Salinity gradient power (SGP) derived from sea and fresh water through reverse electrodialysis (RED) is an emerging discipline with huge potential for carbon-free energy harvesting. SGP technology is still in an infant stage and there is a need for accurate mathematical tools to study its energy harvesting process. Previous models assume a constant salinity gradient with a continuous flow of sea water with constant salinity. In the case of recycling used sea water, such assumption is no longer valid because the salinity gradient reduces with operating time. This paper presents a generalized RED model that covers both of the continuous and recycle modes. It combines an improved kinetic battery module (KiBaM) with an electrical circuit module (ECM), for capturing the behaviors of both RED stacks operating in continuous mode (C-mode) and those in recycle mode (R-mode). To intuitively describe the compound effects of salinity variation and concentration polarization on electrical performance of the R-mode RED stack, nonlinear capacity effects (i.e., recovery effect and rate capacity effect) and self-consumed effect are introduced into the proposed model. The derivation and extraction procedures of the proposed model are included. An RED stack prototype with 50 pairs of alternating membranes is constructed for model validation. Various pulsed and constant current discharge experimental tests are performed to validate the accuracy of the proposed model.