The add on generator idea has been brought up and pursued repeatedly in the past on many EV forums and groups. One of the more successful can be seen at: Mto's 1996 Chevrolet S-10 King-Cab In that case, it was a Chevy S-10 electric conversion with a 5-Kilowatt DC generator. The fellow that built it later built a larger version as a trailer with a generator powered by a small car engine. There was a wonderful write up on the project, but his website is gone now. The real trick is to get away from the losses of converting from AC to DC by building a DC output into the generator and making the generator output voltage match the EV pack voltage. I had planned on building something similar though a bit larger for my Honda Civic conversion, and had purchased many of the parts, but never pursued it. Do a search on range extending trailer and you will see quite a few ideas and attempts. The real challenge is interfacing the generator output into the EV.