Accurate models capable to predict the dynamic behavior and the State-of-Charge (SoC) of Battery Energy Storage Systems (BESSs) is a key aspect for the definition of model-based controls in electric vehicles and in power grid applications of these energy storage systems. In this context, the paper presents an enhanced electrical BESS model capable to accurately represent the effects of charge redistribution in Lithium-based cells. In fact, this phenomenon is the main source of nonlinearity in the behavior of such devices. The improvement of the proposed model is achieved by a virtual DC current generator accounting for the internal charge transfer. The behavior of this virtual generator is inferred from experimental results describing the effects of the charge redistribution during different sub-phases, namely charge, discharge and rest phases. The proposed model, along with its parameter assessment, has been experimentally validated for: i) two types of Li-ion chemistry, ii) aged cells; iii) different cell operating temperatures ranging from -20°C up to 55°C, iv) a complex battery cell pack of 25 kWh rated energy.