Method and apparatus for estimating bio-impedance at electrode-electrolyte interface by injecting a single low-intensity bi-phasic current stimulus having an selected inter-pulse delay first and second current pulse phases, which involves acquiring transient electrode voltage along the bi-phasic current stimulus waveform. Determining equivalent circuit parameters of an electrode, at the electrode-electrolyte/tissue interface, based on transient electrode voltage across said multiple temporal locations is also performed.