There aint no such thing as a free lunch. The ICE is burning gasoline to propel the car. If you want to charge the battery as well as propel the car the ICE has to do extra work, in order to do the extra work the ICE has to consume extra gas. When you think about it burning extra gas is just common sense. If you were driving at a steady 55 MpH on a flat road and you had to climb up a hill you are going to press down on the accelerator further going up the hill in order to maintain your speed of 55 MPH. More work requires more energy usage in the form of gas, The extra work it takes to charge the battery works on the same principle of requiring extra gas to do the extra work to charge the battery. I hope I was able to explain things in away that made sense and was understandable.