That of course makes a hybrid. You can charge the batteries while underway with the engine. In fact if the battery/controller arent in circuit you risk blowing the caps in the controller. My wife and I rode Jekyll on bikes, hers was an ebike. She forgot her batteries. She rode the whole island charging the caps in the controller and blew them out. I should have thought about it and pulled and isolated the phase wires. Live and learn.To me the easiest way to get maximum range, 1 idea besides carrying 2/3/4 extra batteries. Have a hub rear drive Ebke with a good AH batt. Then when the batt goes kaput use a rear friction drive in the rear to extend your range. 1st I've thought of this, any downsides? UF, I wouldn't mess with a 25cc at all, too anemic. 35cc is the same size almost and gives you the minimum needed, my opinion. Every cc counts on these small motors.