here's the math
watts=amps*volts
7.5 horsepower=5595 watts

for 240 volts
5595=a*240
23.31=Amperes
Because motors are NOT 100 percent efficient, we can say 25 amps respectively.

for 480 volts.
5595=a*480
11.65=amps
again, add a few amps respectively to get 13 or 14 amps.

So, it would be convenient if you had a 14 amp 480 volt generator or a 25 amp 240 volt generator that just happened to fit the alternator spot.
then you could use some kind of a converter to charge the battery in place of the alternator.

the problem with this is that the engine won't be running at peak efficiency (about 2-3 Krpm) and if it were then I think it would be making more than 15 horsepower.
that is why I think it is a better idea to use a smaller engine rather than go electric.

just a cool little thing I found.
https://www.unitconversion.org/unit_c.../power-ex.html

I see what you mean about the gasoline generator being inefficient- I was thinking a "plug in" electric drive that could be used around town and save the ICE power train for high speeds.

are those motors you have ac or dc?
it would be much easier to use dc because then there isn't any inverter/separate generator to deal with.
but you would still have to have 20 deep cycle batteries and isolator circuits to charge them all off of low voltage.

thecheese- they are AC (3 phase). I just mentioned them because kamesama pointed out that hybrids typically use 340-580 volts to power a 3 phase 10 hp motor

I'm not serious about doing a conversion (yet). But I'd be willing to cheer someone on who has the time and $ to do it- and if they were successful with using a similar 3 phase motor, then I might give it a try. Yeah, yeah, call me a chicken, because its true in this case:)

clencher- do you know of any 5th wheel/trailer success stories? Or did they all turn out "just as good as stock ICE"

A worry of mine would be that if a converted vehicle was in an accident- the lawyers might try to use that to get some $$$ or to place blame even if the mods had nothing to do with the accident.

My insurance company always asks this question when I get coverage on another vehicle: "Has the vehicle been modified in any way?" This is part of the insurance policy "application".

Maybe I'm on a one track mind or something, but I keep seeing how much easier it would be to put in a diesel or gas ice in the trunk. the smaller the engine, the less fuel used. the closer to peak power, the better the efficiency. with the main ICE under the hood, you would be using all of the cylinders rather than one or two of a small engine. you also have to deal with the fact that you are using fuel to open the valves when you are at idle, where you would be using this. I guess that what I'm thinking of is extreme P&G going entire street blocks at a time till stopped by a light.
A .5 litre two cylinder engine is more efficient than a 1.5 L 4 cylinder at idle.
maybe even using the smaller ICE to run a generator and then a motor of some kind.
Am I alone on these thoughts?

I would think it would help city mileage better than highway...

First you want to find a light diesel engine . . . good luck!
Battery power - forget about running on the highway on electric - the Prius can't even do that unless it is down hill - you just don't have a big enough battery to provide enough power - only maybe at 30 mph speeds on level roads it could work which is ok because that is when the ICE is wasting a lot of fuel and not putting out a lot of power. Beef up the alternator with a high output 3 phase generator on the main engine that can output high enough voltage to charge a second higher voltage battery and run an electric drive and you can operate the ICE under low rpm higer load more efficient operation and maybe if you keep the losses low enough you could come out ahead. You still have the issue of the battery life. It all comes back to making the gas burn better in the stock motor as the most cost effective solution. Remember that even a small engine is going to convert fuel at the same or less efficiency as a big engine - about 25% so you can't gain much that way. As a case in point the TaTa car is still only getting 50mpg which is what the Geo would get with a 3 cylinder 1 liter engine.
If you really want to save some gas and have some fun then build a new vehicle with two or three wheels, electric powered and get away from the extra weight and complexity of a gas vehicle.

How about compressed air?

Electric air compressor, also connected to one rear wheel for regenerative(compressive?) braking, a compressed-air tank, and a simple piston/cylinder/valve setup hooked to the other rear wheel. Then you'd just need a mercury switch to turn the regen on when the car's downhill, and a brake pedal switch to turn it on for regenerative braking.

Plug in the compressor at night, regenerative braking to keep the air cylinder pressurized etc, an extra boost on starts or when needed, should be very light and simple to engineer.

Whaddya think?

Mike in CT built one.

OK, he built it into a Honda Insight so he already had a big battery pack installed. Might have increased that to support the 5th wheel. But you have to admit his site is an interesting read.

His home page: https://www.99mpg.com/

https://www.99mpg.com/5th_wheel_in_place_trial_1.jpg,0