What is the planned battery range, and How far do you think you want to "extend" the range ?...20%, 100%, or unlimited range ??
In most cases, its simpler, cheaper, and more efficient to simply increase the battery capacity to "extend" the range, but if you are thinking of having a range extender that can maintain performance once the battery is flat , until the fuel is empty,....then you want to be thinking of a Plug in Hybrid drive instead.
Google the BMW i3 REX.
