An alternator is something that you spin and out comes electricity. It takes effort/work to spin the alternator directly proportional to the amount of electricity you get out. Let's say some theoretical car where everything in the car was perfectly efficient had a motor and an alternator (electric cars don't actually need an alternator). The motor outputs 10kW of power to keep your car cruising at 50MPH (overcoming wind resistance and rolling resistance of the tyres). You switch on the alternator, it draws 1kW of power from the motor, which you then feed back to the battery. So the motor now draws 11kW and you charge the battery at 1kW. That 1kW hasn't gained you anything, the system is equivalent to running the motor at 10kW. In reality because of inefficiencies, this would waste a load of energy.