  1. High Efficiency - Ultracapacitors have a columbic efficiency that is greater than 99%.  So little is lost during charge/discharge.  Ultracaps have a low equivalent series resistance (ESR).which make them more efficient use of energy (less heating and less cooling for energy storage).

  2. Temperature Range -  Ultracapacitors do not rely on chemical reactions like a battery - so they can operate at a wide range of temperatures.  Typically from 65 degrees Celsius to -40 degrees Celsius.  This means excellent cold performance as a perfect fit for engine starting.  When you put ultracaps with batteries, you can have a system that meets the energy requirements (battery) with the power requirements (ultracapacitor).  A good example is that you can use it to start your engine and power lights and your stereo when the engine is off.

  3. High Current -  Since ultracapacitors are designed with very low equivalent series resistance (ESR) – they deliver and can absorb a high current.   They can be quickly charged making them great for regenerative braking situations (like capturing the energy of a train an other quick charge/ quick release scenarios).  There isn’t a battery made that can tolerate this charge/discharge rate.

  4. Voltage Range - Since we are talking about capacitors, you are not confined to a narrow voltage range.  If you are a designer, you only need to look at the voltage range of your system which is much wider than the narrow voltage required by your battery.  With an ultracapacitor, to get a higher voltage, multiple cells are placed in a series which equate the total maximum voltage.  A great thing is that you never have to worry about over discharging an ultracapacitor.

  5. Long  Life Cycle -   The energy storage of an ultracapacitor is a highly reversible process.   The process will move charge and ions only and does not make or break chemical bonds (like a battery).  This process allows for hundreds of thousands of charge/discharge cycles without minimal change in your performance.   This is perfect for something like a uninterruptible power supply (UPS) that may only charge/discharge fully a few times a year.  Or another example is a hybrid electric vehicle that may be cycled frequently.

  6. Long Life-  Once again.. since there are no chemical reactions like in a battery, the energy storage of an ultracapacior is a highly stable process which is capable of many years of continuous performance.  Ultracapacitors can be installed for the life of the system not on a regular maintenance routine like a battery (costing time an money).  

The Advantage of Low ESR (Equivalent Series Resistance) Ultra and Super Capacitors

ESR, which involves resistances of cell components, for example, electrolyte resistance and contact resistance between current collectors and electrodes affects the power delivery of a cell.  High ESRs restrict the rate at which supercapacitors can be charged and discharged, leading to power reduction and energy dissipation.



The supercapacitor is often misunderstood; it is not a battery replacement to store long-term energy. If, for example, the charge and discharge times are more than 60 seconds, use a battery; if shorter, then the supercapacitor becomes economical.

Supercapacitors are ideal when a quick charge is needed to fill a short-term power need; whereas batteries are chosen to provide long-term energy. Combining the two into a hybrid battery satisfies both needs and reduces battery stress, which reflects in a longer service life.

Supercapacitors are most effective to bridge power gaps lasting from a few seconds to a few minutes and can be recharged quickly. A flywheel offers similar qualities, and an application where the supercapacitor competes against the flywheel is the Long Island Rail Road (LIRR) trial in New York. LIRR is one of the busiest railroads in North America.

To prevent voltage sag during acceleration of a train and to reduce peak power usage, a 2MW supercapacitor bank is being tested in New York against flywheels that deliver 2.5MW of power. Both systems must provide continuous power for 30 seconds at their respective megawatt capacity and fully recharge in the same time. The goal is to achieve a regulation that is within 10 percent of the nominal voltage; both systems must have low maintenance and last for 20 years. (Authorities believe that flywheels are more rugged and energy efficient for this application than batteries. Time will tell.)

Japan also employs large supercapacitors. The 4MW systems are installed in commercial buildings to reduce grid consumption at peak demand times and ease loading. Other applications are to start backup generators during power outages and provide 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 acceleration makes the supercapacitor ideal as a peak-load enhancer for hybrid vehicles as well as for fuel cell applications. Its broad temperature range and long life offers an advantage over the battery.

Typical Applications:

  • Automotive subsystems
  • Burst/Boost power delivery for cold starting diesel or gasoline engines
  • Wind turbine pitch control
  • Hybrid vehicles
  • Railroad braking
  • Industrial motor braking
  • Energy storage recovery
  • UPS & telecom system power stabilization

Next-Generation Energy Storage Devices


Frequently Asked Questions

What is an ultracapacitor?

Electric double-layer capacitors, also known as supercapacitors, electrochemical double layer capacitors (EDLCs) or ultracapacitors are electrochemical capacitors that have an unusually high energy density when compared to common capacitors, typically several orders of magnitude greater than a high-capacity electrolytic capacitor.

The electric double-layer capacitor effect was first noticed in 1957 by General Electric engineers experimenting with devices using porous carbon electrode. It was believed that the energy was stored in the carbon pores and it exhibited "exceptionally high capacitance", although the mechanism was unknown at that time.

How do ultracapacitors differ from battery and traditional capacitors?

The Ultracapacitors reside in between conventional batteries and conventional capacitors. They are typically used in applications where batteries have a short fall when it comes to high power and life, and conventional capacitors cannot be used because of a lack of energy. EDLCs offer a high power density along with adequate energy density for most short term high power applications. Many users compare EDLCs with other energy storage devices including batteries and conventional capacitor technology. Each product has its own advantages and disadvantages compared to other technologies.

What are the key applications for ultracapacitors? 

  • Ultracapacitor Functions
    • Secure power
      • Provides reliable interim power, even if the primary source fails or fluctuates
    • Energy storage
      • Stores energy from low power sources, enabling support for high power loads
    • Pulse power
      • Supplies peak power to the load while drawing average power from the source
  • User Benefits
    • Reduces the size & weight of the battery / power source required
    • Improves run-time & battery life, particularly at cold temperatures
    • Enables more power-hungry features, being used more often
    • Can remove the need for a battery & harvest energy from clean sources
    • Protects against accidental power loss or fluctuations/interruptions
    • Doesn’t need to be replaced like batteries (unlimited discharge cycles)
    • Environmentally friendly & safe

What is end of life and failure mode for an ultracapacitor?

In general ultracapacitors do not have a hard end of life failure similar to batteries. Their end of life is defined as when the capacitance and/or ESR has degraded beyond the application needs.

What is the self discharge or leakage current?

Self Discharge: Is the voltage drop on a charged cell after a set period of time without a load.

Leakage Current: Is the stable parasitic current expected when capacitor is held indefinitely on charge at the rated voltage. This value is voltage and temperature dependent.

How will Ultracapacitors help my product?

Ultracapacitors have very high specific power which is largely unaffected by temperature through their operational range. When ultracapacitors are paired with high energy sources, the combined system peak power can increase dramatically.

How long will Ultracapacitors last?

Ultracapacitors have a design life of 10 years at rated voltage and 25 °C. Reducing voltage and temperature can increase design life, while increasing temperature and voltage will shorten design life. Most applications will use cells at a lower nominal voltage to get longer design life at higher temperatures.

What is the output voltage of an Ultracapacitor?

An ultracapacitor only provides energy as its voltage decreases and absorbs energy as its voltage increases. The output voltage is dependent on the state of charge. An upper and lower voltage limit has to be used to determine working voltage range.

How much energy will an Ultracapacitor provide (how long will my application run)?

Ultracapacitors store much more energy than other traditional capacitors, but substantially less than batteries. To determine the energy stored in an ideal capacitor take the upper voltage and square that, then subtract the square of the lower voltage and multiply the result by one half the cell capacitance to get the energy transferred in joules. E = 1/2C x (Vupper2 – Vlower2)

How do I charge an Ultracapacitor?

Ultracapacitors follow strictly current-based charging rules. An ultracapacitor cell will absorb as much current as is supplied to it, while its voltage increase is based on how much charge it has accumulated. Care must be taken in designs for charging ultracapacitors at a low state of charge, since they will act like a short circuit when their voltage is near zero.

What are the rules for connecting cells in Series?

Most applications require cells to be connected in series to reach higher working voltages. For cells in series, it is best to derate the cells’ rated voltages to reduce the impact of unbalanced cell voltages on system life. As identical cells are connected in series, ESR increases as a multiple of the number of cells and capacitance decreases by the quotient number of cells. (ESRTotal = ESRCell x # of cells, CTotal = CCell / # of cells)

What are the rules for connecting cells in Parallel?

Cells can be connected in parallel if a capacitance is needed that is larger than an available cell size. Cells of different sizes can be connected in parallel, as long as the same types of cells are used for each series connected unit to match capacitance and ESR. For identical cells connected in parallel, ESR decreases by the quotient number of cells and capacitance increases as a multiple of the number of cells. (ESRTotal = ESRCell / # of cells, CTotal = CCell x # of cells)

What is leakage current?

Ultracapacitors have a small amount of self discharge which is referred to as leakage current. Because of small variations in materials and manufacturing, the leakage current of cells can vary a small amount. Over time, as the cells are at a high state of charge, the small variations in leakage currents will cause cell voltages to spread apart. Cells with lower leakage current will increase in voltage and cells with high leakage current will decrease in voltage. Since leakage current increases with voltage, the cell voltages will eventually stop spreading apart when the individual leakage currents become equal. However, enough of a spread can exist which can cause the cells at higher voltages to degrade prematurely when the series-connected cell bank is at a nominal voltage

What is cell balancing and do I need it?

Cell balancing is a way to reduce the voltage spread in cell voltages resulting from an imbalance in leakage currents, an imbalance in capacitance, or an imbalance in power losses from ESR. Cell balancing can range from a simple 1 percent tolerance resistor across the terminals of each cell, which is sized to dissipate 10 times the nominal cell leakage current to complex circuits which shuttle charge between cells. In general, it is a good idea to have cell balancing for applications requiring long cell life.

Can I replace battery XYZ with Ultracapacitors?

Usually no, but there are certain low-energy applications such as short-term ride through for generator starting, engine starting, and motor starting that can be powered by ultracapacitors without a battery. The use of high-efficiency DC-DC converters can greatly extend the working voltage range for increased energy extraction from cells while providing a regulated output.

What cell should I use in my application and how many cells do I need?

Correct sizing of ultracapacitors for an application takes into account many variables, but there are a few rules of thumb that can be used to estimate requirements. Determine the number of cells required in series by dividing the maximum voltage by the rated cell voltage (or derated voltage) and rounding up. Determine how much energy is required for discharge or how much capacitance based on constant current using the equation for capacitance C = Q/V (Capacitance equals current in amps multiplied by time in seconds divided by voltage change in volts). Example: 100 VDC to 70 VDC at 145 A, 10 Wh (36000 J) storage.Capacitance = 2 x 10 Wh x 3600 s / (1002 – 702) = 14.1 FCapacitance = 130 A x 3 s / 30 V = 13 F# of Cells = 100 / (2.7 x 0.95) = 38.9 cells, round up to 39 cellsCell size = 39 x 14.1 F = 550F or 39 x 13 F = 507, round up to closest size: 600 F

Concern - Cost?

In the past decade, the price of higher performing ultracapacitors has fallen by 99 percent. In the same time period, batteries have become only 30 to 40 percent cheaper. That disparity is likely to continue, since the market is adopting ultracapacitors in greater numbers and the cost of related raw materials is falling.  

Concern - Complexity?

A singular energy storage approach might offer simplicity, but its results fail to measure up in almost every way to those of a hybrid component. This is certainly true in regards to power. In most applications, there is a continuous energy demand that is handled by a primary energy source. At times, there are peak power demands. Engineers can either size batteries to handle peak demands or use ultracapacitors to bridge the demand. The latter option has the added benefit of downsizing the primary energy source.

High-power ultracapacitors provide the burst power required by high current demands associated with acceleration, starting, steering and regeneration. Pairing a capacitor with a battery improves the power density of the hybrid supply, which allows the battery to operate without the large current spikes that would exist without the capacitor. 

Concern - Functionality?

Manufacturers know how a battery performs on its own.  The benefits of a dual-energy solution are less familiar to some. For these organizations, it’s important to note that hybridization does not eliminate the value of the battery. The battery actually performs far better and for longer periods of time when paired with an ultracapacitor. The presence of the ultracapacitor enables the battery to do the job for which it was originally designed: provide high-energy density. One example involves hybrid vehicle acceleration, which creates a significant demand for power in the form of amps.  A hybrid solution directs the ultracapacitor to provide high current, enabling the battery to serve strictly as an energy source, rather than as an energy and power source.

Concern - Efficiency & Lifespan?

Ultracapacitors have a much lower internal resistance and, thus, a much faster charge rate than batteries. For this reason, ultracapacitors make battery-powered systems run more efficiently. These components also make batteries last longer because they do the bulk of the work when the load is switched on and allow the battery to pick up load slowly, preventing high current draws. This scenario insulates batteries from high current drains that cause thermal, chemical and mechanical stresses. This reduction of current spikes significantly lowers the internal temperature of batteries, extending their life by as much as 400 percent, depending on the application. Furthermore, while lead-acid batteries offer 70 percent efficiency rates at best, ultracapacitors outshine them with 95 to 98 percent efficiency.  

Batteries rely on a chemical reaction to dissipate stored energy. There is no chemical reaction in ultracapacitors, as they store energy in an electrostatic field. This lack of chemical change enables ultracapacitors to last more than a million cycles, compared to mere hundreds to low thousands of cycles for various batteries. In terms of cycle life, therefore, ultracapacitors deliver greater return than batteries.

Concern - Temperature Resistance?

 The battery has been asked to perform under extreme temperature conditions, with varying degrees of success. Batteries ostensibly withstand temperatures from +60 C to -20 C, but at 0 degrees and below, they lose most of their available energy. The ultracapacitor is more forgiving at the high and low ends of the temperature spectrum, operating comfortably between +70 C and -40 C. This is particularly important to manufacturers who use these components in applications such as jet engine ignition systems that function at high altitudes and within extreme temperatures.

How about the future outlook of the ultracapacitor relative to the battery?

Battery technologies such as Li-ion, NiMH or Lead-Acid are “mature” technologies for which chemical limits to performance are well known and already reached.  Hence, performance increases are incremental, if any, and chiefly come from packaging and process enhancements.

 Ultracapacitor is still an “infant-stage” technology with unexploited potential and because, it relies largely on physical properties to store energy, the technology is not constrained to absolute structural limits as with chemical batteries.  Also, in contrastto major batteries, the ultracapacitor of today has barely scratched the surface of energy storage potential and has barely graduated from “cottage-industry”style manufacturing and “sample quantity”purchase of raw materials.

What’s the down-side of the ultracapacitor relative to the battery?

It’s lower energy density and high price.

This means that the ultracapacitor market is the top-end of the battery market where large premiums can be commanded for outstanding power, life, and reliability features of the ultracapacitor. Today, such market applications are hybrid-electric buses, diesel engine starters, power regulators, wireless devices, digital cameras, battery-less remote controllers, cordless power tools, etc.

What are the advantages of the ultracapacitor relative to the battery?

They are power, life and reliability.

 The power is about the capability to accept or discharge electricity instantaneously.  Many familiar battery applications such as car engine starting or wireless transmissions are actually about power.  It matters less how much energy stored in the battery; but rather how fast it can be discharged.  The ultracapacitor is more than 10 times better in power than the battery devices.  Power where it is critical to the application commands a high premium.

 Rechargeable batteries last less than 1000 full-cycles.  The ultracapacitor last up to one million cycles.  Battery-based applications needs many replacements and expensive servicing; on the other hand, ultracapacitors in a hybrid electric transit bus last beyond the lifetime of the application(15 years and 1 million miles).

The corollary of semi-permanent life and the wide operating temperature band. means high reliabilityand no maintenanceeven under extreme environmental conditions.  In contrast, batteries are fragile devices requiring constant monitoring, protective circuits and thermal management which translates into a lots of $$$.

Where does ultracapacitor fit in among major energy storage devices?

There are two main energy storage markets: the automotive and stationary power segment dominated by Lead-acid battery and the consumer mobile devices segment where Nickel and Lithium batteries are pre-eminent. The ultracapacitor is suited and is already making big inroads into both market segments. Rapid technological advances and cost reductions shall enable it to eventually establish itself as a Top-4 or Top-5 “battery” in the market.


References: ES Components
                     LS Mtron
                     Battery University
                     Green Biz
                      Nesscap Ultracapacitors
                      Maxwell Technologies