The ICE turns a generator and is not mechanically connected to the driving wheels. This isolates the engine from demand, allowing it to consistently operate at its most efficient speed. Since the primary motive power is generated by the battery, a smaller generator/engine can be fitted as compared to a conventional direct drive engine. Electric traction motors can receive electricity from the battery, or directly from the engine/generator or both. Traction motors frequently are powered only by the electric battery, which can be charged from external sources such as the electricity grid.