In this paper, a new deterministic model is developed for the real-time optimization of microgrids with diesel gensets and battery energy storage systems. This new model ensures a better usage of batteries that prevent undesirable behaviour, such as intermittent battery charge and discharge. Furthermore, the model enables implicitly to control battery cycling, based on information provided by upper optimization layers. Also, since some constraints associated with batteries are bilinear (therefore nonlinear) and because performance is critical for real-time optimization, many approximations are proposed to linearize the problem. Among these, the constant voltage approximation and the relative state of charge calculation approximation yield an acceptable performance for real-time optimization, while keeping the approximation error small with respect to the relative state of charge.