Supercapacitor applications guide

• Supercapacitors may vent or rupture if overcharged, reverse charged, incinerated or heated above 150 °C for extensive periods of time.
• Do not crush, mutilate, nail penetrate or disassemble.
• High case temperature (burn hazard) may result from abuse of a supercapacitor.
• When supercapacitors are connected in series and charged, high voltages can be hazardous.

Supercapacitors energy storage devices which function on electrostatic principles with no chemical reactions and no moving parts.  They avoid the lifetime issues associated with chemical storage such as batteries or mechanical issues associated with fly wheels.  They are non-toxic and designed for years of maintenance-free operation. 

Supercapacitors are intended as energy storage with a DC discharge.  The module should not be used for AC charging or discharging.  Discharges may be constant current or constant power.  Example discharges are shown in Figure 1a and 1b.   The voltage drops linearly under a constant current discharge.  

Due to the very low equivalent series resistance (ESR) of supercapacitors, minimal heat is generated during operation.  However, as supercapacitors can handle very high currents, a significant heat rise can occur if the discharges and re-charging is frequent (duty cycle >1.5%) and above the continuous current. 

When determining the capacitance and ESR requirements for an application, it is important to consider both the resistive and capacitive discharge components. In high current applications or short duration charge/discharge, the resistive component is critical. In low current, applications, the capacitive discharge component is typically most critical. The formula for the voltage drop, Vdrop, during a discharge at I current for t seconds is:

    Vdrop = I x (R + t/C).

To minimize voltage drop in a pulse application, use a supercapacitor with low ESR (R value). To minimize voltage drop in a low current application, use a supercapacitor with large capacitance (C value). Please visit https://tools.eatonelectronics.com/ for the supercapacitor calculator for predicting electrical requirements and matching these requirements to various supercapacitor configurations / alternatives.

Supercapacitors can be charged using various methods including constant current, constant power, constant voltage or by paralleling to an energy source, i.e. battery, fuel cell, DC converter, etc. If a supercapacitor is configured in parallel with a battery, adding a low value resistor in series will reduce the charge current to the supercapacitor and will increase the life of the battery. If a series resistor is used, ensure that the voltage outputs of the supercapacitor are connected directly to the application and not through the resistor, otherwise the low impedance of the supercapacitor will be nullified. Many battery systems exhibit decreased lifetime when exposed to high current discharge pulses. The maximum recommended charge current, I, for a supercapacitor where Vw is the charge voltage and R is the supercapacitor impedance is calculated as follows:

    I = Vw 5R

Overheating of the supercapacitor can occur from continuous overcurrent or overvoltage charging. Overheating can lead to increased ESR, gas generation, decreased lifetime, leakage, venting or rupture. Contact the factory if you plan to use a higher charge current. Do not exceed the maximum operating current of a supercapacitor. 

Eaton supercapacitors are designed with symmetrical electrodes, meaning they are similar in composition. When a supercapacitor is first assembled, either electrode can be designated positive or negative. Once the supercapacitor is charged for the first time during the 100% QA testing operation, the electrodes become polarized. Every supercapacitor either has a negative stripe or sign denoting polarity. Although they can be shorted to zero volts, the electrodes maintain a very small amount of charge. Reverse polarity is not recommended, however previously charged supercapacitors have been discharged to -2.5 V with no measurable difference in capacitance or ESR. Note: the longer they are held charged in one direction, the more polarized they become. If reversed charged after prolonged charging in one direction, the life of the supercapacitor will be shortened.

Supercapacitors are rated with a nominal recommended working or applied voltage. The values provided are set for long life at their maximum rated temperature. If the applied voltage exceeds this recommended voltage, the result will be reduced lifetime. If the voltage is excessive for a prolonged time period, gas generation will occur inside the supercapacitor and may result in leakage or rupture of the safety vent. Short-term overvoltage can usually be tolerated by the supercapacitor. 

Temperature in combination with voltage can affect the lifetime of a supercapacitor. In general, raising the ambient temperature by 10 °C will decrease the lifetime of a supercapacitor by a factor of two (although the exact effect will vary by product family). As a result, it is recommended to use the supercapacitor at the lowest temperature possible to decrease internal degradation and ESR increase. If this is not possible, decreasing the applied voltage to the supercapacitor will assist in offsetting the negative effect of the high temperature. For instance, +85 °C ambient temperature can be reached if the applied voltage is reduced to 1.8 V per supercapacitor. At temperatures lower than normal room temperature, it is possible to apply voltages slightly higher than the recommended working voltage without significant increase in degradation and reduction in lifetime. Raising the applied voltage at low temperatures can be useful to offset the increased ESR seen at low temperatures. Increased ESR at higher temperatures is a result of permanent degradation due to electrolyte decomposition inside the supercapacitor. At low temperatures, however, increased ESR is only a temporary phenomenon due to the increased viscosity of the electrolyte and slower movement of the ions. Figure 1 plot shows the time taken for capacitance to drop by 30% at 1.8 V & 2.5 V and by 50% at 2.5 V for continuous operation at a given temperature. This can be used to estimate the operating life for specific applications where the minimum allowable capacitance value is known.

Eaton supercapacitors have a longer lifetime than secondary batteries, but their lifetime is not infinite. The basic end-of-life failure mode for a supercapacitor is an increase in equivalent series resistance (ESR) and a decrease in capacitance. Lifetime depends upon voltage and temperature. The actual end-of-life criteria are dependent on the application requirements. Prolonged exposure to elevated temperatures, high applied voltage and excessive current will lead to increased ESR and decreased capacitance. Reducing these parameters will lengthen the lifetime of a supercapacitor. In general, cylindrical supercapacitors have a similar construction to electrolytic capacitors, having a liquid electrolyte inside an aluminum can sealed with a rubber bung. Over many years, the supercapacitor will dry out, similar to an electrolytic capacitor, causing high ESR and eventually end-of-life.

In specific terms, the end of life is defined as the time when the capacitance reaches the specified condition (typically 20% to 30% of the initial capacitance depending upon the product series) or when the ESR reaches the specified condition (typically 100% to 400% of the rated ESR). The following model is used to estimate life time.

INSERT LIFETIME FORMULA

 

Where

    Constant, b, g are constants found from experiment

    K = Boltzman’s constant

    T = temperature in degrees Kelvin

It can be seen from this equation that the life time depends upon the charge voltage and temperature. The lower the voltage and lower the temperature the longer the life.

Self discharge and leakage current are essentially the same root cause, but parametrically measured in different ways. Due to the supercapacitor construction, there is a high impedance internal current path from the anode to the cathode. In order to maintain the charge on the capacitor, a small amount of additional current is required, during charging this is referred to as leakage current. When the charge voltage is removed, and the capacitor is not loaded, this additional current will discharge the supercapacitor and is referred to as the self discharge current. Self discharge is typically measured as a voltage loss or % voltage loss. In order to get a realistic measurement of leakage or self discharge, the supercapacitor should be charged for 72 hours or more, this again is due to the capacitor construction. The supercapacitor can be modeled as several capacitors connected in parallel, each with an increasing value of series resistance. The capacitors with low values of series resistance charge quickly thus increasing the terminal voltage to the same level as the charge voltage. However, if the charge voltage is removed, these capacitors will discharge into the parallel capacitors with higher series resistance if they are not fully charged. The result of this being that the terminal voltage will fall giving the impression of high self discharge current. It should be noted that the higher the capacitance value the longer it will take for the device to be fully charged, see Measurement Techniques application note for more details. 

Individual supercapacitors are rated up to 3.0 V (higher voltage products contain multiple cells). As many applications require higher voltages, supercapacitors can be configured in series to increase the working voltage. It is important to ensure that the individual voltages of any single supercapacitor do not exceed its maximum recommended working voltage as this could result in electrolyte decomposition, gas generation, ESR increase and reduced lifetime. Capacitor over voltage can occur due to  differences in capacitance during charge and discharge, and, in steady state, by differences in leakage current. During charging, series connected capacitors will act as a voltage divider so lower capacitance devices will receive greater voltage stress. For example, if two 10 F capacitors are connected in series, one at +20% of nominal capacitance the other at –20% the worst-case voltage across the capacitors is given by:

    Vcap1 = Vsupply x (Ccap1 / (Ccap1 + Ccap2)

where Ccap1 has the +20% capacitance.

    So for a Vsupply = 5 V,

    Vcap1 = 5 V x (1.2 / (1.2 + 0.8) = 3.0 V

    And Vcap2 = 2.0 V

From this, it can be seen that, in order to avoid exceeding the supercapacitor surge voltage rating of 3 V, the capacitance values of series connected parts must fall in a +/-20% tolerance range. Alternatively, a suitable voltage balancing circuit can be employed to counteract the voltage imbalance. It should be noted that the most appropriate method of voltage balancing will be application specific. 

Passive voltage balancing uses voltage-dividing resistors in parallel with each supercapacitor (or per set of parallel connected cells. This allows current to flow around the supercapacitor at a higher voltage level into the supercapacitor at the lower voltage level, thus balancing the voltage. It is important to choose balancing resistor values that provide for higher current flow than the anticipated leakage current of the supercapacitors, bearing in mind that the leakage current will increase at higher temperatures. Passive voltage balancing is only recommended for applications that don’t regularly charge and discharge the supercapacitor and that can tolerate the additional load current of the balancing resis-tors. It is suggested that the balancing resistors be selected to give additional current flow of at least 50 times the worst-case supercapacitor leakage current (3.3 kΩ to 22 kΩ depending on maximum operating temperature). Although higher values of balancing resistor will work in most cases they are unlikely to provide adequate protection when significantly mismatched parts are connected in series. 

When series connected supercapacitors are rapidly discharged the voltage on low capacitance value parts can potential go negative. As explained previously, this is not desirable and can reduce the operating life of the supercapacitor. One simple way of protecting against reverse voltage is to add a diode across the capacitor, configured so that it is normally reverse bias. By using a suitably rated zener diode in place of a standard diode the supercapacitor can also be protected against overvoltage events. Care must be taken to ensure that the diode can withstand the available peak current from the power source. 

Please refer to the product datasheet for applicable soldering process guidelines. Excessive heat may cause deterioration of the electrical characteristics of the supercapacitor, electrolyte leakage or an increase in internal pressure. Follow the specific instructions listed in the following detail. In addition:

• Do not dip the supercapacitor body into melted solder.

• Only flux the leads of the supercapacitor.

• Ensure that there is no direct contact between the sleeve of the supercapacitor and the PC board or any other component. Excessive solder temperature may cause sleeve to shrink or crack.

• Avoid exposed circuit board runs under the supercapacitor to prevent electrical shorts.

Manual soldering

Do not touch the supercapacitor’s external sleeve with the soldering rod or the sleeve will melt or crack. The recommended temperature of the soldering rod tip is less than 260 ° C (maximum: 350 °C) and the soldering duration should be less than 5 seconds. Minimize the time that the soldering iron is in direct contact with the terminals of the supercapacitor as excessive heating of the leads may lead to higher equivalent series resistance (ESR). 

Wave soldering

Use a maximum preheating time of 60 seconds for PC boards 0.8 mm or thicker. Preheating temperature should be limited to less than 100 °C. See Table 1 for information on wave soldering leads only.

Reflow soldering

Do not use reflow soldering on the supercapacitors using infrared or convection oven heating methods.

Although Eaton’s supercapacitors have very low resistance in comparison to other supercapacitors, they do have higher resistance than aluminum electrolytic capacitors and are more susceptible to internal heat generation when exposed to ripple current. Heat generation leads to electrolyte decomposition, gas generation, increased ESR and reduced lifetime. In order to ensure long lifetime, the maximum ripple current recommended should not increase the surface temperature of the supercapacitor by more than 3 °C. 

Avoid cleaning of circuit boards, however if the circuit board must be cleaned use static or ultrasonic immersion in a standard circuit board cleaning fluid for no more than 5 minutes and a maximum temperature of 60 °C. Afterwards thoroughly rinse and dry the circuit boards. In general, treat supercapacitors in the same manner you would an aluminum electrolytic capacitor.

Do not store supercapacitors in any of the following environments:

• High temperature and/or high humidity
• Direct contact with water, salt water, oil or other chemicals
• Direct contact with corrosive materials, acids, alkalis, or toxic gases
• Direct exposure to sunlight
• Dusty environment
• Environment subject to excessive shock and/or vibration

If a supercapacitor is found to be overheating or if you smell a sweet odor, immediately disconnect any power or load to the supercapacitor. Allow the supercapacitor to cool down, then dispose of properly. Do not expose your face or hands to an over-heating supercapacitor. Contact the factory for a Material Safety Data Sheet if a supercapacitor leaks or vents. If exposed to electrolyte:

• Skin Contact: Wash exposed area thoroughly with soap and water.
• Eye Contact: Rinse eyes with water for 15 minutes and seek medical attention.
• Ingestion: Drink milk/water and induce vomiting; seek medical attention.

Note: In general the electrolyte, using the NFPA/HMIS (0 to 4) rating system, has slight (1 out of 4) health and fire hazard and minimal (0 out of 4) reactivity hazard. 

Eaton supercapacitors are rated non-hazardous under the OSHA hazard communication standard (29 CFR 1910.1200).

Shipping is governed by UN regulations Transport of Dangerous Goods. Supercapacitors are categorized under regulation 3499. Special provision 361 applies to all of Eaton’s supercapacitors as they are less than 10 Wh in total stored energy. Many part numbers are under 0.3 Wh and therefore do not have any restrictions. Note: The rating is according the energy storage of the base cell, either standalone or in a pack or module.

Import classifications: HS Code: 8532290040

ECCN: 3A991.j.2