pkh, I owe you an apology. As I sometimes do, I didn't think about what I was saying. I completely overlooked discharge rate . I'm at work right now, and have many numbers floating through my head currently. A 650 CA car battery would be more like a 130 amp charge capacity, not 650Ah. Car batteries are not labelled with charge capacity and discharge rates as we're used to in this hobby, and my mind was somewhere else. So, go ahead, say it. I'm a dumbass sometimes.............
I still think that a 10,000 mah battery is too high a capacity to recharge using the car battery, the alternator is not designed to be a recharge tool, and it will ruin it after too long. Either a deep cycle auxillary battery or generator would be the better choice.
If the car battery was discharged even just 50 amps, it would still take the average alternator an hour plus of driving to replace it. So just starting the car and letting it run for a few minutes isn't a good option, especially since you would most likely use even more capacity cranking the engine over than you would replace in a few minutes of ilding. The alternator needs airflow to cool it just like a motor does, and idle speeds are too slow to cool sufficiently at high charge rates.