Every energy transfer/conversion has a loss and in the case of mechanical-to-electrical one that loss is big. Alternators and dynamos are very innefficient and suppose a big energy loss. Of course it's better than anything when there is no other option, but as long as there is an alternative, like just using electricity or just use the combustion engine, then that's better. You say that when idling the engine consumes less. That's, of course, true. They procuce less energie/power too. An idling engine can't produce enough power to drive a car satisfactorily, so after a conversion and its attached energie loss, much less. The batterie would be charged with less energie than the one that the fuel engine was capable of produce, hence it would be able to produce less movement. A combustion engine converts chemical energie into mechanical energie (movement). There is a loss. An electrical engine converts electrical into mechanical. There is a loss. Your suggested setup would follow this conversions and subsequent losses: - Chemical to mechanical. - Mechanical to electrical. - Electrical to mechanical. (I obviated the chemical to electrical one, because it will happen in any case) The second one is just totally unnecessary, you can just swap wich engine does the work and that is much more efficient. Dont they teach you these things in school?? This is 4th grade science. Sorry to use your words against you, but it just made me laugh the way you acted as an authority above him, while being so wrong.