Charging a battery of any voltage from a source of any voltage is possible, but always involves loss of power due to conversion inefficiency. The real problem is that charging from 12V implies a desire to use the engine's conventional alternator as the electrical power source (the generator), and it's just not suitable for the task (as dcb explained).
The proposal sounds like a parallel hybrid: the engine drives the vehicle without using the electric system, and the electric motors can drive the vehicle, so the two drive systems are in parallel. Since this has not been expressed as a plug-in hybrid, I assume that all of the energy comes from the engine, so all of the energy in the battery comes from the engine (by driving a generator) - this is a "traditional" or "non-plug-in" hybrid design. In vehicles of this design, the same machine is normally used as the generator and as the drive motor (or one of the drive motors), because it is efficient and suitably sized for the task.
Where are the 15 kW motors planned to be? One per wheel of the axle which the engine drives? One per wheel of the opposite axle from the one the engine drives? One on each axle? This matters to whether or not you can efficiently use one or both of the motors as generators.
The proposal sounds like a parallel hybrid: the engine drives the vehicle without using the electric system, and the electric motors can drive the vehicle, so the two drive systems are in parallel. Since this has not been expressed as a plug-in hybrid, I assume that all of the energy comes from the engine, so all of the energy in the battery comes from the engine (by driving a generator) - this is a "traditional" or "non-plug-in" hybrid design. In vehicles of this design, the same machine is normally used as the generator and as the drive motor (or one of the drive motors), because it is efficient and suitably sized for the task.
Where are the 15 kW motors planned to be? One per wheel of the axle which the engine drives? One per wheel of the opposite axle from the one the engine drives? One on each axle? This matters to whether or not you can efficiently use one or both of the motors as generators.
