In this paper, we investigate profit maximization problem in wireless powered communication (WPC) systems, where the profit is defined as the difference between the revenue earned from uplink data transmission and total energy cost of the system. Since the efficiencies of energy harvesting (EH) circuits and battery storage are both non-linear, conventional linear models will lead to suboptimal resource allocation. In this paper, we propose to maximize the profit of a WPC system under nonlinear energy harvesting and storage models. In order to solve the non-convex optimization problem, we develop a piecewise linear EH model, which is more accurate than conventional linear EH models and existing non-linear EH models. Numerical results show that the power and time allocation design based on the non-linear energy harvesting and storage models outperforms that based on existing linear models in terms of the profit.