Metrology

ultracapacitor and supercapacitor measurement methodology

Measurement methods

• Cyclic Voltammetry
• Impedance spectroscopy
• Constant current charge

The following chart shows that the capacitance values strongl depend on the measurement methods. The caracteristic measurement time or the frequency give higher value for slow measurements. The high values of bias voltage give higher capacitance values.

IEC 62391 standard

IEC 62391-1:	Fixed electric double layer capacitors for use in electronic equipment - Part 1: Generic specification
IEC 62391-2:	Fixed electric double layer capacitors for use in electronic equipment - Part 2: Sectional specification: Electric double layer capacitors for power application
IEC 62391-2-1:	Fixed electric double layer capacitors for use in electronic equipment - Part 2-1: Blank detail specification: Electric double layer capacitors for power application. Assessment level EZ.

Cyclic Voltammetry

The voltage is swept between a lower limit to an upper limit at a fixed voltage scan rate. The voltage scan rate (v) is calculated from the slope of the line. The current evolution is measured as a function of the voltage.

The characteristics of the linear sweep voltammogram recorded depend on a number of factors including:

• - The rate of the electron transfer reaction(s)
• - The chemical reactivity of the electroactive species
• - The voltage scan rate

Voltammetry applied to Li battery

Impedance spectroscopy

See also Supercapacitor frequency properties

Potentiostatic mode: ΔE(ω) is the imposed ripple voltage amplitude superposed on the bias E₀, and ΔI(ω) is the measured current on a constant current I₀.

Galvanostatic mode: in that case it is a small current ripple amplitude which is imposed on a constant current and a ripple voltage which is measured.

The impedance Z(ω) est un nombre complexe qui peut être ecrit sous deux formes equivalentes :

Z(ω) = Z(ω) e^jΦ(ω) or

Z(ω) = Z_r(ω) + jZ_j(ω) with j = 1,

Z is the impedance module, Φ the phase, Z_r the real part and Z_j the imaginary part.

The relations between the 2 equations is given by:

Z² = Z²_r + Z²_j and Φ = tan¹ Z_j/Z_r

where Z_r = Z cos Φ and Z_j = Z sin Φ

Constant current charge

See also Supercapacitor time properties

C charge: Time between t₀ and t₂

C discharge: Time between t₄ and t₆

ESR charge: Voltage drop between t₂ and t₃ @ U_n (2 s)

EST discharge: Voltage recovery between t₂ and t₃ @ U_min (2 s)

In t₂ there is an instantaneous drop of the voltage due the interruption of the charging current. The product r_si vanished with the cutrrent.

Between t₃ and t₄ the charge redistribution and the selfdischarge contribute to the voltage drop.

Between t₇ and t₀ the charge redistribution contributes to an increase of the voltage and the selfdischarge contribute to a voltage drop.

Source: Garmanage: Roland Gallay