  A novel inverse relaxation technique for supercapacitor characterization is
developed, modeled numerically, and experimentally tested on a number of
commercial supercapacitors. It consists in shorting a supercapacitor for a
short time $\tau$, then switching to the open circuit regime and measuring an
initial rebound and long-time relaxation. The results obtained are: the ratio
of "easy" and "hard" to access capacitance and the dependence $C(\tau)$, that
determines what the capacitance the system responds at time-scale $\tau$; it
can be viewed as an alternative to used by some manufacturers approach to
characterize a supercapacitor by fixed capacitance and time-scale dependent
internal resistance. Among the advantages of proposed technique is that it does
not require a source of fixed current, what simplifies the setup and allows a
high discharge current regime. The approach can be used as a replacement of
low-frequency impedance measurements and the ones of IEC 62391 type, it can be
effectively applied to characterization of supercapacitors and other relaxation
type systems with porous internal structure. The technique can be completely
automated by a microcontroller to measure, analyze, and output the results.
