Powerflow of a rechargeable energy storage system (RESS) is managed according to a method. The RESS has series-connected first and second battery elements with different characteristics. Each element, e.g., a pack, has a corresponding maximum or minimum voltage or current limit. Currents are predicted for each of the first and second battery elements via a controller using a corresponding voltage limit. A requested operating mode of the RESS is used to select a current for the elements. A voltage across the elements is predicted using the selected current and a corresponding battery state space model. The method predicts a total power capability of the RESS over a prediction horizon using the selected current to generate predicted power capability values. The requested operating mode is controlled over the horizon using the power capability values. A powertrain system includes the RESS, an inverter, an electric machine, and the controller.