Abstract: A retrofit system for converting a hybrid vehicle to a plug-in hybrid comprises a battery for storing a charge sufficient to provide a vehicle with an electric-only driving range in excess of 5 miles, a battery management system for monitoring battery parameters indicative of the state of battery charge to produce a signal indicative thereof; and a control unit responsive the battery management system to report an inflated state of battery charge to the hybrid vehicle's hybrid control system to maintain the gasoline engine in a deactivated condition over an extended driving range.