Patent ID: 7857082

Claim:
A method of operating a series hybrid vehicle having a primary power source and a secondary power source, the method comprising: monitoring an amount of available stored energy within an energy storage device; operating an engine at one of a plurality of preselected power levels to generate an amount of primary power from the primary power source when the amount of available stored energy is below a first level, the plurality of preselected power levels including a predefined minimum power level for efficient operation of the engine; converting the amount of primary power from the primary power source into an amount of direct input energy while powering the secondary power source directly with a portion of the amount of direct input energy; only ever using secondary energy to drive the wheels of the vehicle; selecting the one of the plurality of preselected power levels based on the amount of available stored energy; and operating the engine at or near the predefined minimum power level when the amount of available stored energy is within a predetermined range of available stored energy.