Contemplating a DIY hybrid

Hi all, I’m new, and in reading up on batteries it seems to me that options and price likely will change significantly in coming years. So, what I have in mind is an EV with limited battery storage (say, enough for 10 to 20 miles in-town driving) and an on-board generator to recharge batteries on longer trips.

Also, my current vehicle is fairly heavy, so lots of batteries would be an issue (1994 Subaru wagon, 3,220 pounds curb weight). I realize this is less than ideal, but the cost of buying a different vehicle on top of EV components puts that option out of reach.

The questions I have about this design:

– How big a generator would I need to be able to replenish batteries on a 144-volt DC system?
– Is it feasible to have a system that is charging batteries while the electric motor is pulling power?


  1. I am not talking about the wheel generator/perpetual motion machine idea – I’m thinking ICE range extender.

  2. I found this thread; perhaps there is no further information to add:

  3. The above-cited thread speaks of having an ICE directly assist the electric motor, but I’d prefer a generator system because I believe the loss from a small ICE running at peak efficiency while generating electricity will be less than the inefficiency of a larger ICE having to run at variable speeds to power the vehicle (I’m assuming I’d need a larger ICE to move the vehicle than I would for producing electricity).


I’m not sure you’re right in your perceptions of ICE range extenders. I have recently converted a pick-up (a New Zealand version of the Ford Ranger). I have the option of putting a 5 kW ICE generator on the back of the pickup. But the more I run the numbers, the more I think that an ICE “pusher” would be more effective. Using an electrical generator you have a few transform inefficiencies. If I put the power into the battery pack as AC, then the maximum current I can inject is 19A. My EV uses about 90A just to run at 30 MPH on flat ground. At highway speed on flat ground the EV is pushing aside much more air and draws about 200A. So, there is no way that an AC generator on the back of the pickup can inject more than a bit of a the power being consumed. I also have to worry that the generator doesn’t produce AC harmonics that damage the on-board charger. Chargers are design for mains power rather than the sometimes variable currents of generators.

In contrast, a pusher can push my EV at highway speed and it is only doing the major class of transformation from the engine to the pusher drive wheels. Pushers sound complex, but they are just front-wheel-drive cars cut off at the fire-wall and coupled to the EV using a conventional A-frame and tow ball coupling. The pollution control is fitted and the engine is efficient for the ranges of RPM used.

Hope this opinion helps.

I disagree with Jens, I think that using an ICE run generator to provide power to move the car is a fine idea. Apparently GM does as well, since this is the concept behind the Chevy Volt. It is to have enough batteries to go 40 miles all electric, then an onboard generator kicks in to keep you going. This method of moving a car has been in use for 30 years. What with 30 years of advances in electrical generation and ICE efficiency, should be able to do better than 45MPH unlimited on flat terrain.

I think the key factor is that some members are looking at putting the electric power generated into the batteries for the EV motor to pull out. Such fast-charging systems likely are not feasible. But if a generator can provide power for the EV motor to pull (and I know that motors pull different amounts of power at different times), then it would be a better option.

My idea is to use a trailer to tow the generator so the weight isn’t present when not needed (in-town use, etc.).

If an 11,000 watt generator could provide sufficient power to run the electric motor (as some members have indicated in other posts), then one like this should work:
Generator specs are: 11,000 watts continuous output, 91.7 Amps at 120 volts or 45.8 Amps at 240 volts.

Fuel consumption using gasoline is about 1 gallon per hour, and LP gas and natural gas also may be used as fuel.

If gasoline is used, and the unit puts out enough power to keep an EV moving at 60 mph, then I’d be getting about 60 mpg in hybrid mode – much better than the 30 mpg I currently get with my ICE vehicle. If the unit also could be charging batteries, then mpg might improve further once batteries are topped off and the vehicle could go back to EV mode.

That all depends on whether the generator would produce enough power to run the EV motor and whether the power supplied by the generator could be converted to supply the motor. Anyone know whether such a system could be set up?

Yes, it can. The generator would need batteries to act as a buffer, no more. The generator will provide the long term current needed, straight to the EM, with any extra going to charge the batteries. In the event of an acceleration event where the speed changes, the batteries would either provide the extra power needed, or absorb the extra power generated before the generator can react to the sudden reduced load. A minimal battery pack is all that would be required.

If you wanted to go with a larger battery pack to go longer on a plugin, you’d need some sort of a power controller that can monitor battery levels, and turn the generator on as necessary. In fact, this would be a better way to go since the controller could then operate the generator in either its most efficient max charge speed or off, but it would take someone that knows how to build such a system to come up with it, not something I’m capable of.

This pretty much describes how the Chevy Volt is intended to operate.

If you wanted to go with a larger battery pack to go longer on a plugin, you’d need some sort of a power controller that can monitor battery levels, and turn the generator on as necessary.

That’s what I’m thinking of. I’m stuck at how to get the power from the generator to the batteries – is a separate system from the charger needed?

As for the controller being the part that turns on the generator – if need be, the driver could manually turn on the generator when the battery pack has been depleted and cut it off when the pack’s charged again. A gauge and some blinking lights to warn of low battery charge and full charge would be sufficient to alert the driver as needed. Not the most elegant solution, but workable.

Getting the power to the batteries would be the same way a car does it now. The alternator has a wire going from the output lead to the battery input. Anything the car needs is drawn at the same point, so the load either goes to the battery or goes to the vehicle.

People think that the alternator only charges the battery and the vehicle can only run off the battery, but in actuality the battery doesn’t have to be there once the vehicle is running. Well, at least that was true on the older vehicles, but today’s computerized vehicles need a battery to supply a constant, unchanging voltage to the computer and an alternator doesn’t have a clean output on its own.

A bit of trivia, a science teacher in high school gave a test I took with one of the questions being about whether or not a car would run without a battery. I naturally answered yes it would, and she marked it wrong. I argued about it, she said that if the battery is taken out of the car that it would no longer run. I immediately offered a practical demonstration in the parking lot, and she threatened to send me to the principal’s office for being disruptive.

It would be easiest if the generator output were the same as the battery load (ie 120V generator, ten 12V batteries in series) as then you could just hook it up the same way. If you had some other voltage you’d need a converter that would adjust voltage to match the smallest series battery pack. An alternator has built in regulator that will add or remove exiter voltage so as to make exactly what the vehicle needs to charge the battery and operate the vehicle, which is what you’d need for this as well. Any generator will have the same feedback control, so there are parts out there that might be adapted.

As to whether or not an automatic system is needed, anything I make has to be wife-friendly, so it would be a requirement that it be automatic.

Battery monitor with generator autostart output
Battery/generator controls in general

I’ve been emailing the folks at, and a recommendation they have is a 108-volt EV system:

If you had a 108 volt system, you could use recterfiers and convert it straight over from AC to DC from the generator