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EDLC Principles and Uses

supercapacitors
Figure 1: Structure of an electric double layer capacitor (EDLC).

Authors: Julio Gallego-Lopez, Business Development Manager, and Christian Kasper, Technical Support, Rutronik, akos labady, Senior Field Application Engineer at EATON

EDLCs, also known as supercapacitors, boost caps, or gold caps, are electrochemical capacitors that combine high capacitance with low internal resistance. This makes them ideal in a wide variety of applications. But, what are they and what should be taken into account. An EDLC (Electronic Double-Layer Capacitor) consists of two carbon-coated aluminum electrodes (Figure 1). Its highly porous structure results in a very large surface area, which is primarily responsible for the high capacitance. The separator paper between the electrodes simultaneously acts as a reservoir for the electrolyte. It is made up primarily of acetonitrile (ACN), which is used as a solvent to dissolve conductivity-increasing salts. When a DC voltage is applied, charge carriers accumulate according to polarity at an extremely short distance from the carbon surface. This effect, known as the Helmholtz layer, forms the dielectric. Since this happens at the positive and negative electrodes, the components are called double layer capacitors.

Supercapacitor-based energy storage systems

There are three applications that are relevant to supercapacitors: energy storage, pulse power, and backup (backup). The goal is to develop an energy store based exclusively on supercapacitors or combined with a battery, with the supercapacitor as a secondary store to cover power peaks.

energy accumulation

In energy storage, the primary energy source, eg a solar module, requires a method of storing the generated energy and recovering it on demand, eg in a supercapacitor. However, it is important to note that supercapacitors have higher leakage current or self-discharge ratings than batteries. During the accumulation or charging phase, the charging current must be at least ten times greater than the leakage current.

Pulse power or power boost (boost)

If an application repeatedly demands power spikes, a supercapacitor can cover them, thus extending the life of the primary power source such as a battery many times over (Figure 2). Here, the ESR (equivalent serial resistance – equivalent series resistance) of the capacitor is the most important parameter and becomes the basis for the choice of the supercapacitor. As a "rule of thumb," the ESR of the capacitor should be around 25 percent of the ESR of the primary cell.

high-energy
Figure 2: The primary cell consistently delivers relatively low current, with the supercapacitor covering peak demand.

Power failure or backup (backup)

Supercapacitors are capable of supplying power for a specified period of time (Figure 3). The system does not necessarily have to have a power source such as a battery. Examples include real-time clock systems that require a few microwatts of power for several days or even weeks, but also very energy “intensive” applications, such as trams that have to travel a relatively short distance without a catenary. Supercapacitors are also especially suitable for supporting driverless transportation systems (AGV – automated guided vehicles – automatic guided vehicles) for a certain distance without the need for a battery. 

backup power
Figure 3: If the power source fails (for example, the electrical grid), supercapacitors can act as backup storage to provide the power required to, for example, safely shut down servers.

Important parameters in the development of supercapacitors

Certain parameters must be taken into consideration in designs with supercapacitors. The most important are the following:

  • Temperature range
  • Lifespan
  • Charge and discharge cycles during the useful life
  • usable voltage range
  • Costs
  • Load parameters
  • Balance

Temperature range: Due to their physical and chemical composition, supercapacitors have very constant power over a very wide operating temperature range. Capacitance and ESR curves as a function of temperature are available from the manufacturer (upon request).

Lifespan: The operating voltage and temperature are the most influential aspects in the lifespan of supercapacitors. If they are stored in a no-load condition, their useful life is virtually unlimited. As characteristic data in the data sheets, the suppliers specify the change that usually occurs in performance when decreasing the capacitance and increasing the resistance.

Cycles Over Life: Under typical operating conditions, depending on the cell, a supercapacitor can perform up to one million duty cycles with a reduction in nominal capacitance of 20 to 30 percent in most cases.

Usable voltage range: Supercapacitors have a nominal voltage of 2,7 or 3 V (hybrid models: 3,8 V). They can operate until 0 V is reached (hybrid variants only drop to 2,2 V). Hybrid components combine the properties of both a battery and a supercapacitor. However, its use should be carefully considered due to the minimum tension required. Some manufacturers also specify voltage peaks in their data sheets. This describes the absolute maximum voltage for a time of up to one second. Random voltage spikes above the nominal voltage do not affect the capacitor immediately, but depending on the frequency and duration, they can significantly reduce its life.

Theoretically, the total energy content of a capacitor is E=½C V2 (C= capacitance, V= voltage). Since most electronic devices require a certain minimum voltage to operate, this means the following: the voltage range never extends from nominal voltage to zero. Using a voltage range of half the nominal voltage allows about 75 percent of the available power to be drawn from the supercapacitor.

The energy content with partial discharge is calculated using the following formula:

E = ½C (V2Max - V2min)

Costs: The price per watt-hour of a supercapacitor is relatively high compared to, for example, lithium-ion batteries, which is why it is advisable to consider very carefully whether development with a supercapacitor makes sense and is feasible. The costs must be weighed against the benefits of its use.

Charging parameters: Supercapacitors do not store energy through a chemical reaction in the same way as batteries, but electrostatically, allowing them to be charged and discharged in the same way and with the same current value. This is possible with constant current or constant power from a DC source.

Balance: The operating voltage of those applications for which supercapacitors are suitable typically far exceeds the nominal voltage of supercapacitor cells, which sits at 2,7 or 3V. Multiple capacitors must be connected in series to achieve the voltage demanded, for example, 12, 24 or 48 V. The different capacitance and leakage current tolerances of the individual capacitor cells have to be balanced when charged using cell balance, so the individual cells do not drop below its maximum voltage range. Passive and active methods are available. In simple terms, passive balancing is best for applications with low load, while active balancing is more suitable for systems with high load and faster charge/discharge cycle sequences.

Passive balance involves connecting a resistance of bypass or a Zener diode in parallel to each cell in order to compensate the leakage current of the cell itself and, therefore, reduce the difference in capacitance between the cells. If all the resistances connected in parallel are identical, the cells with a higher voltage must discharge faster than the cells with a lower voltage due to resistance, thus achieving balance of the individual cell voltages.

For its part, active balancing involves the integration of voltage comparators, individually or in a circuit combined with other monitoring/load functions. This active balance does not continuously regulate all the compensation current as it happens in the passive balance, but it only does so when the voltage exceeds a predefined threshold. This makes active balancing highly effective and efficient, but also more complex and expensive. In general, a supercapacitor module that already has most of the balance built in is usually the best choice. In this case, the balance is optimally adapted to the cells. 

Scaling up a supercapacitor

In the first phase of a development with supercapacitors, it is essential to define the values ​​of the parameters in the Figure 4, since they determine the operation of the supercapacitors in the application.

Supercap discharge
Figure 4: Discharge curve of a supercapacitor.

VMax: Maximum operating voltage

Vmin: Minimum voltage below which the application will not work

Time: Time during which the supercapacitor must deliver a voltage and a current between the maximum and minimum voltages

Vdrop: Voltage drop across capacitor due to ESR (determined later)

It: Current required for the operation of the device. Although most applications demand near constant current, some have variable current, an average current is specified here.

End of life of a supercapacitor

A supercapacitor reaches its EOL (end of life – end of useful life ) when, in comparison with the data sheet,

or capacitance has dropped by 30 percent (in some industries, such as aviation, 20 percent is applicable, and in others, such as automotive, 50 percent)

or the ESR has doubled (usually with a capacitance reduction of about 30 percent, in button cells a 400 percent increase applies).

The relevant EOL criteria are usually found under the heading “dc life – DC useful life” in the data sheet.

capacitance calculation

The following formula can be used (solved for IT or C) when determining the required capacitance of a supercapacitor:

IT = C · dV/dT = C · (VMax - Vmin - Vdrop) / T

C=It T/(V)Max - Vmin - Vdrop)

However, in the first step, the voltage drop (Vdrop):

C=It T/(V)Max - Vmin)

Then, a standard capacitance is selected, which lies above the calculated value. For example, if the result of the formula is 13,2 F, a 15 F capacitor should be chosen.

The next step is a second calculation that takes into account the maximum ESR for DC (or at low frequency). DC ESR is generally defined in the data sheet; an example is shown in the Figure 5.

answer esr
Figure 5: ESR response (in ohms) as a function of frequency.

Voltage drop (ESR drop)

Vdrop = DC ESR It

C=It T/(V)Max - Vmin - Vdrop)

Taking this into consideration, the ESR Vdrop results in a higher final capacitance because the usable voltage range is reduced. This new calculated value must be less than the previously selected capacitance. For example, if it is 13,8F, the selected 15F capacitor is still suitable. However, if the calculated capacitance is greater than 15F, a capacitor with a higher capacitance will have to be chosen. It is always important to consider capacitance degradation and ESR (Figure 6).

esr super capacitor
Figure 6: While typical capacitance decreases over the life of a supercapacitor, ESR increases.

For example, if we take 80 percent of the original capacitance as the EOL and if the calculated capacitance value is 13,8 F, the capacitance has to be 20 percent above this, i.e. 16,56 F This means that, for example, a 25 F capacitor should be chosen.

super capacitor capacitance
Figure 7: Capacitance loss of a supercapacitor; according to the data sheet, the end of useful life arrives when between 65 and 80 percent of the original capacitance is reached.

temperature profile

A crucial factor when selecting a supercapacitor is the temperature profile of the application, as it greatly influences various properties of the capacitor, especially ESR, capacitance, and leakage current. In turn, this has an impact on its lifespan.

The internal resistance (ESR) is almost stable in the medium temperature range and even drops slightly at higher temperatures (Figure 8). If the component performs in temperatures below 68°F (20°C), the increased ESR must be considered and incorporated into the calculations. On the contrary, the capacitance is relatively stable throughout the temperature progression and only drops a little at low temperatures.

increased esr
Figure 8: For applications operating at temperatures below 68°F (20°C), the increase in ESR must always be considered.

In simple language, the leakage current is the minimum current that has to be supplied to a supercapacitor to permanently maintain the same charge or the same voltage level. It largely depends on the temperature and applied voltage. With each 50 °F (10 °C) rise in temperature, it grows two to three times, rising sharply beyond 104 °F (40 °C) (Figure 9). A voltage drop of 0,2 V causes a drop of approximately 50 percent. It is proportional to the capacitance of the capacitor and depends on the type of electrolyte. When using ACN (acetonitrile), it is slightly higher than when using PC (propylene carbonate).

leakage current
Figure 9: Leakage current of supercapacitors as a function of temperature.

Leakage current plays a leading role in applications where voltage is continuously applied because the cells always have to receive current to keep the voltage at a constant level. For capacitors connected in series with a permanent voltage, the leakage current is also important. Its change over the lifetime causes the system to lose balance over time. Cells with lower leakage current are slightly charged, while cells with higher leakage current are minimally discharged. This results in further uneven aging of cells in the system. Passive balance here would be a remedy. As a rule of thumb: the balance current should be ten times the leakage current.

Example of a system with a supercapacitor

In warehouses for modern e-commerce services (e-trade), the highly automated transfer to inventory and order preparation processes (stock & picking) is usually carried out by driverless transport systems (AGV). They take boxes or pallets from the shelves and take them to a packing station. An operation like this usually lasts two or three minutes. The power supply comes from guide rails or from an integrated energy store (battery or supercapacitors). This accumulator can supply power to the entire area and be charged after the shift or cover only part of the route and be recharged regularly at a charging station between jobs.

work cycle
Figure 10: Duty cycle of a driverless transportation system with a 48 V / 65 F supercapacitor energy storage system, weighing approximately 7 kilograms and having a volume of 9 liters.

Supercapacitors have progressed to become a popular power solution in these types of driverless vehicles, particularly in refrigerated warehouses or where maintenance-free 24/7 operation is required. This is because they can be charged during vehicle operation for two to three minutes of use in 10 to 30 seconds, thus allowing warehouse professionals to achieve almost XNUMX percent efficiency. the use of the vehicle, due to the short inactivity times for charging. In addition, they can perform without maintenance for more than ten years and do not represent any safety problem, as can be the case with batteries.