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IEC 62391 — Electric Double-Layer Capacitors for Power Storage
Standardising performance, testing and application classification of supercapacitors for industrial and consumer electronics
IEC 62391 is the primary international standard for electric double-layer capacitors (EDLCs), commonly known as supercapacitors or ultracapacitors. Unlike conventional electrolytic capacitors that store charge dielectrically, EDLCs store energy through electrostatic charge separation at the electrode-electrolyte interface (Helmholtz double layer), enabling capacitance values ranging from fractions of a farad to several thousand farads at low voltage. The standard series comprises multiple parts covering terminology, performance characterisation, test methods, and application-specific requirements for these devices.
The fundamental working principle of an EDLC differs fundamentally from a battery: charge storage is purely physical (electrostatic), not electrochemical (faradaic). This gives EDLCs near-infinite cycle life (typically >500,000 cycles), extremely high power density (up to 15 kW/kg), but lower energy density (typically 3–10 Wh/kg) compared to lithium-ion batteries.
1. Standard Scope and Classification by Application
IEC 62391-1 establishes the generic specification, while part 2-x series define sectional specifications for different application classes. The standard classifies EDLCs into four application categories based on their duty profile, which directly determines the test regime:
| Application Class | Typical Use Case | Characteristic Duty | Key Test Parameter |
|---|---|---|---|
| Class 1 — Memory backup | RTC, SRAM data retention | Low current, long duration (hours–days) | Self-discharge ≤ 50% in 30 days |
| Class 2 — Energy storage | UPS, regenerative braking | Moderate current, seconds–minutes | Capacitance change ≤ 30% after 10,000 h DC life |
| Class 3 — Power delivery | Engine start, pulse power | High current, 0.1–10 seconds | Internal resistance change ≤ 100% after 500,000 cycles |
| Class 4 — Instantaneous power | Camera flash, actuator drive | Very high peak current, < 100 ms | Peak current capability test |
A common design mistake is selecting a Class 2 (energy storage) EDLC for a Class 3 (power delivery) application. The internal resistance (ESR) requirements differ drastically: Class 3 demands ESR typically ≤ 1 mΩ per 100 F, while Class 2 permits up to 5 mΩ per 100 F. Using a Class 2 device in a cranking application will cause excessive I²R heating and premature failure.
2. Performance Characterisation and Measurement Methods
IEC 62391-1 defines the standard methods for measuring the four fundamental parameters of an EDLC: nominal capacitance, internal resistance (DC and AC), leakage current, and self-discharge characteristics.
Capacitance measurement: The standard specifies a constant-current charge/discharge method. The capacitance is calculated from the linear portion of the discharge curve between 80% and 40% of rated voltage using the formula C = I × Δt / ΔV. This differs from the 20%–80% window used in some industry practices and must be adhered to for standard-compliant data sheets.
Internal resistance: Two methods are specified:
- DC internal resistance (DC-IR): Calculated from the instantaneous voltage drop at the start of a constant-current discharge (typically measured at 50 ms).
- AC internal resistance (AC-IR or ESR): Measured at 1 kHz using an impedance bridge, equating to the real part of the complex impedance at that frequency.
The standard mandates that both values be reported, as they serve different design purposes. DC-IR is more relevant for power delivery applications, while AC-IR indicates high-frequency behaviour.
A critical nuance in EDLC characterisation is the «voltage derating» effect: the rated voltage is the maximum continuous voltage, but lifetime can be dramatically extended by operating below this limit. A general rule validated by the standard’s DC life test methodology is that every 0.1 V reduction below rated voltage doubles the operational lifetime — following an exponential Arrhenius-like acceleration model similar to electrolytic capacitors but with voltage rather than temperature as the primary stressor.
3. Engineering Design Insights: DC Life Testing and Reliability Modelling
The DC life test specified in IEC 62391 is the cornerstone of EDLC reliability qualification. The test involves applying the rated voltage at the upper category temperature (typically 65 °C for standard types, 85 °C for high-temperature grades) for a specified duration (1,000 h to 10,000 h depending on class). End-of-life criteria are:
- Capacitance decreases by more than 30% from initial value.
- DC internal resistance increases by more than 100% from initial value.
- Leakage current exceeds twice the initial specification limit.
Thermal management in EDLC banks: When multiple EDLC cells are series-connected to achieve higher voltage (e.g., 48 V or 400 V bus), voltage balancing is mandatory. Passive balancing using shunt resistors (typically 1–10 kΩ) is adequate for low-power applications, while active balancing using switched-capacitor or flyback converter topologies is essential for high-power systems to prevent any single cell from exceeding its rated voltage during charging. The standard references IEC 62391-2 for balancing circuit design guidance.
Lifetime estimation model: EDLC ageing follows two main degradation mechanisms: electrolyte decomposition at the positive electrode (accelerated by voltage and temperature) and pore blocking due to decomposition by-products. The standard-endorsed lifetime model is:
L = L0 × 2(Tmax − T)/10 × 2(Vrated − V)/0.1
Where L0 is the rated life at maximum temperature Tmax and rated voltage Vrated. This model demonstrates that every 10 °C reduction in operating temperature doubles the lifetime, and every 0.1 V reduction similarly doubles it, making derating the single most effective strategy for extending EDLC bank service life.
4. Application Design Examples
The versatility of EDLCs is best illustrated through practical application scenarios where IEC 62391-compliant devices are deployed:
| Application | Class | Typical Configuration | Key Design Considerations |
|---|---|---|---|
| Wind turbine pitch control | Class 3 | 48 V bank, 6 series × 2 parallel, 3,000 F cells | Wide temperature range (−30 to +65 °C), 500k+ cycle life |
| Portable power tool | Class 3 | 18 V pack, 5 series, 100 F cells | Fast charge (< 30 s), high surge current (200 A peak) |
| Smart meter backup | Class 1 | 5.5 V, 1 F coin cell | Ultra-low leakage (≤ 1 μA), 10-year life |
| Regenerative braking in HEV | Class 2 | 160 V bank, 40 series, 1,200 F cells | Active balancing, liquid cooling, CAN monitoring |
A critical safety warning: EDLCs can store enormous energy at low voltage — a 48 V, 165 F bank stores approximately 190 kJ (53 Wh). A dead short across the terminals can deliver thousands of amperes, causing explosive vaporisation of bus bars and severe arcing. All EDLC bank designs must include pre-charge circuits, current-limiting fuses, and mechanical interlocks per IEC 62391-1 safety annex.
5. Conclusion
IEC 62391 provides the essential framework for specifying, testing, and applying electric double-layer capacitors across a wide spectrum of power storage applications. By defining clear classification categories, rigorous test methodologies including the crucial DC life test, and performance boundary conditions, the standard enables engineers to confidently select EDLCs for applications ranging from microseconds of backup power to minutes of peak shaving. As supercapacitor energy density continues to improve and costs decline, IEC 62391-compliant devices will play an increasingly central role in the hybrid energy storage systems that power our increasingly electrified world.
Q1: Can EDLCs replace batteries entirely?
Not in general — EDLCs have 3–10 Wh/kg vs 150–250 Wh/kg for Li-ion batteries. However, for applications requiring high power density (> 5 kW/kg), cycle life exceeding 100,000 cycles, or reliable operation at extreme temperatures (−40 to +85 °C), EDLCs outperform batteries. Hybrid systems combining both technologies are increasingly common.
Not in general — EDLCs have 3–10 Wh/kg vs 150–250 Wh/kg for Li-ion batteries. However, for applications requiring high power density (> 5 kW/kg), cycle life exceeding 100,000 cycles, or reliable operation at extreme temperatures (−40 to +85 °C), EDLCs outperform batteries. Hybrid systems combining both technologies are increasingly common.
Q2: What does the voltage derating rule mean in practical design?
If a 2.7 V EDLC rated for 1,000 h at 65 °C is operated at 2.5 V and 45 °C, the estimated lifetime becomes: 1,000 h × 2(65−45)/10 × 2(2.7−2.5)/0.1 = 1,000 h × 4 × 4 = 16,000 h — a sixteenfold improvement.
If a 2.7 V EDLC rated for 1,000 h at 65 °C is operated at 2.5 V and 45 °C, the estimated lifetime becomes: 1,000 h × 2(65−45)/10 × 2(2.7−2.5)/0.1 = 1,000 h × 4 × 4 = 16,000 h — a sixteenfold improvement.
Q3: How is leakage current different from self-discharge?
Leakage current is the steady-state current flowing into a fully charged EDLC to maintain voltage, measured after 30 minutes of constant-voltage charging. Self-discharge is the voltage decay rate when the open-circuited device is stored, measured over 30 days. Both are related but specified separately in IEC 62391-1.
Leakage current is the steady-state current flowing into a fully charged EDLC to maintain voltage, measured after 30 minutes of constant-voltage charging. Self-discharge is the voltage decay rate when the open-circuited device is stored, measured over 30 days. Both are related but specified separately in IEC 62391-1.
Q4: Is balancing always required for series-connected EDLCs?
Yes, unless the operating voltage is very low (≤ 5 V). Individual cell capacitance and ESR tolerances (±20% and ±25% respectively per IEC 62391-1) cause uneven voltage distribution. Without balancing, one cell will exceed its rated voltage and fail prematurely.
Yes, unless the operating voltage is very low (≤ 5 V). Individual cell capacitance and ESR tolerances (±20% and ±25% respectively per IEC 62391-1) cause uneven voltage distribution. Without balancing, one cell will exceed its rated voltage and fail prematurely.
