I have always seen battery technology as the weak link in EVs.
I thought I might shed some pounds from the vehicle if I were to replace the battery bank with a Kohler or Onan marine generator – I have one of each and they both run about four hours on a gallon of gas. [@ 50mph = 200+mpg !]
The problem is that these generator engines are not very efficient and are certainly not very clean. In the end you may end up doing a whole bunch of work to make your own “hybrid” and end up with something that pollutes more than the original engine AND gets worse gas mileage.
Also, unfortunately, you can’t always do math like: 4hours * 50mph = 200mpg. Why not drive 100mph and get 400mpg?! But seriously, 4 hours supplying how many watts? At what voltage? My first EV sucked down 75amps at 120 volts while driving on flat terrain at 35mph. That’s around 9,000 watts. Generators usually put out AC, which will need to be converted to DC (you lose power at each conversion step).
I did a quick search and found a 10kw generator for around $2,200. It weighs 300lbs which is more weight that needs to be hauled around, more weight, lower car efficiency (even with a gas auto). Also, I suspect if you ran a typical generator set at full load it would suck gas like there were no tomorrow and die an early death. You can get heavier duty gensets, but they will cost more, weigh more and still not be as efficient as a purpose made setup like a Toyota Prius.
The bottom line is that trying to simply turn gas into electricity to power an electric motor will typically use MORE gas than just keeping your existing gas engine. The reason hybrid vehicles do a good job is that they are precisely engineered to use electric power (via batteries or generator) to power the vehicle at take off, slow speeds and during heavy loads. This way the gas engine can run at its most efficient, with the electric filling in the rest of the time. They can then use a smaller and more finely optimized gas engine that way and, thereby, get better gas mileage. More info here