Abstract: A charging system for a hybrid vehicle comprises an internal combustion engine, an electric motor generator coupled to the internal combustion engine and operable to be driven as a generator to produce a charging voltage and a battery coupled to the electric motor generator and configured to receive a charge voltage. The charging system further comprises an engine control computer coupled to the internal combustion engine, the electric motor generator and the battery. The engine control computer is configured to determine a threshold CSFC, calculate an instantaneous CSFC, and initiate active charging of the battery if the instantaneous CSFC is less than or equal to the threshold CSFC.