A method of operating a vehicle powertrain system where the powertrain system includes a fuel controlled engine, an electric motor, and a battery, the fuel, controlled engine and the electric motor capable of powering the powertrain system. The powertrain system is capable of operating in a plurality of operational modes. The method includes detecting a current value representative of a current flowing through a portion of the battery, calculating a first value representative of the current over a predetermined amount of time, and de-rating at least one of the plurality of operational modes in response to the calculated first value.