[Christ]

You might consider storing the voltage in a capacitor to use for starting the engine, the single largest momentary tax on your battery/charging system.

There are high current long-store, quick discharge capacitors out there, if I can find the stupid links again. You might be able to set up a circuit which allows for the 300 amp draw (most car starters don't require this even momentarily, I don't think) to occur over a 5 second period, more than enough to start a well-tuned engine. (And even some not so well tuned ones.)