Ultracapacitors, also known as electrochemical double-layer capacitors (EDLCs), are electrochemical capacitors that possess an unusually high power and energy density when compared with traditional capacitors—typically several orders of magnitude greater than a high-capacity electrolytic capacitor. Most ultracapacitors are rated in farads, and individual cells can typically be found in the 100F to 500F range, though they can extend higher or lower. Depending on the application needed, ultracapacitors may be used as battery replacements or enable smaller, more economical battery selection. Ultracapacitors have low equivalent series resistance (ESR), allowing them to deliver and absorb high currents. Their mechanical (rather than chemical) charge-carrier mechanisms enable their long, predictable life with a smooth performance change over time.
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Ultracapacitors are not a new technology. The ultracapacitor effect was first noticed in by General Electric engineers experimenting with devices using porous carbon electrodes. It was believed that the energy was stored in the carbon pores and exhibited exceptionally high capacitance, although the cause was unknown at that time.
General Electric did not immediately follow up on this work, and the modern version of the devices was eventually developed by researchers at Standard Oil of Ohio in , after they accidentally rediscovered the effect while working on experimental fuel cell designs. Their cell design used two layers of activated charcoal separated by a thin porous insulator, and this mechanical design remains the basis for most ultracapacitors today. They first appeared as a low-energy, long-life power backup for consumer electronic devices like VCRs.
The past decade has seen substantial advances in ultracapacitor material science, construction, and manufacturing techniques, which has made them highly desirable products, especially for mission-critical applications. In recent years, these devices have found their way into consumer electronics, industrial products, and automotive applications. Today, the best ultracapacitors are extremely high-power devices with power densities of up to 20kW/kg. Compact in size (small-cell ultracapacitors are often no bigger than a postage stamp), they can store much more energy than conventional capacitors and can release that energy quickly or slowly. They have long life and are designed to last the lifetime of the end product.
Generally, capacitors are constructed with a dielectric (insulator) placed between two metal plates that serve as conductors; energy is stored when, during the charging process, electrons leave one plate and accumulate on the other, building a positive charge on one plate and a negative charge on the other as the dielectric prevents the negatively charged electrons from returning to the positively charged plate. This charge separation creates a potential between the two plates, which can be harnessed in an external circuit. The total energy accumulated in this fashion is a combination of the number of charges stored and the potential between the plates. The number of charges stored is essentially a function of the size and material properties of the plates, while the potential between the plates depends on the dielectric material used.
Ultracapacitors, by contrast, generally do not have any dielectrics but rather utilize a thin coating on the metal plates to keep the positive and negative charges in place. The porous nature of the carbon coating used in this electric double layer gives the plates a larger surface area, which allows for a higher number of charges to be stored on them. The carbon coating used is also much thinner than any dielectric used in a traditional capacitor, which means that the distance between the separated charges is much smaller in ultracapacitors. Combined, this very small charge separation and increased plate surface area give ultracapacitors a much higher energy density than that of traditional capacitors.
To charge them, ultracapacitors are submerged in an electrolyte consisting of positive and negative ions dissolved in a solvent. There are two types of electrolytes used by ultracapacitor manufacturers: One is water-soluble, and the other is non-water-soluble. The non-water-soluble electrolyte increases the withstand voltage per cell compared to that of a water-soluble electrolyte, producing a higher energy density. Tecate Group cells are made with non-water-soluble electrolytes.
The specifics of ultracapacitor construction are dependent on the manufacturer and the intended application. The materials may also differ slightly between manufacturers or due to specific application requirements. The commonality among all ultracapacitors is that they consist of a positive electrode, a negative electrode, a separator between these two electrodes, and an electrolyte filling the porosities of the two electrodes and separators.
Most manufacturers have adopted a cylindrical construction method for their ultracapacitors; however, there are still products on the market that use a prismatic design. Tecate's products use a cylindrical construction method. The cells are constructed from activated carbon particles, which are mixed with a binder and then deposited on aluminum foil. In this method, as shown in Figure 3, the electrodes are wound into a jelly roll configuration very similar to an aluminum electrolytic capacitor. The electrodes have foil extensions that are then welded to the terminals to enable a current path to the outside of the capacitor.
Ultracapacitors experience some level of internal resistance, referred to as equivalent series resistance (ESR), which occurs when the materials used in the ultracapacitor resist the passage of the charges from one plate to another. The amount of ESR varies depending on the particular characteristics (materials used, quality, etc.) of the ultracapacitor.
ESR (referred to as Rs in Figure 4, below) is the main contributor to power loss during ultracapacitor charging and discharging. The other contributing factors are parallel resistance (Rp), capacitance (C), and the series inductor (Ls).
Since Rp is always much larger than Rs, it can be ignored. Also, because of the porous material used on the electrodes of ultracapacitors, they exhibit non-ideal behavior, which causes the capacitance and resistance to be distributed such that the electrical response mimics transmission line behavior. Therefore, it is necessary to use a more general circuit, as shown in Figure 5, to represent the real electrical response and ESR effect.
An ultracapacitor cell can only withstand low voltages. In general, if cells are operated above their rated voltages for a long period of time, their life is reduced. This is a result of the electrolyte breaking down after exposure to high voltage. The amount of damage varies based on the voltage and the amount of time the cell is exposed to the over-voltage condition. Thus, occasional spikes above rated voltage will not immediately affect an ultracapacitor, but prolonged high-voltage uses will.
The voltage rating of Tecate's ultracapacitor cells is 2.7V or 3.0V. This higher-than-usual rating is mainly derived from the electrochemical stability of the electrolyte and electrode materials. The Tecate family of products uses an organic electrolyte. The key advantage of an organic electrolyte versus other (aqueous) electrolytes is its higher voltage stability. Yet even the higher-than-usual voltage rating of Tecate's ultracapacitor cells is not enough to meet the required voltage of most applications.
Placing multiple ultracapacitor cells in series overcomes this limitation. This means that ultracapacitors rated for higher voltages must be made of matched, series-connected individual capacitors, much like series-connected cells in higher-voltage batteries. Depending on the required energy, there could be a need to then place multiple cells in parallel. When ultracapacitor cells are placed in series or parallel, they react very similarly to conventional capacitors. Below is a summary of how key attributes respond when multiple cells are placed in series/parallel formation:
Voltage
Series connection: When placing cells in series, the overall voltage is increased directly by the number of cells in series.
Example: 4 cells (rated at 2.7V each) connected in series will have a maximum voltage of 10.8V.
Parallel connection: Placing cells in parallel will not affect the voltage.
Example: 4 cells (rated at 2.7V each) connected in parallel will have a maximum voltage of 2.7V.
Capacitance
Series connection: When placing same-value cells in series, the system capacitance is reduced by the number of cells placed in series based on the following formula: Csys = Ccell/n.
Example: 4 cells (rated at 10F each) connected in series will have a capacitance of 2.5F.
Parallel connection: Placing same-value cells in parallel will increase the overall system capacitance directly by the number of cells placed in parallel.
Example: 4 cells (rated at 10F each) connected in parallel will have a capacitance of 40F.
ESR
Series connection: When placing same-value cells in series, the overall system ESR will increase directly by the number of cells placed in series.
Example: 4 cells (DC ESR 75 mΩ each) connected in series will have a total ESR of 300 mΩ.
Parallel connection: Placing same-value cells in parallel will decrease the overall system ESR proportionally to the number of cells placed in parallel, according to the following formula: ESRsys = ESRcell/n.
Example: 4 cells (DC ESR 75 mΩ each) connected in parallel will have a total ESR of 18.75 mΩ.
Leakage Current
Series connection: Placing same-value cells in series will not affect the leakage current.*
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Example: 4 cells (each with a leakage current of 0.03mA) connected in series will have a total leakage current of 0.03mA.
Parallel connection: Placing same-value cells in parallel will increase the overall leakage current directly by the number of cells placed in parallel.*
Example: 4 cells (each with a leakage current of 0.03mA) connected in parallel will have a total leakage current of 0.12mA.
*It should be noted that this does not take into account any leakage current induced as a result of cell balancing. In the case of passive balancing, the leakage current will be dominated by the bypass resistor value.
For most applications, a single ultracapacitor cell at low voltage is not very useful, so multiple ultracapacitor cells placed in series are required. Since ultracapacitor cells have a tolerance difference in capacitance and leakage current, there will always be an imbalance in the cell voltages of an ultracapacitor series stack. It is important to ensure that the individual voltage of any single ultracapacitor cell does not exceed its maximum recommended working voltage, as this could result in electrolyte decomposition, gas generation, ESR increase, and ultimately reduced life.
This voltage imbalance is immediately dominated by the capacitance difference between the ultracapacitor cells because a cell with a lower capacitance will charge to a higher voltage in a series string. For example, if two ultracapacitor cells of 10F each are connected in series, with one cell at +20% of nominal capacitance and the other at -10%, then the highest possible voltage for each ultracapacitor cell can be calculated as follows:
Vcap1=Vsupply x (Ccap1/(Ccap1 + Ccap2))
Assuming Vsupply=5.4V
Vcap1=5.4 x (12/(12+9))
Vcap1=3.08V
A voltage of 3.08V is too high for a single ultracapacitor cell; thus, series-connected ultracapacitor cells require a proper cell-balancing scheme to ensure that no cell sees a higher-than-rated voltage.
When an ultracapacitor series stack is on charge for a period of time, leakage current may also affect the voltage distribution among the cells. In this case, a cell with a higher leakage current will go to a lower voltage, distributing the remaining voltage among the other cells and resulting in an over-voltage condition. Proper cell balancing can eliminate this imbalance.
There are two balancing schemes to tackle uneven voltage distribution and ensure a properly balanced ultracapacitor module: passive balancing and active balancing.
Within the passive-balancing scheme, one technique to compensate for variations in individual cells is to place a same-valued bypass resistor in parallel with each cell, sized to dominate the total cell leakage current. This effectively reduces the variation in equivalent parallel resistance between the cells, which eliminates differences in the leakage current. For example, if the cells have an average leakage current of 10uA +/- 3uA, using a 1% resistor that will bypass 100uA in parallel to each cell will change the average leakage current to 110uA +/- 4uA and decrease the variation in leakage current from 30% to 3.6%. Having the same-value resistor in parallel with all cells also allows the cells with higher voltages to discharge through the parallel resistor at a higher rate than the cells with lower voltages. Together, these effects help to distribute the total voltage evenly across the entire series of ultracapacitor cells.
Passive voltage balancing is only recommended for applications that do not regularly charge and discharge the ultracapacitor cells and that can tolerate the additional load current of the balancing resistors. If this method is appropriate, the balancing resistors should be selected to give an additional current flow of at least 10 times the worst-case cell leakage current. Higher ratios can be used to balance the cells faster, if the series can tolerate the increased current load. Once the system is balanced, the time it takes to keep the system in balance is less of an issue unless it is being severely cycled.
By contrast, an active balancing circuit works by forcing the voltage at the nodes of series-connected cells to stay below a fixed reference voltage. Active circuits typically draw much lower levels of current in a steady state and only require larger currents when the cell voltages go out of balance. These characteristics make active voltage-balancing circuits ideal for applications that charge and discharge the cells frequently, as well as those with a finite energy source.
Tecate Group carries a variety of different ultracapacitor cells. Most of these products are designed to be mounted to PC boards by way of soldering, and all of them are RoHS-compliant and thus have lead-free preparations that require adjustments in the processes used to interconnect them with their substrates.
Please note that the process and parameters described here were developed on specific equipment and apply most fully to that equipment. It is very likely that alternative pieces of equipment will require fine-tuning of the parameters to achieve optimal results.
There is a risk of the ultracapacitor cells deteriorating due to excessive heat exposure during soldering, and there may also be a buildup of internal pressure. Depending on the type and size of the board, overheating the ultracapacitor cells may also cause the safety vent to burst. These outcomes will greatly shorten the life of the cells and may cause leaking problems.
Be sure not to dip the entirety of the ultracapacitor cell bodies into melted solder and to only flux the leads of the cells. Ensure that there is no direct contact between the sleeves of the ultracapacitor cells and the PC board or any other component as excessive heat during soldering may cause sleeves to shrink or crack. Similarly, do not touch the sleeves with the soldering rod, as that can cause them to melt or crack.
In order to solder ultracapacitor cells by hand, good soldering practices must be used. The solderer should have experience with hand soldering of electronic components and understand fundamental soldering processes. In general, lead-free soldering by hand requires higher heat and more active fluxes than solder containing lead as a constituent. Therefore, new thermal profiles must be adopted and new cleaning agents should be used.
The recommended temperature of the soldering rod tip is no more than 350°C. The soldering duration should be shorter than 3 seconds. Minimize the time that the soldering iron is in direct contact with the terminals of the ultracapacitor cells, as excessive heating of the leads may result in higher ESR.
In order to wave solder components, special attention must be paid to the dwell time and total time at temperature, since ultracapacitors are temperature-sensitive components. Preheat the board from the bottom side only, bringing the top of the board to 100°C, maximum, immediately before soldering. The preheat time will depend upon the heating efficiency, but use a maximum preheating time of 60 seconds.
The recommended conveyor speed is 2.8cm per second, and the soldering duration time should be shorter than 2.5 seconds. Minimize the time that the soldering iron is in direct contact with the terminals of the ultracapacitor, as excessive heating of the leads may result in higher ESR.
Due to the relatively high thermal masses of the components, and especially if the total number or density of the components is high, the use of a standard thermo-profiling device is strongly recommended to avoid excessively high temperatures.
Soldering may be automated with a hot-bar soldering jig, with soldering irons mounted on an automated raise-lower device with time and pressure controls. It is also possible to mount cells using conductive adhesive, ultrasonic welding, or laser welding.
Do not use solvent cleaners, as these may damage the device packaging; improper solvents include acetone, benzene, isopropyl alcohol, and halogenated solvents. Instead, use only aqueous cleaning solutions based on deionized water.
Washing should take place at elevated temperatures not exceeding 70°C. Spray pressures should not exceed 50psi.
Post-wash drying should be kept to the minimum necessary duration, at temperatures not exceeding 80°C. Rapid airflow around the modules during drying will assist in the removal of moisture trapped in the packages.
[Julian] needed to weld a bit of nickel to some steel and decided to use a spot welding technique. Of course he didn’t have a spot welder sitting around. Since these are fairly simple machines so [Julian] set out to build a spot welder using a charged supercapacitor. The fundamentals all seem to be there — the supercap is a 100 Farad unit and with a charge of 2.6V, that works out to over 300 joules — yet it simply doesn’t work.
The problem is in how the discharge energy is being directed. Just using the capacitor would cause the charge to flow out as a spark when you got near the point to discharge. To combat this, [Julian] put a microswitch between the capacitor and the copper point he expected to use as the welding tip. The microswitch, of course, is probably not the best for carrying a large surge of current, so we suspect that may be part of why he didn’t get great results.
The other thing we noticed is that he used a single point and used the workpiece as a ground return. Most spot welders use two points near each other or on each side of the workpiece. The current from the capacitor is probably just absorbed by the relatively large piece of metal.
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