This paper deals with advanced characterization approaches of thin piezoelectric materials enabling accurate modelling of low frequency (<50Hz) bimorph-based cantilever beam. Such structures had been widely studied and actual devices using bulk PZT materials had exhibited the best FoM (Figure of Merit) compared to thin and thick piezoelectric films generally used in MEMS industry. Bulk piezoelectric cantilever beam are mostly assembled under serial bimorph topologies where the piezoelectric material is laminated around an inner shim material. The two PZT skins exhibit an individual thickness ranging from few tens to hundreds of micrometers according to the targeted application and its miniaturization requests. Designing such devices become a challenge and one needs FEM (finite element model) to properly define material thicknesses and overall piezoelectric energy harvester (PEH) geometries. Unfortunately, bulk piezoelectric material are characterized using thicker samples which provide results slightly different of the actual characteristics of thinned-bulk piezoelectric materials. We present here two approaches, one analytical and one numerical, to study the thin piezoelectric material layer relying on the electrical admittance analysis. By using these methods, the effective coefficients of thinned-bulk piezoelectric material are identified and compared with those of a thicker bulk material of the same composition. The set of parameters is then entered in a 3D FEM of the PZT layer, and the electrical admittance is calculated, showing a good agreement with the experimental measurements.