Currently the exact recombination mechanism of free carriers in organic photovoltaic (OPV) devices is poorly understood. Often a reduced Langevin model is used to describe the decay behavior of electrons and holes. Here we propose a novel, simple kinetic model that accurately describes the decay behavior of free carriers in the PTB7:PCBM organic photovoltaic blend. This model needs to only take into account free and trapped holes in the polymer, and free electrons in the fullerene, to accurately describe the recombination behavior of free carriers as measured by time-resolved microwave conductivity (TRMC). The model is consistent for different PTB7:PCBM blend ratios and spans a light intensity range of over 3 orders of magnitude. The model demonstrates that dark carriers exist in the polymer and interact with photoinduced charge carriers, and that the trapping and detrapping rates of the holes are of high importance to the overall carrier lifetime. (Figure Presented).