Powerflow is managed using a method, e.g., in a powertrain system having a multi-pack rechargeable energy storage system (RESS) with parallel battery packs. Each pack has a corresponding maximum electrical (current or voltage) limit. The method includes predicting a corresponding terminal voltage for each pack using the corresponding maximum electrical limit. The method includes selecting a terminal voltage as a selected voltage based on a requested operating mode, including selecting a maximum of the terminal voltages when the requested operating mode is a discharging mode and a minimum of the same when the requested operating mode is a charging mode. A pack current through each pack is predicted using the selected voltage and a corresponding battery state space model. A total power capability of the RESS is predicted over a predetermined prediction horizon using the selected voltage, with the operating mode controlled over the prediction horizon via the controller.